3.0.4.md

# OpenAPI Specification

#### Version 3.0.4

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [BCP 14](https://tools.ietf.org/html/bcp14) [RFC2119](https://tools.ietf.org/html/rfc2119) [RFC8174](https://tools.ietf.org/html/rfc8174) when, and only when, they appear in all capitals, as shown here.

This document is licensed under [The Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-2.0.html).

## Introduction

The OpenAPI Specification (OAS) defines a standard, language-agnostic interface to RESTful APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection. When properly defined, a consumer can understand and interact with the remote service with a minimal amount of implementation logic.

An OpenAPI definition can then be used by documentation generation tools to display the API, code generation tools to generate servers and clients in various programming languages, testing tools, and many other use cases.

For examples of OpenAPI usage and additional documentation, please visit [learn.openapis.org](https://learn.openapis.org/).

For extension registries and other specifications published by the OpenAPI Initiative, please visit [spec.openapis.org](https://spec.openapis.org/)

## Table of Contents
<!-- TOC depthFrom:1 depthTo:3 withLinks:1 updateOnSave:1 orderedList:0 -->

- [Definitions](#definitions)
	- [OpenAPI Description](#oasDocument)
	- [Path Templating](#pathTemplating)
	- [Media Types](#mediaTypes)
	- [HTTP Status Codes](#httpCodes)
- [Specification](#specification)
	- [Versions](#versions)
	- [Format](#format)
	- [OpenAPI Description Structure](#documentStructure)
	- [Data Types](#dataTypes)
	- [Rich Text Formatting](#richText)
	- [Relative References In URLs](#relativeReferences)
	- [Schema](#schema)
		- [OpenAPI Object](#oasObject)
		- [Info Object](#infoObject)
		- [Contact Object](#contactObject)
		- [License Object](#licenseObject)
		- [Server Object](#serverObject)
		- [Server Variable Object](#serverVariableObject)
		- [Components Object](#componentsObject)
		- [Paths Object](#pathsObject)
		- [Path Item Object](#pathItemObject)
		- [Operation Object](#operationObject)
		- [External Documentation Object](#externalDocumentationObject)
		- [Parameter Object](#parameterObject)
		- [Request Body Object](#requestBodyObject)
		- [Media Type Object](#mediaTypeObject)
		- [Encoding Object](#encodingObject)
		- [Responses Object](#responsesObject)
		- [Response Object](#responseObject)
		- [Callback Object](#callbackObject)
		- [Example Object](#exampleObject)
		- [Link Object](#linkObject)
		- [Header Object](#headerObject)
		- [Tag Object](#tagObject)
		- [Reference Object](#referenceObject)
		- [Schema Object](#schemaObject)
		- [Discriminator Object](#discriminatorObject)
		- [XML Object](#xmlObject)
		- [Security Scheme Object](#securitySchemeObject)
		- [OAuth Flows Object](#oauthFlowsObject)
		- [OAuth Flow Object](#oauthFlowObject)
		- [Security Requirement Object](#securityRequirementObject)
	- [Specification Extensions](#specificationExtensions)
	- [Security Filtering](#securityFiltering)
- [Appendix A: Revision History](#revisionHistory)
	

<!-- /TOC -->

## Definitions

##### <a name="oasDocument"></a>OpenAPI Description
An OpenAPI Description (OAD) formally describes the surface of an API and its semantics. It is composed of an [entry document](#documentStructure) and any/all of its referenced documents. An OAD uses and conforms to the OpenAPI Specification.

##### <a name="pathTemplating"></a>Path Templating
Path templating refers to the usage of template expressions, delimited by curly braces ({}), to mark a section of a URL path as replaceable using path parameters.

Each template expression in the path MUST correspond to a path parameter that is included in the [Path Item](#path-item-object) itself and/or in each of the Path Item's [Operations](#operation-object).

##### <a name="mediaTypes"></a>Media Types
Media type definitions are spread across several resources.
The media type definitions SHOULD be in compliance with [RFC6838](https://tools.ietf.org/html/rfc6838).

Some examples of possible media type definitions:
```
  text/plain; charset=utf-8
  application/json
  application/vnd.github+json
  application/vnd.github.v3+json
  application/vnd.github.v3.raw+json
  application/vnd.github.v3.text+json
  application/vnd.github.v3.html+json
  application/vnd.github.v3.full+json
  application/vnd.github.v3.diff
  application/vnd.github.v3.patch
```
##### <a name="httpCodes"></a>HTTP Status Codes
The HTTP Status Codes are used to indicate the status of the executed operation. 
Status codes SHOULD be selected from the available status codes registered in the [IANA Status Code Registry](https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml).

##### <a name="httpAndCaseSensitivity"></a>HTTP and Case Sensitivity

As most field names and values in the OpenAPI Specification are case-sensitive, this document endeavors to call out any case-insensitive names and values.
However, the case sensitivity of field names and values that map directly to HTTP concepts follow the case sensitivity rules of HTTP, even if this document does not make a note of every concept.

##### <a name="undefinedAndImplementationDefinedBehavior"></a>Undefined and Implementation-Defined Behavior

This specification deems certain situations to have either _undefined_ or _implementation-defined_ behavior.

Behavior described as _undefined_ is likely, at least in some circumstances, to result in outcomes that contradict the specification.
This description is used when detecting the contradiction is impossible or impractical.
Implementations MAY support undefined scenarios for historical reasons, including ambiguous text in prior versions of the specification.
This support might produce correct outcomes in many cases, but relying on it is NOT RECOMMENDED as there is no guarantee that it will work across all tools or with future specification versions, even if those versions are otherwise strictly compatible with this one.

Behavior described as _implementation-defined_ allows implementations to choose which of several different-but-compliant approaches to a requirement to implement.
This documents ambiguous requirements that API description authors are RECOMMENDED to avoid in order to maximize interoperability.
Unlike undefined behavior, it is safe to rely on implementation-defined behavior if _and only if_ it can be guaranteed that all relevant tools support the same behavior.

## Specification

### Versions

The OpenAPI Specification is versioned using a `major`.`minor`.`patch` versioning scheme. The `major`.`minor` portion of the version string (for example `3.1`) SHALL designate the OAS feature set. *`.patch`* versions address errors in, or provide clarifications to, this document, not the feature set. Tooling which supports OAS 3.1 SHOULD be compatible with all OAS 3.1.\* versions. The patch version SHOULD NOT be considered by tooling, making no distinction between `3.1.0` and `3.1.1` for example.

Occasionally, non-backwards compatible changes may be made in `minor` versions of the OAS where impact is believed to be low relative to the benefit provided.

An OpenAPI description document compatible with OAS 3.\*.\* contains a required [`openapi`](#oasVersion) field which designates the version of the OAS that it uses.

### Format

An OpenAPI description document that conforms to the OpenAPI Specification is itself a JSON object, which may be represented either in JSON or YAML format.

For example, if a field has an array value, the JSON array representation will be used:

```json
{
   "field": [ 1, 2, 3 ]
}
```
All field names in the specification are **case sensitive**.
This includes all fields that are used as keys in a map, except where explicitly noted that keys are **case insensitive**.

The schema exposes two types of fields: Fixed fields, which have a declared name, and Patterned fields, which declare a regex pattern for the field name.

Patterned fields MUST have unique names within the containing object. 

In order to preserve the ability to round-trip between YAML and JSON formats, YAML version [1.2](https://yaml.org/spec/1.2/spec.html) is RECOMMENDED along with some additional constraints:

- Tags MUST be limited to those allowed by [YAML's JSON schema ruleset](https://yaml.org/spec/1.2/spec.html#id2803231), which defines a subset of the YAML syntax and is unrelated to [JSON Schema](https://tools.ietf.org/html/draft-wright-json-schema-00).
- Keys used in YAML maps MUST be limited to a scalar string, as defined by the [YAML Failsafe schema ruleset](https://yaml.org/spec/1.2/spec.html#id2802346).

**Note:** While APIs may be described by OpenAPI documents in either YAML or JSON format, the API request and response bodies and other content are not required to be JSON or YAML.

### <a name="documentStructure"></a>OpenAPI Description Structure

An OpenAPI Description (OAD) MAY be made up of a single document or be divided into multiple, connected parts at the discretion of the author. In the latter case, [Reference Object](#referenceObject) and [Path Item Object](#pathItemObject) `$ref` keywords, as well as the [Link Object](#linkObject) `operationRef` keyword, are used.

Any document consisting entirely of an OpenAPI Object is known as a **complete OpenAPI document**.
In a multi-document description, the document containing the [OpenAPI Object](#oasObject) where parsing begins for a specific API's description is known as that API's **entry OpenAPI document**, or simply **entry document**.

It is RECOMMENDED that the entry OpenAPI document be named: `openapi.json` or `openapi.yaml`.

#### <a name="structuralInteroperability"></a>Structural Interoperability

When parsing an OAD, JSON or YAML objects are parsed into specific Objects (such as [Operation Objects](#operationObject), [Response Objects](#responseObject), [Reference Objects](#referenceObject), etc.) based on the parsing context.  Depending on how references are arranged, a given JSON or YAML object can be parsed in multiple different contexts:

* As a full OpenAPI Description document (an [OpenAPI Object](#oasObject) taking up an entire document)
* As the Object type implied by its parent Object within the document
* As a reference target, with the Object type matching the reference source's context

If the same JSON/YAML object is parsed multiple times and the respective contexts require it to be parsed as _different_ Object types, the resulting behavior is _implementation defined_, and MAY be treated as an error if detected.  An example would be referencing an empty Schema Object under `#/components/schemas` where a Path Item Object is expected, as an empty object is valid for both types.  For maximum interoperability, it is RECOMMENDED that OpenAPI Description authors avoid such scenarios.

#### <a name="resolvingImplicitConnections"></a>Resolving Implicit Connections

### <a name="dataTypes"></a>Data Types

Primitive data types in the OAS are based on the types supported by the [JSON Schema Specification Wright Draft 00](https://tools.ietf.org/html/draft-wright-json-schema-00#section-4.2). 
Note that `integer` as a type is also supported and is defined as a JSON number without a fraction or exponent part. 
`null` is not supported as a type (see [`nullable`](#schemaNullable) for an alternative solution).
Models are defined using the [Schema Object](#schemaObject), which is an extended subset of JSON Schema Specification Wright Draft 00.

<a name="dataTypeFormat"></a>Primitives have an optional modifier property: `format`.
OAS uses several known formats to define in fine detail the data type being used.
However, to support documentation needs, the `format` property is an open `string`-valued property, and can have any value.
Formats such as `"email"`, `"uuid"`, and so on, MAY be used even though they are not defined by this specification.
The OpenAPI Initiative also hosts a [Format Registry](https://spec.openapis.org/registry/format/) for formats defined by OAS users and other specifications.  Support for any registered format is strictly OPTIONAL, and support for one registered format does not imply support for any others.

Types that are not accompanied by a `format` property follow the type definition in the JSON Schema. Tools that do not recognize a specific `format` MAY default back to the `type` alone, as if the `format` is not specified.

The formats defined by the OAS are:

[`type`](#dataTypes) | [`format`](#dataTypeFormat) | Comments
------ | -------- | --------
`integer` | `int32` | signed 32 bits
`integer` | `int64` | signed 64 bits (a.k.a long)
`number` | `float` | |
`number` | `double` | |
`string` | `byte` | base64 encoded characters - [RFC4648](https://www.rfc-editor.org/rfc/rfc4648#section-4)
`string` | `binary` | any sequence of octets
`string` | `date` | As defined by `full-date` - [RFC3339](https://www.rfc-editor.org/rfc/rfc3339#section-5.6)
`string` | `date-time` | As defined by `date-time` - [RFC3339](https://www.rfc-editor.org/rfc/rfc3339#section-5.6)
`string` | `password` | A hint to obscure the value.

#### <a name="binaryData"></a>Working With Binary Data

Two formats, `binary` and `byte`, describe different ways to work with binary data:

* `binary` is used where unencoded binary data is allowed, such as when sending a binary payload as an HTTP message body, or as part of a `multipart/*` payload that allows binary parts
* `byte` is used where binary data is embedded in a text-only format such as `application/json` or `application/x-www-form-urlencoded`

The `maxLength` keyword MAY be used to set an expected upper bound on the length of a streaming payload.  The keyword can be applied to either string data, including encoded binary data, or to unencoded binary data.  For unencoded binary, the length is the number of octets.

Note that the encoding indicated by `byte`, which inflates the size of data in order to represent it as 7-bit ASCII text, is unrelated to HTTP's `Content-Encoding` header, which indicates whether and how a message body has been compressed.

### <a name="richText"></a>Rich Text Formatting
Throughout the specification `description` fields are noted as supporting CommonMark markdown formatting.
Where OpenAPI tooling renders rich text it MUST support, at a minimum, markdown syntax as described by [CommonMark 0.27](https://spec.commonmark.org/0.27/). Tooling MAY choose to ignore some CommonMark or extension features to address security concerns. 

While the framing of CommonMark 0.27 as a minimum requirement means that tooling MAY choose to implement extensions on top of it, note that any such extensions are by definition implementation-defined and will not be interoperable.
OpenAPI Description authors SHOULD consider how text using such extensions will be rendered by tools that offer only the minimum support.


### <a name="relativeReferences"></a>Relative References in URLs

Unless specified otherwise, all properties that are URLs MAY be relative references as defined by [RFC3986](https://tools.ietf.org/html/rfc3986#section-4.2).
Relative references are resolved using the URLs defined in the [`Server Object`](#serverObject) as a Base URI.

Relative references used in `$ref` are processed as per [JSON Reference](https://tools.ietf.org/html/draft-pbryan-zyp-json-ref-03), using the URL of the current document as the base URI. See also the [Reference Object](#referenceObject).

Relative references in CommonMark hyperlinks are resolved in their rendered context, which might differ from the context of the API description.

### Schema

This section describes the structure of the OpenAPI Description format.
This text is the only normative description of the format.
A JSON Schema is hosted on [spec.openapis.org](https://spec.openapis.org) for informational purposes.
If the JSON Schema differs from this section, then this section MUST be considered authoritative.

In the following description, if a field is not explicitly **REQUIRED** or described with a MUST or SHALL, it can be considered OPTIONAL.

#### <a name="oasObject"></a>OpenAPI Object

This is the root document object of the [OpenAPI document](#oasDocument).

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="oasVersion"></a>openapi | `string` | **REQUIRED**. This string MUST be the [semantic version number](https://semver.org/spec/v2.0.0.html) of the [OpenAPI Specification version](#versions) that the OpenAPI document uses. The `openapi` field SHOULD be used by tooling specifications and clients to interpret the OpenAPI document. This is *not* related to the API [`info.version`](#infoVersion) string.
<a name="oasInfo"></a>info | [Info Object](#infoObject) | **REQUIRED**. Provides metadata about the API. The metadata MAY be used by tooling as required.
<a name="oasServers"></a>servers | [[Server Object](#serverObject)] | An array of Server Objects, which provide connectivity information to a target server. If the `servers` property is not provided, or is an empty array, the default value would be a [Server Object](#serverObject) with a [url](#serverUrl) value of `/`.
<a name="oasPaths"></a>paths | [Paths Object](#pathsObject) | **REQUIRED**. The available paths and operations for the API.
<a name="oasComponents"></a>components | [Components Object](#componentsObject) | An element to hold various schemas for the specification.
<a name="oasSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used across the API. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a request. Individual operations can override this definition. To make security optional, an empty security requirement (`{}`) can be included in the array.
<a name="oasTags"></a>tags | [[Tag Object](#tagObject)] | A list of tags used by the specification with additional metadata. The order of the tags can be used to reflect on their order by the parsing tools. Not all tags that are used by the [Operation Object](#operationObject) must be declared. The tags that are not declared MAY be organized randomly or based on the tools' logic. Each tag name in the list MUST be unique.
<a name="oasExternalDocs"></a>externalDocs | [External Documentation Object](#externalDocumentationObject) | Additional external documentation.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

#### <a name="infoObject"></a>Info Object

The object provides metadata about the API.
The metadata MAY be used by the clients if needed, and MAY be presented in editing or documentation generation tools for convenience.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="infoTitle"></a>title | `string` | **REQUIRED**. The title of the API.
<a name="infoDescription"></a>description | `string` | A short description of the API. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="infoTermsOfService"></a>termsOfService | `string` | A URL to the Terms of Service for the API. MUST be in the format of a URL.
<a name="infoContact"></a>contact | [Contact Object](#contactObject) | The contact information for the exposed API.
<a name="infoLicense"></a>license | [License Object](#licenseObject) | The license information for the exposed API.
<a name="infoVersion"></a>version | `string` | **REQUIRED**. The version of the OpenAPI document (which is distinct from the [OpenAPI Specification version](#oasVersion) or the version of API being described).


This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Info Object Example

```json
{
  "title": "Sample Pet Store App",
  "description": "This is a sample server for a pet store.",
  "termsOfService": "http:https://example.com/terms/",
  "contact": {
    "name": "API Support",
    "url": "http:https://www.example.com/support",
    "email": "support@example.com"
  },
  "license": {
    "name": "Apache 2.0",
    "url": "https://www.apache.org/licenses/LICENSE-2.0.html"
  },
  "version": "1.0.1"
}
```

```yaml
title: Sample Pet Store App
description: This is a sample server for a pet store.
termsOfService: http:https://example.com/terms/
contact:
  name: API Support
  url: http:https://www.example.com/support
  email: support@example.com
license:
  name: Apache 2.0
  url: https://www.apache.org/licenses/LICENSE-2.0.html
version: 1.0.1
```

#### <a name="contactObject"></a>Contact Object

Contact information for the exposed API.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="contactName"></a>name | `string` | The identifying name of the contact person/organization.
<a name="contactUrl"></a>url | `string` | The URL pointing to the contact information. MUST be in the format of a URL.
<a name="contactEmail"></a>email | `string` | The email address of the contact person/organization. MUST be in the format of an email address.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Contact Object Example

```json
{
  "name": "API Support",
  "url": "http:https://www.example.com/support",
  "email": "support@example.com"
}
```

```yaml
name: API Support
url: http:https://www.example.com/support
email: support@example.com
```

#### <a name="licenseObject"></a>License Object

License information for the exposed API.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="licenseName"></a>name | `string` | **REQUIRED**. The license name used for the API.
<a name="licenseUrl"></a>url | `string` | A URL to the license used for the API. MUST be in the format of a URL.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### License Object Example

```json
{
  "name": "Apache 2.0",
  "url": "https://www.apache.org/licenses/LICENSE-2.0.html"
}
```

```yaml
name: Apache 2.0
url: https://www.apache.org/licenses/LICENSE-2.0.html
```

#### <a name="serverObject"></a>Server Object

An object representing a Server.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="serverUrl"></a>url | `string` | **REQUIRED**. A URL to the target host.  This URL supports Server Variables and MAY be relative, to indicate that the host location is relative to the location where the OpenAPI document is being served. Variable substitutions will be made when a variable is named in `{`brackets`}`.
<a name="serverDescription"></a>description | `string` | An optional string describing the host designated by the URL. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="serverVariables"></a>variables | Map[`string`, [Server Variable Object](#serverVariableObject)] | A map between a variable name and its value.  The value is used for substitution in the server's URL template.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Server Object Example

A single server would be described as:

```json
{
  "url": "https://development.gigantic-server.com/v1",
  "description": "Development server"
}
```

```yaml
url: https://development.gigantic-server.com/v1
description: Development server
```

The following shows how multiple servers can be described, for example, at the OpenAPI Object's [`servers`](#oasServers):

```json
{
  "servers": [
    {
      "url": "https://development.gigantic-server.com/v1",
      "description": "Development server"
    },
    {
      "url": "https://staging.gigantic-server.com/v1",
      "description": "Staging server"
    },
    {
      "url": "https://api.gigantic-server.com/v1",
      "description": "Production server"
    }
  ]
}
```

```yaml
servers:
- url: https://development.gigantic-server.com/v1
  description: Development server
- url: https://staging.gigantic-server.com/v1
  description: Staging server
- url: https://api.gigantic-server.com/v1
  description: Production server
```

The following shows how variables can be used for a server configuration:

```json
{
  "servers": [
    {
      "url": "https://{username}.gigantic-server.com:{port}/{basePath}",
      "description": "The production API server",
      "variables": {
        "username": {
          "default": "demo",
          "description": "this value is assigned by the service provider, in this example `gigantic-server.com`"
        },
        "port": {
          "enum": [
            "8443",
            "443"
          ],
          "default": "8443"
        },
        "basePath": {
          "default": "v2"
        }
      }
    }
  ]
}
```

```yaml
servers:
- url: https://{username}.gigantic-server.com:{port}/{basePath}
  description: The production API server
  variables:
    username:
      # note! no enum here means it is an open value
      default: demo
      description: this value is assigned by the service provider, in this example `gigantic-server.com`
    port:
      enum:
        - '8443'
        - '443'
      default: '8443'
    basePath:
      # open meaning there is the opportunity to use special base paths as assigned by the provider, default is `v2`
      default: v2
```


#### <a name="serverVariableObject"></a>Server Variable Object

An object representing a Server Variable for server URL template substitution.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="serverVariableEnum"></a>enum | [`string`] | An enumeration of string values to be used if the substitution options are from a limited set. The array SHOULD NOT be empty.
<a name="serverVariableDefault"></a>default | `string` |  **REQUIRED**. The default value to use for substitution, which SHALL be sent if an alternate value is _not_ supplied. Note this behavior is different than the [Schema Object's](#schemaObject) treatment of default values, because in those cases parameter values are optional. If the [`enum`](#serverVariableEnum) is defined, the value SHOULD exist in the enum's values.
<a name="serverVariableDescription"></a>description | `string` | An optional description for the server variable. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

#### <a name="componentsObject"></a>Components Object

Holds a set of reusable objects for different aspects of the OAS.
All objects defined within the components object will have no effect on the API unless they are explicitly referenced from properties outside the components object.


##### Fixed Fields

Field Name | Type | Description
---|:---|---
<a name="componentsSchemas"></a> schemas | Map[`string`, [Schema Object](#schemaObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Schema Objects](#schemaObject).
<a name="componentsResponses"></a> responses | Map[`string`, [Response Object](#responseObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Response Objects](#responseObject).
<a name="componentsParameters"></a> parameters | Map[`string`, [Parameter Object](#parameterObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Parameter Objects](#parameterObject).
<a name="componentsExamples"></a> examples | Map[`string`, [Example Object](#exampleObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Example Objects](#exampleObject).
<a name="componentsRequestBodies"></a> requestBodies | Map[`string`, [Request Body Object](#requestBodyObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Request Body Objects](#requestBodyObject).
<a name="componentsHeaders"></a> headers | Map[`string`, [Header Object](#headerObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Header Objects](#headerObject).
<a name="componentsSecuritySchemes"></a> securitySchemes| Map[`string`, [Security Scheme Object](#securitySchemeObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Security Scheme Objects](#securitySchemeObject).
<a name="componentsLinks"></a> links | Map[`string`, [Link Object](#linkObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Link Objects](#linkObject).
<a name="componentsCallbacks"></a> callbacks | Map[`string`, [Callback Object](#callbackObject) \| [Reference Object](#referenceObject)] | An object to hold reusable [Callback Objects](#callbackObject).

This object MAY be extended with [Specification Extensions](#specificationExtensions).

All the fixed fields declared above are objects that MUST use keys that match the regular expression: `^[a-zA-Z0-9\.\-_]+$`.

Field Name Examples:

```
User
User_1
User_Name
user-name
my.org.User
```

##### Components Object Example

```json
"components": {
  "schemas": {
    "GeneralError": {
      "type": "object",
      "properties": {
        "code": {
          "type": "integer",
          "format": "int32"
        },
        "message": {
          "type": "string"
        }
      }
    },
    "Category": {
      "type": "object",
      "properties": {
        "id": {
          "type": "integer",
          "format": "int64"
        },
        "name": {
          "type": "string"
        }
      }
    },
    "Tag": {
      "type": "object",
      "properties": {
        "id": {
          "type": "integer",
          "format": "int64"
        },
        "name": {
          "type": "string"
        }
      }
    }
  },
  "parameters": {
    "skipParam": {
      "name": "skip",
      "in": "query",
      "description": "number of items to skip",
      "required": true,
      "schema": {
        "type": "integer",
        "format": "int32"
      }
    },
    "limitParam": {
      "name": "limit",
      "in": "query",
      "description": "max records to return",
      "required": true,
      "schema" : {
        "type": "integer",
        "format": "int32"
      }
    }
  },
  "responses": {
    "NotFound": {
      "description": "Entity not found."
    },
    "IllegalInput": {
      "description": "Illegal input for operation."
    },
    "GeneralError": {
      "description": "General Error",
      "content": {
        "application/json": {
          "schema": {
            "$ref": "#/components/schemas/GeneralError"
          }
        }
      }
    }
  },
  "securitySchemes": {
    "api_key": {
      "type": "apiKey",
      "name": "api_key",
      "in": "header"
    },
    "petstore_auth": {
      "type": "oauth2",
      "flows": {
        "implicit": {
          "authorizationUrl": "http:https://example.org/api/oauth/dialog",
          "scopes": {
            "write:pets": "modify pets in your account",
            "read:pets": "read your pets"
          }
        }
      }
    }
  }
}
```

```yaml
components:
  schemas:
    GeneralError:
      type: object
      properties:
        code:
          type: integer
          format: int32
        message:
          type: string
    Category:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
    Tag:
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
  parameters:
    skipParam:
      name: skip
      in: query
      description: number of items to skip
      required: true
      schema:
        type: integer
        format: int32
    limitParam:
      name: limit
      in: query
      description: max records to return
      required: true
      schema:
        type: integer
        format: int32
  responses:
    NotFound:
      description: Entity not found.
    IllegalInput:
      description: Illegal input for operation.
    GeneralError:
      description: General Error
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/GeneralError'
  securitySchemes:
    api_key:
      type: apiKey
      name: api_key
      in: header
    petstore_auth:
      type: oauth2
      flows:
        implicit:
          authorizationUrl: http:https://example.org/api/oauth/dialog
          scopes:
            write:pets: modify pets in your account
            read:pets: read your pets
```


#### <a name="pathsObject"></a>Paths Object

Holds the relative paths to the individual endpoints and their operations.
The path is appended to the URL from the [`Server Object`](#serverObject) in order to construct the full URL.  The Paths MAY be empty, due to [ACL constraints](#securityFiltering).

##### Patterned Fields

Field Pattern | Type | Description
---|:---:|---
<a name="pathsPath"></a>/{path} | [Path Item Object](#pathItemObject) | A relative path to an individual endpoint. The field name MUST begin with a forward slash (`/`). The path is **appended** (no relative URL resolution) to the expanded URL from the [`Server Object`](#serverObject)'s `url` field in order to construct the full URL. [Path templating](#pathTemplating) is allowed. When matching URLs, concrete (non-templated) paths would be matched before their templated counterparts. Templated paths with the same hierarchy but different templated names MUST NOT exist as they are identical. In case of ambiguous matching, it's up to the tooling to decide which one to use.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Path Templating Matching

Assuming the following paths, the concrete definition, `/pets/mine`, will be matched first if used:

```
  /pets/{petId}
  /pets/mine
```

The following paths are considered identical and invalid:

```
  /pets/{petId}
  /pets/{name}
```

The following may lead to ambiguous resolution:

```
  /{entity}/me
  /books/{id}
```

##### Paths Object Example

```json
{
  "/pets": {
    "get": {
      "description": "Returns all pets from the system that the user has access to",
      "responses": {
        "200": {
          "description": "A list of pets.",
          "content": {
            "application/json": {
              "schema": {
                "type": "array",
                "items": {
                  "$ref": "#/components/schemas/pet"
                }
              }
            }
          }
        }
      }
    }
  }
}
```

```yaml
/pets:
  get:
    description: Returns all pets from the system that the user has access to
    responses:
      '200':
        description: A list of pets.
        content:
          application/json:
            schema:
              type: array
              items:
                $ref: '#/components/schemas/pet'
```

#### <a name="pathItemObject"></a>Path Item Object

Describes the operations available on a single path.
A Path Item MAY be empty, due to [ACL constraints](#securityFiltering).
The path itself is still exposed to the documentation viewer but they will not know which operations and parameters are available.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="pathItemRef"></a>$ref | `string` | Allows for an external definition of this path item. The referenced structure MUST be in the format of a [Path Item Object](#pathItemObject).  In case a Path Item Object field appears both in the defined object and the referenced object, the behavior is undefined.
<a name="pathItemSummary"></a>summary| `string` | An optional string summary, intended to apply to all operations in this path.
<a name="pathItemDescription"></a>description | `string` | An optional string description, intended to apply to all operations in this path. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="pathItemGet"></a>get | [Operation Object](#operationObject) | A definition of a GET operation on this path.
<a name="pathItemPut"></a>put | [Operation Object](#operationObject) | A definition of a PUT operation on this path.
<a name="pathItemPost"></a>post | [Operation Object](#operationObject) | A definition of a POST operation on this path.
<a name="pathItemDelete"></a>delete | [Operation Object](#operationObject) | A definition of a DELETE operation on this path.
<a name="pathItemOptions"></a>options | [Operation Object](#operationObject) | A definition of a OPTIONS operation on this path.
<a name="pathItemHead"></a>head | [Operation Object](#operationObject) | A definition of a HEAD operation on this path.
<a name="pathItemPatch"></a>patch | [Operation Object](#operationObject) | A definition of a PATCH operation on this path.
<a name="pathItemTrace"></a>trace | [Operation Object](#operationObject) | A definition of a TRACE operation on this path.
<a name="pathItemServers"></a>servers | [[Server Object](#serverObject)] | An alternative `server` array to service all operations in this path. If an alternative server object is specified at the Root level, it will be overridden by this value.
<a name="pathItemParameters"></a>parameters | [[Parameter Object](#parameterObject) \| [Reference Object](#referenceObject)] | A list of parameters that are applicable for all the operations described under this path. These parameters can be overridden at the operation level, but cannot be removed there. The list MUST NOT include duplicated parameters. A unique parameter is defined by a combination of a [name](#parameterName) and [location](#parameterIn). The list can use the [Reference Object](#referenceObject) to link to parameters that are defined at the [OpenAPI Object's components/parameters](#componentsParameters). 


This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Path Item Object Example

```json
{
  "get": {
    "description": "Returns pets based on ID",
    "summary": "Find pets by ID",
    "operationId": "getPetsById",
    "responses": {
      "200": {
        "description": "pet response",
        "content": {
          "*/*": {
            "schema": {
              "type": "array",
              "items": {
                "$ref": "#/components/schemas/Pet"
              }
            }
          }
        }
      },
      "default": {
        "description": "error payload",
        "content": {
          "text/html": {
            "schema": {
              "$ref": "#/components/schemas/ErrorModel"
            }
          }
        }
      }
    }
  },
  "parameters": [
    {
      "name": "id",
      "in": "path",
      "description": "ID of pet to use",
      "required": true,
      "schema": {
        "type": "array",
        "items": {
          "type": "string"
        }
      },
      "style": "simple"
    }
  ]
}
```

```yaml
get:
  description: Returns pets based on ID
  summary: Find pets by ID
  operationId: getPetsById
  responses:
    '200':
      description: pet response
      content:
        '*/*':
          schema:
            type: array
            items:
              $ref: '#/components/schemas/Pet'
    default:
      description: error payload
      content:
        text/html:
          schema:
            $ref: '#/components/schemas/ErrorModel'
parameters:
- name: id
  in: path
  description: ID of pet to use
  required: true
  schema:
    type: array
    items:
      type: string
  style: simple
```

#### <a name="operationObject"></a>Operation Object

Describes a single API operation on a path.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="operationTags"></a>tags | [`string`] | A list of tags for API documentation control. Tags can be used for logical grouping of operations by resources or any other qualifier.
<a name="operationSummary"></a>summary | `string` | A short summary of what the operation does.
<a name="operationDescription"></a>description | `string` | A verbose explanation of the operation behavior. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="operationExternalDocs"></a>externalDocs | [External Documentation Object](#externalDocumentationObject) | Additional external documentation for this operation.
<a name="operationId"></a>operationId | `string` | Unique string used to identify the operation. The id MUST be unique among all operations described in the API. The operationId value is **case-sensitive**. Tools and libraries MAY use the operationId to uniquely identify an operation, therefore, it is RECOMMENDED to follow common programming naming conventions.
<a name="operationParameters"></a>parameters | [[Parameter Object](#parameterObject) \| [Reference Object](#referenceObject)] | A list of parameters that are applicable for this operation. If a parameter is already defined at the [Path Item](#pathItemParameters), the new definition will override it but can never remove it. The list MUST NOT include duplicated parameters. A unique parameter is defined by a combination of a [name](#parameterName) and [location](#parameterIn). The list can use the [Reference Object](#referenceObject) to link to parameters that are defined at the [OpenAPI Object's components/parameters](#componentsParameters).
<a name="operationRequestBody"></a>requestBody | [Request Body Object](#requestBodyObject) \| [Reference Object](#referenceObject) | The request body applicable for this operation.  The `requestBody` is only supported in HTTP methods where the HTTP 1.1 specification [RFC7231](https://tools.ietf.org/html/rfc7231#section-4.3.1) has explicitly defined semantics for request bodies.  In other cases where the HTTP spec is vague, `requestBody` SHALL be ignored by consumers.
<a name="operationResponses"></a>responses | [Responses Object](#responsesObject) | **REQUIRED**. The list of possible responses as they are returned from executing this operation.
<a name="operationCallbacks"></a>callbacks | Map[`string`, [Callback Object](#callbackObject) \| [Reference Object](#referenceObject)] | A map of possible out-of band callbacks related to the parent operation. The key is a unique identifier for the Callback Object. Each value in the map is a [Callback Object](#callbackObject) that describes a request that may be initiated by the API provider and the expected responses.
<a name="operationDeprecated"></a>deprecated | `boolean` | Declares this operation to be deprecated. Consumers SHOULD refrain from usage of the declared operation. Default value is `false`.
<a name="operationSecurity"></a>security | [[Security Requirement Object](#securityRequirementObject)] | A declaration of which security mechanisms can be used for this operation. The list of values includes alternative security requirement objects that can be used. Only one of the security requirement objects need to be satisfied to authorize a request. To make security optional, an empty security requirement (`{}`) can be included in the array. This definition overrides any declared top-level [`security`](#oasSecurity). To remove a top-level security declaration, an empty array can be used.
<a name="operationServers"></a>servers | [[Server Object](#serverObject)] | An alternative `server` array to service this operation. If an alternative `server` object is specified at the Path Item Object or Root level, it will be overridden by this value.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Operation Object Example

```json
{
  "tags": [
    "pet"
  ],
  "summary": "Updates a pet in the store with form data",
  "operationId": "updatePetWithForm",
  "parameters": [
    {
      "name": "petId",
      "in": "path",
      "description": "ID of pet that needs to be updated",
      "required": true,
      "schema": {
        "type": "string"
      }
    }
  ],
  "requestBody": {
    "content": {
      "application/x-www-form-urlencoded": {
        "schema": {
          "type": "object",
          "properties": {
            "name": {
              "description": "Updated name of the pet",
              "type": "string"
            },
            "status": {
              "description": "Updated status of the pet",
              "type": "string"
            }
          },
          "required": ["status"]
        }
      }
    }
  },
  "responses": {
    "200": {
      "description": "Pet updated.",
      "content": {
        "application/json": {},
        "application/xml": {}
      }
    },
    "405": {
      "description": "Method Not Allowed",
      "content": {
        "application/json": {},
        "application/xml": {}
      }
    }
  },
  "security": [
    {
      "petstore_auth": [
        "write:pets",
        "read:pets"
      ]
    }
  ]
}
```

```yaml
tags:
- pet
summary: Updates a pet in the store with form data
operationId: updatePetWithForm
parameters:
- name: petId
  in: path
  description: ID of pet that needs to be updated
  required: true
  schema:
    type: string
requestBody:
  content:
    application/x-www-form-urlencoded:
      schema:
       properties:
         name:
           description: Updated name of the pet
           type: string
         status:
           description: Updated status of the pet
           type: string
       required:
         - status
responses:
  '200':
    description: Pet updated.
    content:
      application/json: {}
      application/xml: {}
  '405':
    description: Method Not Allowed
    content:
      application/json: {}
      application/xml: {}
security:
- petstore_auth:
  - write:pets
  - read:pets
```


#### <a name="externalDocumentationObject"></a>External Documentation Object

Allows referencing an external resource for extended documentation.

##### Fixed Fields

Field Name | Type | Description
---|:---:|---
<a name="externalDocDescription"></a>description | `string` | A short description of the target documentation. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="externalDocUrl"></a>url | `string` | **REQUIRED**. The URL for the target documentation. Value MUST be in the format of a URL.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### External Documentation Object Example

```json
{
  "description": "Find more info here",
  "url": "https://example.com"
}
```

```yaml
description: Find more info here
url: https://example.com
```

#### <a name="parameterObject"></a>Parameter Object

Describes a single operation parameter.

A unique parameter is defined by a combination of a [name](#parameterName) and [location](#parameterIn).

See [Appendix E](#percentEncodingAndFormMediaTypes) for a detailed examination of percent-encoding concerns, including interactions with the `application/x-www-form-urlencoded` query string format.

##### Parameter Locations
There are four possible parameter locations specified by the `in` field:
* path - Used together with [Path Templating](#pathTemplating), where the parameter value is actually part of the operation's URL. This does not include the host or base path of the API. For example, in `/items/{itemId}`, the path parameter is `itemId`.
* query - Parameters that are appended to the URL. For example, in `/items?id=###`, the query parameter is `id`.
* header - Custom headers that are expected as part of the request. Note that [RFC7230](https://tools.ietf.org/html/rfc7230#page-22) states header names are case insensitive.
* cookie - Used to pass a specific cookie value to the API.

##### Fixed Fields

The rules for serialization of the parameter are specified in one of two ways.
Parameter Objects MUST include either a `content` field or a `schema` field, but not both.
See [Appendix B](#dataTypeConversion) for a discussion of converting values of various types to string representations.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

###### Common Fixed Fields

These fields MAY be used with either `content` or `schema`.

Field Name | Type | Description
---|:---:|---
<a name="parameterName"></a>name | `string` | **REQUIRED**. The name of the parameter. Parameter names are *case sensitive*. <ul><li>If [`in`](#parameterIn) is `"path"`, the `name` field MUST correspond to a template expression occurring within the [path](#pathsPath) field in the [Paths Object](#pathsObject). See [Path Templating](#pathTemplating) for further information.<li>If [`in`](#parameterIn) is `"header"` and the `name` field is `"Accept"`, `"Content-Type"` or `"Authorization"`, the parameter definition SHALL be ignored.<li>For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameterIn) property.</ul>
<a name="parameterIn"></a>in | `string` | **REQUIRED**. The location of the parameter. Possible values are `"query"`, `"header"`, `"path"` or `"cookie"`.
<a name="parameterDescription"></a>description | `string` | A brief description of the parameter. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="parameterRequired"></a>required | `boolean` | Determines whether this parameter is mandatory. If the [parameter location](#parameterIn) is `"path"`, this property is **REQUIRED** and its value MUST be `true`. Otherwise, the property MAY be included and its default value is `false`.
<a name="parameterDeprecated"></a> deprecated | `boolean` | Specifies that a parameter is deprecated and SHOULD be transitioned out of usage. Default value is `false`.
<a name="parameterAllowEmptyValue"></a> allowEmptyValue | `boolean` | If `true`, clients MAY pass a zero-length string value in place of parameters that would otherwise be omitted entirely, which the server SHOULD interpret as the parameter being unused. Default value is `false`. If [`style`](#parameterStyle) is used, and if [behavior is _n/a_ (cannot be serialized)](#style-examples), the value of `allowEmptyValue` SHALL be ignored. Interactions between this field and the parameter's [Schema Object](#schemaObject) are implementation-defined. This field is valid only for `query` parameters. Use of this property is NOT RECOMMENDED, and it is likely to be removed in a later revision.

Note that while `"Cookie"` as a `name` is not forbidden with `in: header`, the effect of defining a cookie parameter that way is undefined; use `in: cookie` instead.

###### Fixed Fields for use with `schema`

For simpler scenarios, a [`schema`](#parameterSchema) and [`style`](#parameterStyle) can describe the structure and syntax of the parameter.
When `example` or `examples` are provided in conjunction with the `schema` object, the example SHOULD match the specified schema and follow the prescribed serialization strategy for the parameter.
The `example` and `examples` fields are mutually exclusive, and if either is present it SHALL _override_ any `example` in the schema.

Serializing with `schema` is NOT RECOMMENDED for `in: cookie` parameters, `in: header` parameters that use HTTP header parameters (name=value pairs following a `;`) in their values, or `in: header` parameters where values might have non-URL-safe characters; see [Appendix D](#serializingHeadersAndCookies) for details.

Field Name | Type | Description
---|:---:|---
<a name="parameterStyle"></a>style | `string` | Describes how the parameter value will be serialized depending on the type of the parameter value. Default values (based on value of `in`): for `query` - `form`; for `path` - `simple`; for `header` - `simple`; for `cookie` - `form`.
<a name="parameterExplode"></a>explode | `boolean` | When this is true, parameter values of type `array` or `object` generate separate parameters for each value of the array or key-value pair of the map. For other types of parameters this property has no effect. When [`style`](#parameterStyle) is `form`, the default value is `true`. For all other styles, the default value is `false`. Note that despite `false` being the default for `deepObject`, the combination of `false` with `deepObject` is undefined.
<a name="parameterAllowReserved"></a>allowReserved | `boolean` | When this is true, parameter values are serialized  using reserved expansion, as defined by [RFC6570](https://datatracker.ietf.org/doc/html/rfc6570#autoid-20), which allows [RFC3986's reserved character set](https://datatracker.ietf.org/doc/html/rfc3986#autoid-13), as well as percent-encoded triples, to pass through unchanged, while still percent-encoding all other disallowed characters (including `%` outside of percent-encoded triples). Applications are still responsible for percent-encoding reserved characters that are [not allowed in the query string](https://datatracker.ietf.org/doc/html/rfc3986#autoid-24) (`[`, `]`, `#`), or have a special meaning in `application/x-www-form-urlencoded` (`-`, `&`, `+`); see Appendices [C](#usingRFC6570Implementations) and [E](#percentEncodingAndFormMediaTypes) for details. This property only applies to parameters with an `in` value of `query`. The default value is `false`.
<a name="parameterSchema"></a>schema | [Schema Object](#schemaObject) \| [Reference Object](#referenceObject) | The schema defining the type used for the parameter.
<a name="parameterExample"></a>example | Any | Example of the parameter's potential value; see [Working With Examples](#working-with-examples).
<a name="parameterExamples"></a>examples | Map[ `string`, [Example Object](#exampleObject) \| [Reference Object](#referenceObject)] | Examples of the parameter's potential value; see [Working With Examples](#working-with-examples).

See also [Appendix C: Using RFC6570 Implementations](#usingRFC6570Implementations) for additional guidance.

###### Fixed Fields for use with `content`

For more complex scenarios, the [`content`](#parameterContent) property can define the media type and schema of the parameter, as well as give examples of its use.
Using `content` with a `text/plain` media type is RECOMMENDED for `in: header` and `in: cookie` parameters where the `schema` strategy is not appropriate.

Field Name | Type | Description
---|:---:|---
<a name="parameterContent"></a>content | Map[`string`, [Media Type Object](#mediaTypeObject)] | A map containing the representations for the parameter. The key is the media type and the value describes it. The map MUST only contain one entry.


##### Style Values

In order to support common ways of serializing simple parameters, a set of `style` values are defined.

`style` | [`type`](#dataTypes) | `in` | Comments
----------- | ------ | -------- | --------
matrix | `primitive`, `array`, `object` | `path` | Path-style parameters defined by [RFC6570](https://tools.ietf.org/html/rfc6570#section-3.2.7)
label | `primitive`, `array`, `object` | `path` | Label style parameters defined by [RFC6570](https://tools.ietf.org/html/rfc6570#section-3.2.5)
simple | `primitive`, `array`, `object` | `path`, `header` | Simple style parameters defined by [RFC6570](https://tools.ietf.org/html/rfc6570#section-3.2.2). This option replaces `collectionFormat` with a `csv` value from OpenAPI 2.0.
form | `primitive`, `array`, `object` | `query`, `cookie` | Form style parameters defined by [RFC6570](https://tools.ietf.org/html/rfc6570#section-3.2.8). This option replaces `collectionFormat` with a `csv` (when `explode` is false) or `multi` (when `explode` is true) value from OpenAPI 2.0.
spaceDelimited | `array`, `object` | `query` | Space separated array values or object properties and values. This option replaces `collectionFormat` equal to `ssv` from OpenAPI 2.0.
pipeDelimited | `array`, `object` | `query` | Pipe separated array values or object properties and values. This option replaces `collectionFormat` equal to `pipes` from OpenAPI 2.0.
deepObject | `object` | `query` | Allows objects with scalar properties to be represented using form parameters. The representation of array or object properties is not defined.

See [Appendix E](#percentEncodingAndFormMediaTypes) for a discussion of percent-encoding, including when delimiters need to be percent-encoded and options for handling collisions with percent-encoded data.

##### Style Examples

Assume a parameter named `color` has one of the following values:

```
   string -> "blue"
   array -> ["blue","black","brown"]
   object -> { "R": 100, "G": 200, "B": 150 }
```

The following table shows examples, as would be shown with the `example` or `examples` keywords, of the different serializations for each value.

* The value _empty_ denotes the empty string, and is unrelated to the `allowEmptyValue` field
* The behavior of combinations marked _n/a_ is undefined
* The `undefined` replaces the `empty` column in previous versions of this specification in order to better align with [RFC6570 §2.3](https://www.rfc-editor.org/rfc/rfc6570.html#section-2.3) terminology, which describes certain values including but not limited to `null` as "undefined" values with special handling; notably, the empty string is _not_ undefined
* For `form` and the non-RFC6570 query string styles `spaceDelimited`, `pipeDelimited`, and `deepObject`, each example is shown prefixed with `?` as if it were the only query parameter; see [Appendix C](#usingRFC6570Implementations) for more information on constructing query strings from multiple parameters, and [Appendix D](#serializingHeadersAndCookies) for warnings regarding `form` and cookie parameters
* Note that the `?` prefix is not appropriate for serializing `application/x-www-form-urlencoded` HTTP message bodies, and MUST be stripped or (if constructing the string manually) not added when used in that context; see the [Encoding Object](#encodingObject) for more information
* The examples are percent-encoded as required by RFC6570 and RFC3986; see [Appendix E](#percentEncodingAndFormMediaTypes) for a thorough discussion of percent-encoding concerns, including why unencoded `|` (`%7C`), `[` (`%5B`), and `]` (`%5D`) seem to work in some environments despite not being compliant.

[`style`](#dataTypeFormat) | `explode` | `empty` | `string` | `array` | `object`
----------- | ------ | -------- | -------- | -------- | -------
matrix | false | ;color | ;color=blue | ;color=blue,black,brown | ;color=R,100,G,200,B,150
matrix | true | ;color | ;color=blue | ;color=blue;color=black;color=brown | ;R=100;G=200;B=150
label | false | . | .blue | .blue,black,brown | .R,100,G,200,B,150
label | true | . | .blue | .blue.black.brown | .R=100.G=200.B=150
simple | false | _empty_ | blue | blue,black,brown | R,100,G,200,B,150
simple | true | _empty_ | blue | blue,black,brown | R=100,G=200,B=150
form | false | ?color= | ?color=blue | ?color=blue,black,brown | ?color=R,100,G,200,B,150
form | true | ?color= | ?color=blue | ?color=blue&color=black&color=brown | ?R=100&G=200&B=150
spaceDelimited | false | _n/a_ | _n/a_ | ?color=blue%20black%20brown | ?color=R%20100%20G%20200%20B%20150
spaceDelimited | true | _n/a_ | _n/a_ | _n/a_ | _n/a_
pipeDelimited | false | _n/a_ | _n/a_ | ?color=blue%7Cblack%7Cbrown | ?color=R%7C100%7CG%7C200%7CB%7C150
pipeDelimited | true | _n/a_ | _n/a_ | _n/a_ | _n/a_
deepObject | false | _n/a_ | _n/a_ | _n/a_ | _n/a_
deepObject | true | _n/a_ | _n/a_ | _n/a_ | ?color%5BR%5D=100&color%5BG%5D=200&color%5BB%5D=150

##### Parameter Object Examples

A header parameter with an array of 64 bit integer numbers:

```json
{
  "name": "token",
  "in": "header",
  "description": "token to be passed as a header",
  "required": true,
  "schema": {
    "type": "array",
    "items": {
      "type": "integer",
      "format": "int64"
    }
  },
  "style": "simple"
}
```

```yaml
name: token
in: header
description: token to be passed as a header
required: true
schema:
  type: array
  items:
    type: integer
    format: int64
style: simple
```

A path parameter of a string value:
```json
{
  "name": "username",
  "in": "path",
  "description": "username to fetch",
  "required": true,
  "schema": {
    "type": "string"
  }
}
```

```yaml
name: username
in: path
description: username to fetch
required: true
schema:
  type: string
```

An optional query parameter of a string value, allowing multiple values by repeating the query parameter:
```json
{
  "name": "id",
  "in": "query",
  "description": "ID of the object to fetch",
  "required": false,
  "schema": {
    "type": "array",
    "items": {
      "type": "string"
    }
  },
  "style": "form",
  "explode": true
}
```

```yaml
name: id
in: query
description: ID of the object to fetch
required: false
schema:
  type: array
  items:
    type: string
style: form
explode: true
```

A free-form query parameter, allowing undefined parameters of a specific type:
```json
{
  "in": "query",
  "name": "freeForm",
  "schema": {
    "type": "object",
    "additionalProperties": {
      "type": "integer"
    },
  },
  "style": "form"
}
```

```yaml
in: query
name: freeForm
schema:
  type: object
  additionalProperties:
    type: integer
style: form
```

A complex parameter using `content` to define serialization:

```json
{
  "in": "query",
  "name": "coordinates",
  "content": {
    "application/json": {
      "schema": {
        "type": "object",
        "required": [
          "lat",
          "long"
        ],
        "properties": {
          "lat": {
            "type": "number"
          },
          "long": {
            "type": "number"
          }
        }
      }
    }
  }
}
```

```yaml
in: query
name: coordinates
content:
  application/json:
    schema:
      type: object
      required:
        - lat
        - long
      properties:
        lat:
          type: number
        long:
          type: number
```

#### <a name="requestBodyObject"></a>Request Body Object

Describes a single request body.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="requestBodyDescription"></a>description | `string` | A brief description of the request body. This could contain examples of use.  [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="requestBodyContent"></a>content | Map[`string`, [Media Type Object](#mediaTypeObject)] | **REQUIRED**. The content of the request body. The key is a media type or [media type range](https://tools.ietf.org/html/rfc7231#appendix-D) and the value describes it.  For requests that match multiple keys, only the most specific key is applicable. e.g. text/plain overrides text/*
<a name="requestBodyRequired"></a>required | `boolean` | Determines if the request body is required in the request. Defaults to `false`.


This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Request Body Examples

A request body with a referenced schema definition.
```json
{
  "description": "user to add to the system",
  "content": {
    "application/json": {
      "schema": {
        "$ref": "#/components/schemas/User"
      },
      "examples": {
        "user" : {
          "summary": "User Example",
          "externalValue": "http:https://foo.bar/examples/user-example.json"
        }
      }
    },
    "application/xml": {
      "schema": {
        "$ref": "#/components/schemas/User"
      },
      "examples": {
        "user" : {
          "summary": "User example in XML",
          "externalValue": "http:https://foo.bar/examples/user-example.xml"
        }
      }
    },
    "text/plain": {
      "examples": {
        "user" : {
          "summary": "User example in Plain text",
          "externalValue": "http:https://foo.bar/examples/user-example.txt"
        }
      }
    },
    "*/*": {
      "examples": {
        "user" : {
          "summary": "User example in other format",
          "externalValue": "http:https://foo.bar/examples/user-example.whatever"
        }
      }
    }
  }
}
```

```yaml
description: user to add to the system
content:
  application/json:
    schema:
      $ref: '#/components/schemas/User'
    examples:
      user:
        summary: User Example
        externalValue: http:https://foo.bar/examples/user-example.json
  application/xml:
    schema:
      $ref: '#/components/schemas/User'
    examples:
      user:
        summary: User Example in XML
        externalValue: http:https://foo.bar/examples/user-example.xml
  text/plain:
    examples:
      user:
        summary: User example in text plain format
        externalValue: http:https://foo.bar/examples/user-example.txt
  '*/*':
    examples:
      user:
        summary: User example in other format
        externalValue: http:https://foo.bar/examples/user-example.whatever
```

#### <a name="mediaTypeObject"></a>Media Type Object
Each Media Type Object provides schema and examples for the media type identified by its key.

When `example` or `examples` are provided, the example SHOULD match the specified schema and be in the correct format as specified by the media type and its encoding.
The `example` and `examples` fields are mutually exclusive, and if either is present it SHALL _override_ any `example` in the schema.
See [Working With Examples](#working-with-examples) for further guidance regarding the different ways of specifying examples, including non-JSON/YAML values.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="mediaTypeSchema"></a>schema | [Schema Object](#schemaObject) \| [Reference Object](#referenceObject) | The schema defining the content of the request, response, or parameter.
<a name="mediaTypeExample"></a>example | Any | Example of the media type; see [Working With Examples](#working-with-examples).
<a name="mediaTypeExamples"></a>examples | Map[ `string`, [Example Object](#exampleObject) \| [Reference Object](#referenceObject)] | Examples of the media type; see [Working With Examples](#working-with-examples).
<a name="mediaTypeEncoding"></a>encoding | Map[`string`, [Encoding Object](#encodingObject)] | A map between a property name and its encoding information. The key, being the property name, MUST exist in the schema as a property. The encoding attribute SHALL only apply to [Request Body Objects](#requestBodyObject), and only when the media type is `multipart` or `application/x-www-form-urlencoded`.  If no Encoding Object is provided for a property, the behavior is determined by the default values documented for the Encoding Object.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Media Type Examples

```json
{
  "application/json": {
    "schema": {
      "$ref": "#/components/schemas/Pet"
    },
    "examples": {
      "cat" : {
        "summary": "An example of a cat",
        "value": {
          "name": "Fluffy",
          "petType": "Cat",
          "color": "White",
          "gender": "male",
          "breed": "Persian"
        }
      },
      "dog": {
        "summary": "An example of a dog with a cat's name",
        "value" : {
          "name": "Puma",
          "petType": "Dog",
          "color": "Black",
          "gender": "Female",
          "breed": "Mixed"
        }
      },
      "frog": {
        "$ref": "#/components/examples/frog-example"
      }
    }
  }
}
```

```yaml
application/json:
  schema:
    $ref: "#/components/schemas/Pet"
  examples:
    cat:
      summary: An example of a cat
      value:
        name: Fluffy
        petType: Cat
        color: White
        gender: male
        breed: Persian
    dog:
      summary: An example of a dog with a cat's name
      value:
        name: Puma
        petType: Dog
        color: Black
        gender: Female
        breed: Mixed
    frog:
      $ref: "#/components/examples/frog-example"
```

##### Considerations for File Uploads

In contrast with the 2.0 specification, `file` input/output content in OpenAPI is described with the same semantics as any other schema type. Specifically:

```yaml
# content transferred in binary (octet-stream):
schema:
  type: string
  format: binary
```

These examples apply to either input payloads of file uploads or response payloads.

A `requestBody` for submitting a file in a `POST` operation may look like the following example:

```yaml
requestBody:
  content:
    application/octet-stream:
      schema:
        # a binary file of any type
        type: string
        format: binary
```

In addition, specific media types MAY be specified:

```yaml
# multiple, specific media types may be specified:
requestBody:
  content:
      # a binary file of type png or jpeg
    'image/jpeg':
      schema:
        type: string
        format: binary
    'image/png':
      schema:
        type: string
        format: binary
```

To upload multiple files, a `multipart` media type MUST be used:

```yaml
requestBody:
  content:
    multipart/form-data:
      schema:
        properties:
          # The property name 'file' will be used for all files.
          file:
            type: array
            items:
              type: string
              format: binary

```

##### Support for x-www-form-urlencoded Request Bodies

To submit content using form url encoding via [RFC1866](https://tools.ietf.org/html/rfc1866), the following
definition may be used:

```yaml
requestBody:
  content:
    application/x-www-form-urlencoded:
      schema:
        type: object
        properties:
          id:
            type: string
            format: uuid
          address:
            # complex types are stringified to support RFC 1866
            type: object
            properties: {}
```

In this example, the contents in the `requestBody` MUST be encoded per [RFC1866](https://tools.ietf.org/html/rfc1866) when passed to the server.  In addition, the `address` field complex object will be serialized to a string representation prior to encoding.

When passing complex objects in the `application/x-www-form-urlencoded` content type, the default serialization strategy of such properties is described in the [Encoding Object](#encodingObject)'s [`style`](#encodingStyle) property as `form`.

With this example, given an `id` of `f81d4fae-7dec-11d0-a765-00a0c91e6bf6` and a US-style address (with ZIP+4) as follows:

```json
{
  "streetAddress": "123 Example Dr.",
  "city": "Somewhere",
  "state": "CA",
  "zip": "99999+1234"
}
```

Assuming the most compact representation of the JSON value (with unnecessary whitespace removed), we would expect to see the following request body, where space characters have been replaced with `+` and `+`, `"`, `{`, and `}` have been percent-encoded to `%2B`, `%22`, `%7B`, and `%7D`, respectively:

```urlencoded
id=f81d4fae-7dec-11d0-a765-00a0c91e6bf6&address=%7B%22streetAddress%22:%22123+Example+Dr.%22,%22city%22:%22Somewhere%22,%22state%22:%22CA%22,%22zip%22:%2299999%2B1234%22%7D
```

Note that the `id` keyword is treated as `text/plain` per the [Encoding Object](#encodingObject)'s default behavior, and is serialized as-is.
If it were treated as `application/json`, then the serialized value would be a JSON string including quotation marks, which would be percent-encoded as `%22`.

Here is the `id` parameter (without `address`) serialized as `application/json` instead of `text/plain`, and then encoded per RFC1866:

```urlencoded
id=%22f81d4fae-7dec-11d0-a765-00a0c91e6bf6%22
```

See [Appendix E](#percentEncodingAndFormMediaTypes) for a detailed examination of percent-encoding concerns for form media types.

##### Special Considerations for `multipart` Content

It is common to use `multipart/form-data` as a `Content-Type` when transferring request bodies to operations.  In contrast to 2.0, a `schema` is REQUIRED to define the input parameters to the operation when using `multipart` content.  This supports complex structures as well as supporting mechanisms for multiple file uploads.

When passing in `multipart` types, boundaries MAY be used to separate sections of the content being transferred — thus, the following default `Content-Type`s are defined for `multipart`:

* If the property is a primitive, or an array of primitive values, the default Content-Type is `text/plain`
* If the property is complex, or an array of complex values, the default Content-Type is `application/json`
* If the property is a `type: string` with `format: binary` or `format: byte` (aka a file object), the default Content-Type is `application/octet-stream`

Note that only `multipart/*` media types with named parts can be described as shown here.  Note also that while `multipart/form-data` originally defined a per-part `Content-Transfer-Encoding` header that could indicate base64 encoding (`format: byte`), it has been deprecated for use with HTTP as of [RFC7578](https://www.rfc-editor.org/rfc/rfc7578#section-4.7).

Examples:

```yaml
requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          id:
            type: string
            format: uuid
          address:
            # default Content-Type for objects is `application/json`
            type: object
            properties: {}
          profileImage:
            # default Content-Type for string/binary is `application/octet-stream`
            type: string
            format: binary
          children:
            # default Content-Type for arrays is based on the `inner` type (text/plain here)
            type: array
            items:
              type: string
          addresses:
            # default Content-Type for arrays is based on the `inner` type (object shown, so `application/json` in this example)
            type: array
            items:
              $ref: '#/components/schemas/Address'
```

An `encoding` attribute is introduced to give you control over the serialization of parts of `multipart` request bodies.  This attribute is _only_ applicable to request bodies, and _only_ for `multipart` and `application/x-www-form-urlencoded` media types.

#### <a name="encodingObject"></a>Encoding Object

A single encoding definition applied to a single schema property.
See [Appendix B](#dataTypeConversion) for a discussion of converting values of various types to string representations.

Properties are correlated with `multipart` parts using the `name` parameter to `Content-Disposition: form-data`, and with `application/x-www-form-urlencoded` using the query string paramter names.
In both cases, their order is implementation-defined.

See [Appendix E](#percentEncodingAndFormMediaTypes) for a detailed examination of percent-encoding concerns for form media types.

###### Common Fixed Fields

These fields MAY be used either with or without the RFC6570-style serialization fields defined in the next section below.

Field Name | Type | Description
---|:---:|---
<a name="encodingContentType"></a>contentType | `string` | The `Content-Type` for encoding a specific property. The value is a comma-separated list, each element of which is either a specific media type (e.g. `image/png`) or a wildcard media type (e.g. `image/*`). Default value depends on the property type as shown in the table below.
<a name="encodingHeaders"></a>headers | Map[`string`, [Header Object](#headerObject) \| [Reference Object](#referenceObject)] | A map allowing additional information to be provided as headers. `Content-Type` is described separately and SHALL be ignored in this section. This property SHALL be ignored if the request body media type is not a `multipart`.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

The default values for `contentType` are as follows, where an _n/a_ in the `format` column means that the presence or value of `format` is irrelevant:

Property `type` | Property `format` | Default `contentType`
------------- | --------------- | ---------------------
`string` | `binary` | `application/octet-stream`
`string` | _none, or any except `binary`_ | `text/plain`
`number`, `integer`, or `boolean` | _n/a_ | `text/plain`
`object` | _n/a_ | `application/json`
`array` | _n/a_ | according to the `type` and `format` of the `items` schema

Determining how to handle `null` values if `nullable: true` is present depends on how `null` values are being serialized.
If `null` values are entirely omitted, then the `contentType` is irrelevant.
See [Appendix B](#dataTypeConversion) for a discussion of data type conversion options.

##### Fixed Fields for RFC6570-style Serialization

Field Name | Type | Description
---|:---:|---
<a name="encodingStyle"></a>style | `string` | Describes how a specific property value will be serialized depending on its type.  See [Parameter Object](#parameterObject) for details on the [`style`](#parameterStyle) property. The behavior follows the same values as `query` parameters, including default values. Note that the initial `?` using in query strings is not used in `applcation/x-www-form-urlencoded` message bodies, and MUST be removed (if using an RFC6570 implementation) or simply not added (if constructing the string manually). This property SHALL be ignored if the request body media type is not `application/x-www-form-urlencoded`.
<a name="encodingExplode"></a>explode | `boolean` | When this is true, property values of type `array` or `object` generate separate parameters for each value of the array, or key-value-pair of the map.  For other types of properties this property has no effect. When [`style`](#encodingStyle) is `form`, the default value is `true`. For all other styles, the default value is `false`. Note that despite `false` being the default for `deepObject`, the combination of `false` with `deepObject` is undefined. This property SHALL be ignored if the request body media type is not `application/x-www-form-urlencoded`.
<a name="encodingAllowReserved"></a>allowReserved | `boolean` | When this is true, parameter values are serialized using reserved expansion, as defined by [RFC6570](https://datatracker.ietf.org/doc/html/rfc6570#autoid-20), which allows [RFC3986's reserved character set](https://datatracker.ietf.org/doc/html/rfc3986#autoid-13), as well as percent-encoded triples, to pass through unchanged, while still percent-encoding all other disallowed characters (including `%` outside of percent-encoded triples). Applications are still responsible for percent-encoding reserved characters that are [not allowed in the query string](https://datatracker.ietf.org/doc/html/rfc3986#autoid-24) (`[`, `]`, `#`), or have a special meaning in `application/x-www-form-urlencoded` (`-`, `&`, `+`); see Appendices [C](#usingRFC6570Implementations) and [E](#percentEncodingAndFormMediaTypes) for details. This property only applies to parameters with an `in` value of `query`. The default value is `false`. This property SHALL be ignored if the request body media type is not `application/x-www-form-urlencoded`.

See also [Appendix C: Using RFC6570 Implementations](#usingRFC6570Implementations) for additional guidance.

The role of `contentType` with `application/x-www-form-urlencoded` request bodies was not described in detail in version 3.0.3 and earlier of this specification.
To match the intent of these fields and be compatible with version 3.1 of this specification, it is RECOMMENDED that whenever any of `style`, `explode`, or `allowReserved` are present with an explicit value:

* The value of `contentType`, whether it is explicitly defined or has the default value, is to be ignored
* If any of `style`, `explode`, or `allowReserved` are _not_ present with explicit values, then they are to be treated as if they were present with their default values

However, if all three of `style`, `explode`, and `allowReserved` fields are absent, it is RECOMMENDED that:

* All three keywords are to be entirely ignored, rather than treated as having their default values
* Encoding is to be based on `contentType` alone, whether it is present with an explicit value or absent and treated as having its default value

This makes the presence of at least one of `style`, `explode`, or `allowReserved` with an explicit value equivalent to using `schema` with `in: query` Parameter Objects, and their absence the equivalent of using `content`, but with the media type specified in `contentType` rather than through a Media Type Object.

###### Encoding `multipart` Media Types

The `form-data` disposition and its `name` parameter are mandatory for `multipart/form-data` ([RFC7578 §4.2](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.2)).
Array properties are handled by applying the same `name` to multiple parts, as is recommended by [RFC7578 §4.3](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.3) for supplying multiple values per form field.
See [RFC7578 §5](https://www.rfc-editor.org/rfc/rfc7578.html#section-5) for guidance regarding non-ASCII part names.

Various other `multipart` types, most notable `multipart/mixed` ([RFC2046 §5.1.3](https://www.rfc-editor.org/rfc/rfc2046.html#section-5.1.3)) neither require nor forbid specific `Content-Disposition` values, which means care must be taken to ensure that any values used are supported by all relevant software.
It is not currently possible to correlate schema properties with unnamed, ordered parts in media types such as `multipart/mixed`, but implementations MAY choose to support such types when `Content-Disposition: form-data` is used with a `name` parameter.

Note that there are significant restrictions on what headers can be used with `multipart` media types in general ([RFC2046 §5.1](https://www.rfc-editor.org/rfc/rfc2046.html#section-5.1)) and `multi-part/form-data` in particular ([RFC7578 §4.8](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.8)).

Note also that `Content-Transfer-Encoding` is deprecated for `multipart/form-data` ([RFC7578 §4.7](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.7)) where binary data is supported, as it is in HTTP.
Using `format: byte` for a multipart field is equivalent to setting `Content-Transfer-Encoding: base64`.
If `format: byte` is used along with setting a different `Content-Transfer-Encoding` value with the `headers` field, the result is undefined.

##### Encoding Object Example

`multipart/form-data` allows for binary parts:

```yaml
requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          id:
            # default is text/plain
            type: string
            format: uuid
          address:
            # default is application/json
            type: object
            properties: {}
          historyMetadata:
            # need to declare XML format!
            description: metadata in XML format
            type: object
            properties: {}
          profileImage:
            # default is application/octet-stream, need to declare an image type only!
            type: string
            format: binary
      encoding:
        historyMetadata:
          # require XML Content-Type in utf-8 encoding
          contentType: application/xml; charset=utf-8
        profileImage:
          # only accept png/jpeg
          contentType: image/png, image/jpeg
          headers:
            X-Rate-Limit-Limit:
              description: The number of allowed requests in the current period
              schema:
                type: integer
```

`application/x-www-form-urlencoded` is a text format, which requires base64-encoding any binary data:

```YAML
requestBody:
  content:
    application/x-www-form-urlencoded:
      schema:
        type: object
        properties:
          name:
            type: string
          icon:
            # default is text/plain, need to declare an image type only!
            type: string
            format: byte
  encoding:
    icon:
      contentType: image/png, image/jpeg
```

#### <a name="responsesObject"></a>Responses Object

A container for the expected responses of an operation.
The container maps a HTTP response code to the expected response.

The documentation is not necessarily expected to cover all possible HTTP response codes because they may not be known in advance.
However, documentation is expected to cover a successful operation response and any known errors.

The `default` MAY be used as a default response object for all HTTP codes 
that are not covered individually by the specification.

The `Responses Object` MUST contain at least one response code, and it 
SHOULD be the response for a successful operation call.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="responsesDefault"></a>default | [Response Object](#responseObject) \| [Reference Object](#referenceObject) | The documentation of responses other than the ones declared for specific HTTP response codes. Use this field to cover undeclared responses. A [Reference Object](#referenceObject) can link to a response that the [OpenAPI Object's components/responses](#componentsResponses) section defines.

##### Patterned Fields
Field Pattern | Type | Description
---|:---:|---
<a name="responsesCode"></a>[HTTP Status Code](#httpCodes) | [Response Object](#responseObject) \| [Reference Object](#referenceObject) | Any [HTTP status code](#httpCodes) can be used as the property name, but only one property per code, to describe the expected response for that HTTP status code.  A [Reference Object](#referenceObject) can link to a response that is defined in the [OpenAPI Object's components/responses](#componentsResponses) section. This field MUST be enclosed in quotation marks (for example, "200") for compatibility between JSON and YAML. To define a range of response codes, this field MAY contain the uppercase wildcard character `X`. For example, `2XX` represents all response codes between `[200-299]`. Only the following range definitions are allowed: `1XX`, `2XX`, `3XX`, `4XX`, and `5XX`. If a response is defined using an explicit code, the explicit code definition takes precedence over the range definition for that code.


This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Responses Object Example

A 200 response for a successful operation and a default response for others (implying an error):

```json
{
  "200": {
    "description": "a pet to be returned",
    "content": {
      "application/json": {
        "schema": {
          "$ref": "#/components/schemas/Pet"
        }
      }
    }
  },
  "default": {
    "description": "Unexpected error",
    "content": {
      "application/json": {
        "schema": {
          "$ref": "#/components/schemas/ErrorModel"
        }
      }
    }
  }
}
```

```yaml
'200':
  description: a pet to be returned
  content:
    application/json:
      schema:
        $ref: '#/components/schemas/Pet'
default:
  description: Unexpected error
  content:
    application/json:
      schema:
        $ref: '#/components/schemas/ErrorModel'
```

#### <a name="responseObject"></a>Response Object
Describes a single response from an API Operation, including design-time, static 
`links` to operations based on the response.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="responseDescription"></a>description | `string` | **REQUIRED**. A short description of the response. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="responseHeaders"></a>headers | Map[`string`, [Header Object](#headerObject)  \| [Reference Object](#referenceObject)] |  Maps a header name to its definition. [RFC7230](https://tools.ietf.org/html/rfc7230#page-22) states header names are case insensitive. If a response header is defined with the name `"Content-Type"`, it SHALL be ignored.
<a name="responseContent"></a>content | Map[`string`, [Media Type Object](#mediaTypeObject)] | A map containing descriptions of potential response payloads. The key is a media type or [media type range](https://tools.ietf.org/html/rfc7231#appendix-D) and the value describes it.  For responses that match multiple keys, only the most specific key is applicable. e.g. text/plain overrides text/*
<a name="responseLinks"></a>links | Map[`string`, [Link Object](#linkObject) \| [Reference Object](#referenceObject)] | A map of operations links that can be followed from the response. The key of the map is a short name for the link, following the naming constraints of the names for [Component Objects](#componentsObject). 

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Response Object Examples

Response of an array of a complex type:

```json
{
  "description": "A complex object array response",
  "content": {
    "application/json": {
      "schema": {
        "type": "array",
        "items": {
          "$ref": "#/components/schemas/VeryComplexType"
        }
      }
    }
  }
}
```

```yaml
description: A complex object array response
content:
  application/json:
    schema:
      type: array
      items:
        $ref: '#/components/schemas/VeryComplexType'
```

Response with a string type:

```json
{
  "description": "A simple string response",
  "content": {
    "text/plain": {
      "schema": {
        "type": "string"
      }
    }
  }
}
```

```yaml
description: A simple string response
content:
  text/plain:
    schema:
      type: string
```

Plain text response with headers:

```json
{
  "description": "A simple string response",
  "content": {
    "text/plain": {
      "schema": {
        "type": "string",
        "example": "whoa!"
      }
    }
  },
  "headers": {
    "X-Rate-Limit-Limit": {
      "description": "The number of allowed requests in the current period",
      "schema": {
        "type": "integer"
      }
    },
    "X-Rate-Limit-Remaining": {
      "description": "The number of remaining requests in the current period",
      "schema": {
        "type": "integer"
      }
    },
    "X-Rate-Limit-Reset": {
      "description": "The number of seconds left in the current period",
      "schema": {
        "type": "integer"
      }
    }
  }
}
```

```yaml
description: A simple string response
content:
  text/plain:
    schema:
      type: string
    example: 'whoa!'
headers:
  X-Rate-Limit-Limit:
    description: The number of allowed requests in the current period
    schema:
      type: integer
  X-Rate-Limit-Remaining:
    description: The number of remaining requests in the current period
    schema:
      type: integer
  X-Rate-Limit-Reset:
    description: The number of seconds left in the current period
    schema:
      type: integer
```

Response with no return value:

```json
{
  "description": "object created"
}
```

```yaml
description: object created
```

#### <a name="callbackObject"></a>Callback Object

A map of possible out-of band callbacks related to the parent operation.
Each value in the map is a [Path Item Object](#pathItemObject) that describes a set of requests that may be initiated by the API provider and the expected responses.
The key value used to identify the path item object is an expression, evaluated at runtime, that identifies a URL to use for the callback operation.

##### Patterned Fields
Field Pattern | Type | Description
---|:---:|---
<a name="callbackExpression"></a>{expression} | [Path Item Object](#pathItemObject) | A Path Item Object used to define a callback request and expected responses.  A [complete example](../examples/v3.0/callback-example.yaml) is available.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Key Expression

The key that identifies the [Path Item Object](#pathItemObject) is a [runtime expression](#runtimeExpression) that can be evaluated in the context of a runtime HTTP request/response to identify the URL to be used for the callback request.
A simple example might be `$request.body#/url`.
However, using a [runtime expression](#runtimeExpression) the complete HTTP message can be accessed.
This includes accessing any part of a body that a JSON Pointer [RFC6901](https://tools.ietf.org/html/rfc6901) can reference. 

For example, given the following HTTP request:

```http
POST /subscribe/myevent?queryUrl=http:https://clientdomain.com/stillrunning HTTP/1.1
Host: example.org
Content-Type: application/json
Content-Length: 187

{
  "failedUrl" : "http:https://clientdomain.com/failed",
  "successUrls" : [
    "http:https://clientdomain.com/fast",
    "http:https://clientdomain.com/medium",
    "http:https://clientdomain.com/slow"
  ]
}

201 Created
Location: http:https://example.org/subscription/1
```

The following examples show how the various expressions evaluate, assuming the callback operation has a path parameter named `eventType` and a query parameter named `queryUrl`.

Expression | Value 
---|:---
$url | http:https://example.org/subscribe/myevent?queryUrl=http:https://clientdomain.com/stillrunning
$method | POST
$request.path.eventType | myevent
$request.query.queryUrl | http:https://clientdomain.com/stillrunning
$request.header.content-Type | application/json
$request.body#/failedUrl | http:https://clientdomain.com/failed
$request.body#/successUrls/1 | http:https://clientdomain.com/medium
$response.header.Location | http:https://example.org/subscription/1


##### Callback Object Examples

The following example uses the user provided `queryUrl` query string parameter to define the callback URL.  This is an example of how to use a callback object to describe a WebHook callback that goes with the subscription operation to enable registering for the WebHook.

```yaml
myCallback:
  '{$request.query.queryUrl}':
    post:
      requestBody:
        description: Callback payload
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/SomePayload'
      responses:
        '200':
          description: callback successfully processed
```

The following example shows a callback where the server is hard-coded, but the query string parameters are populated from the `id` and `email` property in the request body.

```yaml
transactionCallback:
  'http:https://notificationServer.com?transactionId={$request.body#/id}&email={$request.body#/email}':
    post:
      requestBody:
        description: Callback payload
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/SomePayload'
      responses:
        '200':
          description: callback successfully processed
```

#### <a name="exampleObject"></a>Example Object

An object grouping an internal or external example value with basic `summary` and `description` metadata.
This object is typically used in properties named `examples` (plural), and is a [referenceable](#referenceObject) alternative to older `example` (singular) fields that do not support referencing or metadata.

Examples allow demonstration of the usage of properties, parameters and objects within OpenAPI.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="exampleSummary"></a>summary | `string` | Short description for the example.
<a name="exampleDescription"></a>description | `string` | Long description for the example. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="exampleValue"></a>value | Any | Embedded literal example. The `value` field and `externalValue` field are mutually exclusive. To represent examples of media types that cannot naturally represented in JSON or YAML, use a string value to contain the example, escaping where necessary.
<a name="exampleExternalValue"></a>externalValue | `string` | A URL that points to the literal example. This provides the capability to reference examples that cannot easily be included in JSON or YAML documents.  The `value` field and `externalValue` field are mutually exclusive. 

This object MAY be extended with [Specification Extensions](#specificationExtensions).

In all cases, the example value SHOULD be compatible with the schema of its associated value.
Tooling implementations MAY choose to validate compatibility automatically, and reject the example value(s) if incompatible.

#### Working With Examples

Example Objects can be used in both [Parameter Objects](#parameterObject) and [Media Type Objects](#mediaTypeObject).
In both Objects, this is done through the `examples` (plural) field.
However, there are two other ways to provide examples:  The `example` (singular) field that is mutually exclusive with `examples` in both Objects, and the `example` (singular) field in the [Schema Object](#schemaObject) that appears in the `schema` field of both Objects.
Each of these fields has slightly different considerations.

The Schema Object's `example` field is used to show example values without regard to how they might be formatted as parameters or within media type representations.

The mutually exclusive fields in the Parameter or Media Type Objects are used to show example values which SHOULD both match the schema and be formatted as they would appear as a serialized parameter or within a media type representation.
The exact serialization and encoding is determined by various fields in the Parameter Object, or in the Media Type Object's [Encoding Object](#encodingObject).
Because examples using these fields represent the final serialized form of the data, they SHALL _override_ any `example` in the corresponding Schema Object.

The singular `example` field in the Parameter or Media Type object is concise and convenient for simple examples, but does not offer any other advantages over using Example Objects under `examples`.

Some examples cannot be represented directly in JSON or YAML.
For all three ways of providing examples, these can be shown as string values with any escaping necessary to make the string valid in the JSON or YAML format of the OpenAPI Description document.
With the Example Object, such values can alternatively be handled through the `externalValue` field.

##### Example Object Examples

In a request body:

```yaml
requestBody:
  content:
    'application/json':
      schema:
        $ref: '#/components/schemas/Address'
      examples:
        foo:
          summary: A foo example
          value: {"foo": "bar"}
        bar:
          summary: A bar example
          value: {"bar": "baz"}
    application/xml:
      examples:
        xmlExample:
          summary: This is an example in XML
          externalValue: https://example.org/examples/address-example.xml
    text/plain:
      examples:
        textExample:
          summary: This is a text example
          externalValue: http:https://foo.bar/examples/address-example.txt
```

In a parameter:

```yaml
parameters:
  - name: zipCode
    in: query
    schema:
      type: string
      format: zip-code
    examples:
      zip-example:
        $ref: '#/components/examples/zip-example'
```

In a response:

```yaml
responses:
  '200':
    description: your car appointment has been booked
    content:
      application/json:
        schema:
          $ref: '#/components/schemas/SuccessResponse'
        examples:
          confirmation-success:
            $ref: '#/components/examples/confirmation-success'
```

Two different uses of JSON strings:

First, a request or response body that is just a JSON string (not an object containing a string):

```json
"application/json": {
  "schema": {
    "type": "string"
  },
  "examples": {
    "jsonBody": {
      "description": "A body of just the JSON string \"json\"",
      "value": "json"
    }
  }
}
```

```yaml
application/json:
  schema:
    type: string
  examples:
    jsonBody:
      description: 'A body of just the JSON string "json"'
      value: json
```

In the above example, we can just show the JSON string (or any JSON value) as-is, rather than stuffing a serialized JSON value into a JSON string, which would have looked like `"\"json\""`.


In contrast, a JSON string encoded inside of a URL-style form body:

```json
"application/x-www-form-urlencoded": {
  "schema": {
    "type": "object",
    "properties": {
      "jsonValue": {
        "type": "string"
      }
    }
  },
  "encoding": {
    "jsonValue": {
      "contentType": "application/json"
    }
  },
  "examples": {
    "jsonFormValue": {
      "description": "The JSON string \"json\" as a form value",
      "value": "jsonValue=%22json%22"
    }
  }
}
```

```yaml
application/x-www-form-urlencoded:
  schema:
    type: object
    properties:
      jsonValue:
        type: string
  encoding:
    jsonValue:
      contentType: application/json
  examples:
    jsonFormValue:
      description: 'The JSON string "json" as a form value'
      value: jsonValue=%22json%22
```

In this example, the JSON string had to be serialized before encoding it into the URL form value, so the example includes the quotation marks that are part of the JSON serialization, which are then URL percent-encoded.

#### <a name="linkObject"></a>Link Object

The `Link object` represents a possible design-time link for a response.
The presence of a link does not guarantee the caller's ability to successfully invoke it, rather it provides a known relationship and traversal mechanism between responses and other operations.

Unlike _dynamic_ links (i.e. links provided **in** the response payload), the OAS linking mechanism does not require link information in the runtime response.

For computing links, and providing instructions to execute them, a [runtime expression](#runtimeExpression) is used for accessing values in an operation and using them as parameters while invoking the linked operation.  

##### Fixed Fields

Field Name  |  Type  | Description
---|:---:|---
<a name="linkOperationRef"></a>operationRef | `string` | A relative or absolute URI reference to an OAS operation. This field is mutually exclusive of the `operationId` field, and MUST point to an [Operation Object](#operationObject). Relative `operationRef` values MAY be used to locate an existing [Operation Object](#operationObject) in the OpenAPI definition.
<a name="linkOperationId"></a>operationId  | `string` | The name of an _existing_, resolvable OAS operation, as defined with a unique `operationId`.  This field is mutually exclusive of the `operationRef` field.  
<a name="linkParameters"></a>parameters   | Map[`string`, Any \| [{expression}](#runtimeExpression)] | A map representing parameters to pass to an operation as specified with `operationId` or identified via `operationRef`. The key is the parameter name to be used (optionally qualified with the parameter location, e.g. `path.id` for an `id` parameter in the path), whereas the value can be a constant or an expression to be evaluated and passed to the linked operation.
<a name="linkRequestBody"></a>requestBody | Any \| [{expression}](#runtimeExpression) | A literal value or [{expression}](#runtimeExpression) to use as a request body when calling the target operation.
<a name="linkDescription"></a>description  | `string` | A description of the link. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="linkServer"></a>server       | [Server Object](#serverObject) | A server object to be used by the target operation.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

A linked operation MUST be identified using either an `operationRef` or `operationId`.
In the case of an `operationId`, it MUST be unique and resolved in the scope of the OpenAPI description.
Because of the potential for name clashes, the `operationRef` syntax is preferred 
for multi-document OpenAPI descriptions.

Note that it is not possible to provide a constant value to `parameters` that matches the syntax of a runtime expression.
It is possible to have ambiguous parameter names, e.g. `name: id, in: path` and `name: path.id, in: query`; this is NOT RECOMMENDED and the behavior is implementation-defined, however implementations SHOULD prefer the qualified interpretation (`path.id` as a path parameter), as the names can always be qualified to disambiguate them (e.g. using `query.path.id` for the query paramter).

##### Examples

Computing a link from a request operation where the `$request.path.id` is used to pass a request parameter to the linked operation.

```yaml
paths:
  /users/{id}:
    parameters:
    - name: id
      in: path
      required: true
      description: the user identifier, as userId
      schema:
        type: string
    get:
      responses:
        '200':
          description: the user being returned
          content:
            application/json:
              schema:
                type: object
                properties:
                  uuid: # the unique user id
                    type: string
                    format: uuid
          links:
            address:
              # the target link operationId
              operationId: getUserAddress
              parameters:
                # get the `id` field from the request path parameter named `id`
                userid: $request.path.id
  # the path item of the linked operation
  /users/{userid}/address:
    parameters:
    - name: userid
      in: path
      required: true
      description: the user identifier, as userId
      schema:
        type: string
    # linked operation
    get:
      operationId: getUserAddress
      responses:
        '200':
          description: the user's address
```

When a runtime expression fails to evaluate, no parameter value is passed to the target operation.

Values from the response body can be used to drive a linked operation.

```yaml
links:
  address:
    operationId: getUserAddressByUUID
    parameters:
      # get the `uuid` field from the `uuid` field in the response body
      userUuid: $response.body#/uuid
```

Clients follow all links at their discretion. 
Neither permissions, nor the capability to make a successful call to that link, is guaranteed 
solely by the existence of a relationship.


##### OperationRef Examples

As references to `operationId` MAY NOT be possible (the `operationId` is an optional 
field in an [Operation Object](#operationObject)), references MAY also be made through a relative `operationRef`:

```yaml
links:
  UserRepositories:
    # returns array of '#/components/schemas/repository'
    operationRef: '#/paths/~12.0~1repositories~1%7Busername%7D/get'
    parameters:
      username: $response.body#/username
```

or an absolute `operationRef`:

```yaml
links:
  UserRepositories:
    # returns array of '#/components/schemas/repository'
    operationRef: https://na2.gigantic-server.com/#/paths/~12.0~1repositories~1%7Busername%7D/get
    parameters:
      username: $response.body#/username
```

Note that in the use of `operationRef`, the _escaped forward-slash_ is necessary when 
using JSON Pointers, and it is necessary to URL-encode `{` and `}` as `%7B` and `%7D`, respectively when
using JSON Pointers as URI fragments.


##### <a name="runtimeExpression"></a>Runtime Expressions

Runtime expressions allow defining values based on information that will only be available within the HTTP message in an actual API call.
This mechanism is used by [Link Objects](#linkObject) and [Callback Objects](#callbackObject).

The runtime expression is defined by the following [ABNF](https://tools.ietf.org/html/rfc5234) syntax

```abnf
    expression = ( "$url" / "$method" / "$statusCode" / "$request." source / "$response." source )
    source = ( header-reference / query-reference / path-reference / body-reference )
    header-reference = "header." token
    query-reference = "query." name  
    path-reference = "path." name
    body-reference = "body" ["#" json-pointer ]
    json-pointer    = *( "/" reference-token )
    reference-token = *( unescaped / escaped )
    unescaped       = %x00-2E / %x30-7D / %x7F-10FFFF
       ; %x2F ('/') and %x7E ('~') are excluded from 'unescaped'
    escaped         = "~" ( "0" / "1" )
      ; representing '~' and '/', respectively
    name = *( CHAR )
    token = 1*tchar
    tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
      "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
```

Here, `json-pointer` is taken from [RFC 6901](https://tools.ietf.org/html/rfc6901), `char` from [RFC 7159](https://tools.ietf.org/html/rfc7159#section-7) and `token` from [RFC 7230](https://tools.ietf.org/html/rfc7230#section-3.2.6).

The `name` identifier is case-sensitive, whereas `token` is not. 

The table below provides examples of runtime expressions and examples of their use in a value:

##### <a name="runtimeExpressionExamples"></a>Examples

Source Location | example expression  | notes
---|:---|:---|
HTTP Method            | `$method`         | The allowable values for the `$method` will be those for the HTTP operation.
Requested media type | `$request.header.accept`        |  
Request parameter      | `$request.path.id`        | Request parameters MUST be declared in the `parameters` section of the parent operation or they cannot be evaluated. This includes request headers.
Request body property   | `$request.body#/user/uuid`   | In operations which accept payloads, references may be made to portions of the `requestBody` or the entire body.
Request URL            | `$url`            |  
Response value         | `$response.body#/status`       |  In operations which return payloads, references may be made to portions of the response body or the entire body.
Response header        | `$response.header.Server` |  Single header values only are available

Runtime expressions preserve the type of the referenced value.
Expressions can be embedded into string values by surrounding the expression with `{}` curly braces.

#### <a name="headerObject"></a>Header Object

Describes a single header for [HTTP responses](#responseHeaders) and for [individual parts in `multipart` representations](#encodingHeaders); see the relevant [Response Object](#responseObject) and [Encoding Object](#encodingObject) documentation for restrictions on which headers can be described.

The Header Object follows the structure of the [Parameter Object](#parameterObject), including determining its serialization strategy based on whether `schema` or `content` is present,  with the following changes:

1. `name` MUST NOT be specified, it is given in the corresponding `headers` map.
1. `in` MUST NOT be specified, it is implicitly in `header`.
1. All traits that are affected by the location MUST be applicable to a location of `header` (for example, [`style`](#parameterStyle)).  This means that `allowEmptyValue` and `allowReserved` MUST NOT be used, and `style`, if used, MUST be limited to `simple`.

###### Common Fixed Fields

These fields MAY be used with either `content` or `schema`.

Field Name | Type | Description
---|:---:|---
<a name="headerDescription"></a>description | `string` | A brief description of the header. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="headerRequired"></a>required | `boolean` | Determines whether this header is mandatory. The default value is `false`.
<a name="headerDeprecated"></a> deprecated | `boolean` | Specifies that the header is deprecated and SHOULD be transitioned out of usage. Default value is `false`.

###### Fixed Fields for use with `schema`

For simpler scenarios, a [`schema`](#headerSchema) and [`style`](#headerStyle) can describe the structure and syntax of the header.
When `example` or `examples` are provided in conjunction with the `schema` object, the example MUST follow the prescribed serialization strategy for the header.

Serializing with `schema` is NOT RECOMMENDED for headers with parameters (name=value pairs following a `;`) in their values, or where values might have non-URL-safe characters; see [Appendix D](#serializingHeadersAndCookies) for details.

When `example` or `examples` are provided in conjunction with the `schema` object, the example SHOULD match the specified schema and follow the prescribed serialization strategy for the parameter.
The `example` and `examples` fields are mutually exclusive, and if either is present it SHALL _override_ any `example` in the schema.

Field Name | Type | Description
---|:---:|---
<a name="headerStyle"></a>style | `string` | Describes how the header value will be serialized.  The default (and only legal value for headers) is `simple`.
<a name="headerExplode"></a>explode | `boolean` | When this is true, header values of type `array` or `object` generate a single header whose value is a comma-separated list of the array items or key-value pairs of the map, see [Style Examples](#style-examples). For other data types this property has no effect. The default value is `false`.
<a name="headerSchema"></a>schema | [Schema Object](#schemaObject) \| [Reference Object](#referenceObject) | The schema defining the type used for the header.
<a name="headerExample"></a>example | Any | Example of the header's potential value; see [Working With Examples](#working-with-examples).
<a name="headerExamples"></a>examples | Map[ `string`, [Example Object](#exampleObject) \| [Reference Object](#referenceObject)] | Examples of the header's potential value; see [Working With Examples](#working-with-examples).

See also [Appendix C: Using RFC6570 Implementations](#usingRFC6570Implementations) for additional guidance.

###### Fixed Fields for use with `content`

For more complex scenarios, the [`content`](#headerContent) property can define the media type and schema of the header, as well as give examples of its use.
Using `content` with a `text/plain` media type is RECOMMENDED for headers where the `schema` strategy is not appropriate.

Field Name | Type | Description
---|:---:|---
<a name="headerContent"></a>content | Map[`string`, [Media Type Object](#mediaTypeObject)] | A map containing the representations for the header. The key is the media type and the value describes it. The map MUST only contain one entry.

##### Header Object Example

A simple header of type `integer`:

```json
"X-Rate-Limit-Limit": {
  "description": "The number of allowed requests in the current period",
  "schema": {
    "type": "integer"
  }
}
```

```yaml
X-Rate-Limit-Limit:
  description: The number of allowed requests in the current period
  schema:
    type: integer
```

Requiring that a strong `ETag` header (with a value starting with `"` rather than `W/`) is present.  Note the use of `content`, because using `schema` and `style` would require the `"` to be percent-encoded as `%22`:

```json
"ETag": {
  "required": true,
  "content": {
    "text/plain": {
      "schema": {
        "type": "string",
        "pattern": "^\""
      }
    }
  }
}
```

```yaml
ETag:
  required: true
  content:
    text/plain:
      schema:
        type: string
        pattern: ^" 
```

#### <a name="tagObject"></a>Tag Object

Adds metadata to a single tag that is used by the [Operation Object](#operationObject).
It is not mandatory to have a Tag Object per tag defined in the Operation Object instances.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="tagName"></a>name | `string` | **REQUIRED**. The name of the tag.
<a name="tagDescription"></a>description | `string` | A short description for the tag. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="tagExternalDocs"></a>externalDocs | [External Documentation Object](#externalDocumentationObject) | Additional external documentation for this tag.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Tag Object Example

```json
{
  "name": "pet",
  "description": "Pets operations"
}
```

```yaml
name: pet
description: Pets operations
```


#### <a name="referenceObject"></a>Reference Object

A simple object to allow referencing other components in the specification, internally and externally.

The Reference Object is defined by [JSON Reference](https://tools.ietf.org/html/draft-pbryan-zyp-json-ref-03) and follows the same structure, behavior and rules. 

For this specification, reference resolution is accomplished as defined by the JSON Reference specification and not by the JSON Schema specification.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="referenceRef"></a>$ref | `string` | **REQUIRED**. The reference string.

This object cannot be extended with additional properties and any properties added SHALL be ignored.

##### Reference Object Example

```json
{
  "$ref": "#/components/schemas/Pet"
}
```

```yaml
$ref: '#/components/schemas/Pet'
```

##### Relative Schema Document Example
```json
{
  "$ref": "Pet.json"
}
```

```yaml
$ref: Pet.yaml
```

##### Relative Documents With Embedded Schema Example
```json
{
  "$ref": "definitions.json#/Pet"
}
```

```yaml
$ref: definitions.yaml#/Pet
```

#### <a name="schemaObject"></a>Schema Object

The Schema Object allows the definition of input and output data types.
These types can be objects, but also primitives and arrays.
This object is an extended subset of the [JSON Schema Specification Wright Draft 00](https://json-schema.org/).

For more information about the properties, see [JSON Schema Core](https://tools.ietf.org/html/draft-wright-json-schema-00) and [JSON Schema Validation](https://tools.ietf.org/html/draft-wright-json-schema-validation-00).
Unless stated otherwise, the property definitions follow the JSON Schema.

##### Properties 

The following properties are taken directly from the JSON Schema definition and follow the same specifications:

- title
- multipleOf
- maximum
- exclusiveMaximum
- minimum
- exclusiveMinimum
- maxLength
- minLength
- pattern (This string SHOULD be a valid regular expression, according to the [Ecma-262 Edition 5.1 regular expression](https://www.ecma-international.org/ecma-262/5.1/#sec-15.10.1) dialect)
- maxItems
- minItems
- uniqueItems
- maxProperties
- minProperties
- required
- enum

The following properties are taken from the JSON Schema definition but their definitions were adjusted to the OpenAPI Specification. 
- type - Value MUST be a string. Multiple types via an array are not supported.
- allOf - Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema.
- oneOf - Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema.
- anyOf - Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema.
- not - Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema.
- items - Value MUST be an object and not an array. Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema. `items` MUST be present if the `type` is `array`.
- properties - Property definitions MUST be a [Schema Object](#schemaObject) and not a standard JSON Schema (inline or referenced).
- additionalProperties - Value can be boolean or object. Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema. Consistent with JSON Schema, `additionalProperties` defaults to `true`.
- description - [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
- format - See [Data Type Formats](#dataTypeFormat) for further details. While relying on JSON Schema's defined formats, the OAS offers a few additional predefined formats.
- default - The default value represents what would be assumed by the consumer of the input as the value of the schema if one is not provided. Unlike JSON Schema, the value MUST conform to the defined type for the Schema Object defined at the same level. For example, if `type` is `string`, then `default` can be `"foo"` but cannot be `1`.

Alternatively, any time a Schema Object can be used, a [Reference Object](#referenceObject) can be used in its place. This allows referencing definitions instead of defining them inline.

Additional properties defined by the JSON Schema specification that are not mentioned here are strictly unsupported.

Other than the JSON Schema subset fields, the following fields MAY be used for further schema documentation:

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="schemaNullable"></a>nullable | `boolean` | A `true` value adds `"null"` to the allowed type specified by the `type` keyword, only if `type` is explicitly defined within the same Schema Object. Other Schema Object constraints retain their defined behavior, and therefore may disallow the use of `null` as a value. A `false` value leaves the specified or default `type` unmodified. The default value is `false`.
<a name="schemaDiscriminator"></a>discriminator | [Discriminator Object](#discriminatorObject) | Adds support for polymorphism. The discriminator is an object name that is used to differentiate between other schemas which may satisfy the payload description. See [Composition and Inheritance](#schemaComposition) for more details.
<a name="schemaReadOnly"></a>readOnly | `boolean` | Relevant only for Schema `"properties"` definitions. Declares the property as "read only". This means that it MAY be sent as part of a response but SHOULD NOT be sent as part of the request. If the property is marked as `readOnly` being `true` and is in the `required` list, the `required` will take effect on the response only. A property MUST NOT be marked as both `readOnly` and `writeOnly` being `true`. Default value is `false`.
<a name="schemaWriteOnly"></a>writeOnly | `boolean` | Relevant only for Schema `"properties"` definitions. Declares the property as "write only". Therefore, it MAY be sent as part of a request but SHOULD NOT be sent as part of the response. If the property is marked as `writeOnly` being `true` and is in the `required` list, the `required` will take effect on the request only. A property MUST NOT be marked as both `readOnly` and `writeOnly` being `true`. Default value is `false`.
<a name="schemaXml"></a>xml | [XML Object](#xmlObject) | This MAY be used only on properties schemas. It has no effect on root schemas. Adds additional metadata to describe the XML representation of this property.
<a name="schemaExternalDocs"></a>externalDocs | [External Documentation Object](#externalDocumentationObject) | Additional external documentation for this schema. 
<a name="schemaExample"></a>example | Any | A free-form property to include an example of an instance for this schema. To represent examples that cannot be naturally represented in JSON or YAML, a string value can be used to contain the example with escaping where necessary.
<a name="schemaDeprecated"></a> deprecated | `boolean` | Specifies that a schema is deprecated and SHOULD be transitioned out of usage. Default value is `false`.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

###### <a name="schemaComposition"></a>Composition and Inheritance (Polymorphism)

The OpenAPI Specification allows combining and extending model definitions using the `allOf` property of JSON Schema, in effect offering model composition.
`allOf` takes an array of object definitions that are validated *independently* but together compose a single object. 

While composition offers model extensibility, it does not imply a hierarchy between the models.
To support polymorphism, the OpenAPI Specification adds the `discriminator` field.
When used, the `discriminator` will be the name of the property that decides which schema definition validates the structure of the model.
As such, the `discriminator` field MUST be a required field.
There are two ways to define the value of a discriminator for an inheriting instance.
- Use the schema name.
- Override the schema name by overriding the property with a new value. If a new value exists, this takes precedence over the schema name.
As such, inline schema definitions, which do not have a given id, *cannot* be used in polymorphism.

###### XML Modeling

The [xml](#schemaXml) property allows extra definitions when translating the JSON definition to XML.
The [XML Object](#xmlObject) contains additional information about the available options.

##### Schema Object Examples

###### Primitive Sample

```json
{
  "type": "string",
  "format": "email"
}
```

```yaml
type: string
format: email
```

###### Simple Model

```json
{
  "type": "object",
  "required": [
    "name"
  ],
  "properties": {
    "name": {
      "type": "string"
    },
    "address": {
      "$ref": "#/components/schemas/Address"
    },
    "age": {
      "type": "integer",
      "format": "int32",
      "minimum": 0
    }
  }
}
```

```yaml
type: object
required:
- name
properties:
  name:
    type: string
  address:
    $ref: '#/components/schemas/Address'
  age:
    type: integer
    format: int32
    minimum: 0
```

###### Model with Map/Dictionary Properties

For a simple string to string mapping:

```json
{
  "type": "object",
  "additionalProperties": {
    "type": "string"
  }
}
```

```yaml
type: object
additionalProperties:
  type: string
```

For a string to model mapping:

```json
{
  "type": "object",
  "additionalProperties": {
    "$ref": "#/components/schemas/ComplexModel"
  }
}
```

```yaml
type: object
additionalProperties:
  $ref: '#/components/schemas/ComplexModel'
```

###### Model with Example

```json
{
  "type": "object",
  "properties": {
    "id": {
      "type": "integer",
      "format": "int64"
    },
    "name": {
      "type": "string"
    }
  },
  "required": [
    "name"
  ],
  "example": {
    "name": "Puma",
    "id": 1
  }
}
```

```yaml
type: object
properties:
  id:
    type: integer
    format: int64
  name:
    type: string
required:
- name
example:
  name: Puma
  id: 1
```

###### Models with Composition

```json
{
  "components": {
    "schemas": {
      "ErrorModel": {
        "type": "object",
        "required": [
          "message",
          "code"
        ],
        "properties": {
          "message": {
            "type": "string"
          },
          "code": {
            "type": "integer",
            "minimum": 100,
            "maximum": 600
          }
        }
      },
      "ExtendedErrorModel": {
        "allOf": [
          {
            "$ref": "#/components/schemas/ErrorModel"
          },
          {
            "type": "object",
            "required": [
              "rootCause"
            ],
            "properties": {
              "rootCause": {
                "type": "string"
              }
            }
          }
        ]
      }
    }
  }
}
```

```yaml
components:
  schemas:
    ErrorModel:
      type: object
      required:
      - message
      - code
      properties:
        message:
          type: string
        code:
          type: integer
          minimum: 100
          maximum: 600
    ExtendedErrorModel:
      allOf:
      - $ref: '#/components/schemas/ErrorModel'
      - type: object
        required:
        - rootCause
        properties:
          rootCause:
            type: string
```

###### Models with Polymorphism Support

```json
{
  "components": {
    "schemas": {
      "Pet": {
        "type": "object",
        "discriminator": {
          "propertyName": "petType"
        },
        "properties": {
          "name": {
            "type": "string"
          },
          "petType": {
            "type": "string"
          }
        },
        "required": [
          "name",
          "petType"
        ]
      },
      "Cat": {
        "description": "A representation of a cat. Note that `Cat` will be used as the discriminating value.",
        "allOf": [
          {
            "$ref": "#/components/schemas/Pet"
          },
          {
            "type": "object",
            "properties": {
              "huntingSkill": {
                "type": "string",
                "description": "The measured skill for hunting",
                "default": "lazy",
                "enum": [
                  "clueless",
                  "lazy",
                  "adventurous",
                  "aggressive"
                ]
              }
            },
            "required": [
              "huntingSkill"
            ]
          }
        ]
      },
      "Dog": {
        "description": "A representation of a dog. Note that `Dog` will be used as the discriminating value.",
        "allOf": [
          {
            "$ref": "#/components/schemas/Pet"
          },
          {
            "type": "object",
            "properties": {
              "packSize": {
                "type": "integer",
                "format": "int32",
                "description": "the size of the pack the dog is from",
                "default": 0,
                "minimum": 0
              }
            },
            "required": [
              "packSize"
            ]
          }
        ]
      }
    }
  }
}
```

```yaml
components:
  schemas:
    Pet:
      type: object
      discriminator:
        propertyName: petType
      properties:
        name:
          type: string
        petType:
          type: string
      required:
      - name
      - petType
    Cat:  # "Cat" will be used as the discriminating value
      description: A representation of a cat
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        properties:
          huntingSkill:
            type: string
            description: The measured skill for hunting
            enum:
            - clueless
            - lazy
            - adventurous
            - aggressive
        required:
        - huntingSkill
    Dog:  # "Dog" will be used as the discriminating value
      description: A representation of a dog
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        properties:
          packSize:
            type: integer
            format: int32
            description: the size of the pack the dog is from
            default: 0
            minimum: 0
        required:
        - packSize
```

#### <a name="discriminatorObject"></a>Discriminator Object

When request bodies or response payloads may be one of a number of different schemas, a Discriminator Object gives a hint about the expected schema of the document.
This hint can be used to aid in serialization, deserialization, and validation.
The Discriminator Object does this by implicitly or explicitly associating the possible values of a named property with alternative schemas.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="propertyName"></a>propertyName | `string` | **REQUIRED**. The name of the property in the payload that will hold the discriminating value. This property SHOULD be required in the payload schema, as the behavior when the property is absent is undefined.
<a name="discriminatorMapping"></a> mapping | Map[`string`, `string`] | An object to hold mappings between payload values and schema names or URI references.

##### Conditions for Using the Discriminator Object
The Discriminator Object is legal only when using one of the composite keywords `oneOf`, `anyOf`, `allOf`.
In both the `oneOf` and `anyOf` use cases, where those keywords are adjacent to `discriminator`, all possible schemas MUST be listed explicitly.
To avoid redundancy, the discriminator MAY be added to a parent schema definition, and all schemas building on the parent schema via an `allOf` construct may be used as an alternate schema.
It is implementation-defined as to whether all named [Schema Objects](#schemaObject) under the [Components Object](#componentsObject), or only those that are otherwise directly referenced are searched for `allOf` references to the parent schema.
However, it is RECOMMENDED to search all named schemas in the Components Object because it is common with the `allOf` usage for other parts of the API to only directly reference the parent schema.

The behavior of any configuration of `oneOf`, `anyOf`, `allOf` and `discriminator` that is not described above is undefined.

##### Options for Mapping Values to Schemas
The value of the property named in `propertyName` is used as the name of the associated schema under the [Components Object](#componentsObject), _unless_ a `mapping` is present for that value.
The `mapping` entry maps a specific property value to either a different schema component name, or to a schema identified by a URI.
When using implicit or explicit schema component names, inline `oneOf` or `anyOf` subschemas are not considered.
The behavior of a `mapping` value that is both a valid schema name and a valid relative URI reference is implementation-defined, but it is RECOMMENDED that it be treated as a schema name.
To ensure that an ambiguous value (e.g. `"foo"`) is treated as a relative URI reference by all implementations, authors MUST prefix it with the `"."` path segment (e.g. `"./foo"`).

Mapping keys MUST be string values, but tooling MAY convert response values to strings for comparison.
However, the exact nature of such conversions are implementation-defined.

##### <a name="discriminatorExamples"></a>Examples

For these examples, assume all schemas are in the entry OpenAPI document; for handling of `discriminator` in referenced documents see [Resolving Implicit Connections](#resolvingImplicitConnections).

In OAS 3.0, a response payload MAY be described to be exactly one of any number of types:

```yaml
MyResponseType:
  oneOf:
  - $ref: '#/components/schemas/Cat'
  - $ref: '#/components/schemas/Dog'
  - $ref: '#/components/schemas/Lizard'
```

which means the payload _MUST_, by validation, match exactly one of the schemas described by `Cat`, `Dog`, or `Lizard`.  In this case, a discriminator MAY act as a "hint" to shortcut validation and selection of the matching schema which may be a costly operation, depending on the complexity of the schema. We can then describe exactly which field tells us which schema to use:


```yaml
MyResponseType:
  oneOf:
  - $ref: '#/components/schemas/Cat'
  - $ref: '#/components/schemas/Dog'
  - $ref: '#/components/schemas/Lizard'
  discriminator:
    propertyName: petType
```

The expectation now is that a property with name `petType` _MUST_ be present in the response payload, and the value will correspond to the name of a schema defined in the OpenAPI description.  Thus the response payload:

```json
{
  "id": 12345,
  "petType": "Cat"
}
```

Will indicate that the `Cat` schema be used in conjunction with this payload.

In scenarios where the value of the discriminator field does not match the schema name or implicit mapping is not possible, an optional `mapping` definition MAY be used:

```yaml
MyResponseType:
  oneOf:
  - $ref: '#/components/schemas/Cat'
  - $ref: '#/components/schemas/Dog'
  - $ref: '#/components/schemas/Lizard'
  - $ref: https://gigantic-server.com/schemas/Monster/schema.json
  discriminator:
    propertyName: petType
    mapping:
      dog: '#/components/schemas/Dog'
      monster: https://gigantic-server.com/schemas/Monster/schema.json
```

Here the discriminating value of `dog` will map to the schema `#/components/schemas/Dog`, rather than the default (implicit) value of `#/components/schemas/dog`.  If the discriminating value does not match an implicit or explicit mapping, no schema can be determined and validation SHOULD fail.

When used in conjunction with the `anyOf` construct, the use of the discriminator can avoid ambiguity where multiple schemas may satisfy a single payload.

This example shows the `allOf` usage, which avoids needing to reference all child schemas in the parent:

```yaml
components:
  schemas:
    Pet:
      type: object
      required:
      - petType
      properties:
        petType:
          type: string
      discriminator:
        propertyName: petType
        mapping:
          dog: Dog
    Cat:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Cat`
        properties:
          name:
            type: string
    Dog:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Dog`
        properties:
          bark:
            type: string
    Lizard:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Lizard`
        properties:
          lovesRocks:
            type: boolean
```

a payload like this:

```json
{
  "petType": "Cat",
  "name": "misty"
}
```

will indicate that the `Cat` schema be used.  Likewise this schema:

```json
{
  "petType": "dog",
  "bark": "soft"
}
```

will map to `Dog` because of the definition in the `mappings` element.


#### <a name="xmlObject"></a>XML Object

A metadata object that allows for more fine-tuned XML model definitions.

When using arrays, XML element names are *not* inferred (for singular/plural forms) and the `name` property SHOULD be used to add that information.
See examples for expected behavior.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="xmlName"></a>name | `string` | Replaces the name of the element/attribute used for the described schema property. When defined within `items`, it will affect the name of the individual XML elements within the list. When defined alongside `type` being `array` (outside the `items`), it will affect the wrapping element and only if `wrapped` is `true`. If `wrapped` is `false`, it will be ignored.
<a name="xmlNamespace"></a>namespace | `string` | The URI of the namespace definition. Value MUST be in the form of an absolute URI.
<a name="xmlPrefix"></a>prefix | `string` | The prefix to be used for the [name](#xmlName).
<a name="xmlAttribute"></a>attribute | `boolean` | Declares whether the property definition translates to an attribute instead of an element. Default value is `false`.
<a name="xmlWrapped"></a>wrapped | `boolean` | MAY be used only for an array definition. Signifies whether the array is wrapped (for example, `<books><book/><book/></books>`) or unwrapped (`<book/><book/>`). Default value is `false`. The definition takes effect only when defined alongside `type` being `array` (outside the `items`).

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### XML Object Examples

Each of the following examples represent the value of the `properties` keyword in a [Schema Object](#schemaObject) that is omitted for brevity.
The JSON and YAML representations of the `properties` value are followed by an example XML representation produced for the single property shown.

###### No XML Element

Basic string property:

```json
{
  "animals": {
    "type": "string"
  }
}
```

```yaml
animals:
  type: string
```

```xml
<animals>...</animals>
```

Basic string array property ([`wrapped`](#xmlWrapped) is `false` by default):

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string"
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
```

```xml
<animals>...</animals>
<animals>...</animals>
<animals>...</animals>
```

###### XML Name Replacement

```json
{
  "animals": {
    "type": "string",
    "xml": {
      "name": "animal"
    }
  }
}
```

```yaml
animals:
  type: string
  xml:
    name: animal
```

```xml
<animal>...</animal>
```


###### XML Attribute, Prefix and Namespace

In this example, a full model definition is shown.

```json
{
  "Person": {
    "type": "object",
    "properties": {
      "id": {
        "type": "integer",
        "format": "int32",
        "xml": {
          "attribute": true
        }
      },
      "name": {
        "type": "string",
        "xml": {
          "namespace": "http:https://example.com/schema/sample",
          "prefix": "sample"
        }
      }
    }
  }
}
```

```yaml
Person:
  type: object
  properties:
    id:
      type: integer
      format: int32
      xml:
        attribute: true
    name:
      type: string
      xml:
        namespace: http:https://example.com/schema/sample
        prefix: sample
```

```xml
<Person id="123">
    <sample:name xmlns:sample="http:https://example.com/schema/sample">example</sample:name>
</Person>
```

###### XML Arrays

Changing the element names:

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
```

```xml
<animal>value</animal>
<animal>value</animal>
```

The external `name` property has no effect on the XML:

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    },
    "xml": {
      "name": "aliens"
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
  xml:
    name: aliens
```

```xml
<animal>value</animal>
<animal>value</animal>
```

Even when the array is wrapped, if a name is not explicitly defined, the same name will be used both internally and externally:

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string"
    },
    "xml": {
      "wrapped": true
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
  xml:
    wrapped: true
```

```xml
<animals>
  <animals>value</animals>
  <animals>value</animals>
</animals>
```

To overcome the naming problem in the example above, the following definition can be used:

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    },
    "xml": {
      "wrapped": true
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
  xml:
    wrapped: true
```

```xml
<animals>
  <animal>value</animal>
  <animal>value</animal>
</animals>
```

Affecting both internal and external names:

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string",
      "xml": {
        "name": "animal"
      }
    },
    "xml": {
      "name": "aliens",
      "wrapped": true
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
    xml:
      name: animal
  xml:
    name: aliens
    wrapped: true
```

```xml
<aliens>
  <animal>value</animal>
  <animal>value</animal>
</aliens>
```

If we change the external element but not the internal ones:

```json
{
  "animals": {
    "type": "array",
    "items": {
      "type": "string"
    },
    "xml": {
      "name": "aliens",
      "wrapped": true
    }
  }
}
```

```yaml
animals:
  type: array
  items:
    type: string
  xml:
    name: aliens
    wrapped: true
```

```xml
<aliens>
  <aliens>value</aliens>
  <aliens>value</aliens>
</aliens>
```

#### <a name="securitySchemeObject"></a>Security Scheme Object

Defines a security scheme that can be used by the operations.
Supported schemes are HTTP authentication, an API key (either as a header, a cookie parameter or as a query parameter), OAuth2's common flows (implicit, password, client credentials and authorization code) as defined in [RFC6749](https://tools.ietf.org/html/rfc6749), and [OpenID Connect Core](https://openid.net/specs/openid-connect-core-1_0.html).

##### Fixed Fields
Field Name | Type | Applies To | Description
---|:---:|---|---
<a name="securitySchemeType"></a>type | `string` | Any | **REQUIRED**. The type of the security scheme. Valid values are `"apiKey"`, `"http"`, `"oauth2"`, `"openIdConnect"`.
<a name="securitySchemeDescription"></a>description | `string` | Any | A short description for security scheme. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
<a name="securitySchemeName"></a>name | `string` | `apiKey` | **REQUIRED**. The name of the header, query or cookie parameter to be used.
<a name="securitySchemeIn"></a>in | `string` | `apiKey` | **REQUIRED**. The location of the API key. Valid values are `"query"`, `"header"` or `"cookie"`.
<a name="securitySchemeScheme"></a>scheme | `string` | `http` | **REQUIRED**. The name of the HTTP Authentication scheme to be used in the [Authorization header as defined in RFC7235](https://tools.ietf.org/html/rfc7235#section-5.1).  The values used SHOULD be registered in the [IANA Authentication Scheme registry](https://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml).  The value is case-insensitive, as defined in [RFC7235](https://datatracker.ietf.org/doc/html/rfc7235#section-2.1).
<a name="securitySchemeBearerFormat"></a>bearerFormat | `string` | `http` (`"bearer"`) | A hint to the client to identify how the bearer token is formatted.  Bearer tokens are usually generated by an authorization server, so this information is primarily for documentation purposes.
<a name="securitySchemeFlows"></a>flows | [OAuth Flows Object](#oauthFlowsObject) | `oauth2` | **REQUIRED**. An object containing configuration information for the flow types supported.
<a name="securitySchemeOpenIdConnectUrl"></a>openIdConnectUrl | `string` | `openIdConnect` | **REQUIRED**. [Well-known URL](https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfig) to discover the [OpenID provider metadata](https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata).

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### Security Scheme Object Example

###### Basic Authentication Sample

```json
{
  "type": "http",
  "scheme": "basic"
}
```

```yaml
type: http
scheme: basic
```

###### API Key Sample

```json
{
  "type": "apiKey",
  "name": "api_key",
  "in": "header"
}
```

```yaml
type: apiKey
name: api_key
in: header
```

###### JWT Bearer Sample

```json
{
  "type": "http",
  "scheme": "bearer",
  "bearerFormat": "JWT",
}
```

```yaml
type: http
scheme: bearer
bearerFormat: JWT
```

###### Implicit OAuth2 Sample

```json
{
  "type": "oauth2",
  "flows": {
    "implicit": {
      "authorizationUrl": "https://example.com/api/oauth/dialog",
      "scopes": {
        "write:pets": "modify pets in your account",
        "read:pets": "read your pets"
      }
    }
  }
}
```

```yaml
type: oauth2
flows:
  implicit:
    authorizationUrl: https://example.com/api/oauth/dialog
    scopes:
      write:pets: modify pets in your account
      read:pets: read your pets
```

#### <a name="oauthFlowsObject"></a>OAuth Flows Object

Allows configuration of the supported OAuth Flows.

##### Fixed Fields
Field Name | Type | Description
---|:---:|---
<a name="oauthFlowsImplicit"></a>implicit| [OAuth Flow Object](#oauthFlowObject) | Configuration for the OAuth Implicit flow 
<a name="oauthFlowsPassword"></a>password| [OAuth Flow Object](#oauthFlowObject) | Configuration for the OAuth Resource Owner Password flow 
<a name="oauthFlowsClientCredentials"></a>clientCredentials| [OAuth Flow Object](#oauthFlowObject) | Configuration for the OAuth Client Credentials flow.  Previously called `application` in OpenAPI 2.0.
<a name="oauthFlowsAuthorizationCode"></a>authorizationCode| [OAuth Flow Object](#oauthFlowObject) | Configuration for the OAuth Authorization Code flow.  Previously called `accessCode` in OpenAPI 2.0.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

#### <a name="oauthFlowObject"></a>OAuth Flow Object

Configuration details for a supported OAuth Flow

##### Fixed Fields
Field Name | Type | Applies To | Description
---|:---:|---|---
<a name="oauthFlowAuthorizationUrl"></a>authorizationUrl | `string` | `oauth2` (`"implicit"`, `"authorizationCode"`) | **REQUIRED**. The authorization URL to be used for this flow. This MUST be in the form of a URL.
<a name="oauthFlowTokenUrl"></a>tokenUrl | `string` | `oauth2` (`"password"`, `"clientCredentials"`, `"authorizationCode"`) | **REQUIRED**. The token URL to be used for this flow. This MUST be in the form of a URL.
<a name="oauthFlowRefreshUrl"></a>refreshUrl | `string` | `oauth2` | The URL to be used for obtaining refresh tokens. This MUST be in the form of a URL.
<a name="oauthFlowScopes"></a>scopes | Map[`string`, `string`] | `oauth2` | **REQUIRED**. The available scopes for the OAuth2 security scheme. A map between the scope name and a short description for it. The map MAY be empty.

This object MAY be extended with [Specification Extensions](#specificationExtensions).

##### OAuth Flow Object Examples

```JSON
{
  "type": "oauth2",
  "flows": {
    "implicit": {
      "authorizationUrl": "https://example.com/api/oauth/dialog",
      "scopes": {
        "write:pets": "modify pets in your account",
        "read:pets": "read your pets"
      }
    },
    "authorizationCode": {
      "authorizationUrl": "https://example.com/api/oauth/dialog",
      "tokenUrl": "https://example.com/api/oauth/token",
      "scopes": {
        "write:pets": "modify pets in your account",
        "read:pets": "read your pets"
      }
    }
  }
}
```

```yaml
type: oauth2
flows:
  implicit:
    authorizationUrl: https://example.com/api/oauth/dialog
    scopes:
      write:pets: modify pets in your account
      read:pets: read your pets
  authorizationCode:
    authorizationUrl: https://example.com/api/oauth/dialog
    tokenUrl: https://example.com/api/oauth/token
    scopes:
      write:pets: modify pets in your account
      read:pets: read your pets
```

#### <a name="securityRequirementObject"></a>Security Requirement Object

Lists the required security schemes to execute this operation.
The name used for each property MUST correspond to a security scheme declared in the [Security Schemes](#componentsSecuritySchemes) under the [Components Object](#componentsObject).

Security Requirement Objects that contain multiple schemes require that all schemes MUST be satisfied for a request to be authorized.
This enables support for scenarios where multiple query parameters or HTTP headers are required to convey security information.

When a list of Security Requirement Objects is defined on the [OpenAPI Object](#oasObject) or [Operation Object](#operationObject), only one of the Security Requirement Objects in the list needs to be satisfied to authorize the request.

##### Patterned Fields

Field Pattern | Type | Description
---|:---:|---
<a name="securityRequirementsName"></a>{name} | [`string`] | Each name MUST correspond to a security scheme which is declared in the [Security Schemes](#componentsSecuritySchemes) under the [Components Object](#componentsObject). If the security scheme is of type `"oauth2"` or `"openIdConnect"`, then the value is a list of scope names required for the execution, and the list MAY be empty if authorization does not require a specified scope. For other security scheme types, the array MUST be empty.

##### Security Requirement Object Examples

###### Non-OAuth2 Security Requirement

```json
{
  "api_key": []
}
```

```yaml
api_key: []
```

###### OAuth2 Security Requirement

```json
{
  "petstore_auth": [
    "write:pets",
    "read:pets"
  ]
}
```

```yaml
petstore_auth:
- write:pets
- read:pets
```

###### Optional OAuth2 Security

Optional OAuth2 security as would be defined in an <a href="#openapi-object">OpenAPI Object</a> or an <a href="#operation-object">Operation Object</a>:

```json
{
  "security": [
    {},
    {
      "petstore_auth": [
        "write:pets",
        "read:pets"
      ]
    }
  ]
}
```

```yaml
security:
  - {}
  - petstore_auth:
    - write:pets
    - read:pets
```

### <a name="specificationExtensions"></a>Specification Extensions

While the OpenAPI Specification tries to accommodate most use cases, additional data can be added to extend the specification at certain points.

The extensions properties are implemented as patterned fields that are always prefixed by `"x-"`.

Field Pattern | Type | Description
---|:---:|---
<a name="infoExtensions"></a>^x- | Any | Allows extensions to the OpenAPI Schema. The field name MUST begin with `x-`, for example, `x-internal-id`. The value can be any valid JSON format value (`null`, a primitive, an array or an object.)

The OpenAPI Initiative maintains several [extension registries](https://spec.openapis.org/registry/index.html), including registries for [individual extension keywords](https://spec.openapis.org/registry/extension/) and [extension keyword namespaces](https://spec.openapis.org/registry/namespace/).

Extensions are one of the best ways to prove the viability of proposed additions to the specification.  
It is therefore RECOMMENDED that implementations be designed for extensibility to support community experimentation.

Support for any one extension is OPTIONAL, and support for one extension does not imply support for others.

### <a name="securityFiltering"></a>Security Filtering

Some objects in the OpenAPI Specification MAY be declared and remain empty, or be completely removed, even though they are inherently the core of the API documentation. 

The reasoning is to allow an additional layer of access control over the documentation.
While not part of the specification itself, certain libraries MAY choose to allow access to parts of the documentation based on some form of authentication/authorization.

Two examples of this:

1. The [Paths Object](#pathsObject) MAY be empty. It may be counterintuitive, but this may tell the viewer that they got to the right place, but can't access any documentation. They'd still have access to the [Info Object](#infoObject) which may contain additional information regarding authentication.
2. The [Path Item Object](#pathItemObject) MAY be empty. In this case, the viewer will be aware that the path exists, but will not be able to see any of its operations or parameters. This is different from hiding the path itself from the [Paths Object](#pathsObject), because the user will be aware of its existence. This allows the documentation provider to finely control what the viewer can see.

## Security Considerations

### OpenAPI Document Formats

OpenAPI description documents use JSON, YAML, and JSON Schema, and therefore share their security considerations:
- [JSON](https://www.iana.org/assignments/media-types/application/json)
- [YAML](https://www.iana.org/assignments/media-types/application/yaml)
- [JSON Schema Core](https://json-schema.org/draft/2020-12/json-schema-core#section-13)
- [JSON Schema Validation](https://json-schema.org/draft/2020-12/json-schema-validation#name-security-considerations)

### Tooling and Usage Scenarios

In addition, OpenAPI description documents are processed by a wide variety of tooling for numerous different purposes, such as client code generation, documentation generation, server side routing, and API testing. OpenAPI description authors must consider the risks of the scenarios where the OpenAPI description may be used.

### Security Schemes

An OpenAPI description describes the security schemes used to protect the resources it defines. The security schemes available offer varying degrees of protection. Factors such as the sensitivity of the data and the potential impact of a security breach should guide the selection of security schemes for the API resources. Some security schemes, such as basic auth and OAuth Implicit flow, are supported for compatibility with existing APIs. However, their inclusion in OpenAPI does not constitute an endorsement of their use, particularly for highly sensitive data or operations.

### Handling External Resources

OpenAPI description documents may contain references to external resources that may be dereferenced automatically by consuming tools. External resources may be hosted on different domains that may be untrusted. References in an OpenAPI document, or across OpenAPI documents within a multi-document OpenAPI description, may cause a cycle. Tooling must detect and handle cycles to prevent resource exhaustion.

### Markdown and HTML Sanitization

Certain properties allow the use of Markdown which can contain HTML including script. It is the responsibility of tooling to appropriately sanitize the Markdown.

## <a name="revisionHistory"></a>Appendix A: Revision History

Version   | Date       | Notes
---       | ---        | ---
3.0.4     | TBD        | Patch release of the OpenAPI Specification 3.0.4
3.0.3     | 2020-02-20 | Patch release of the OpenAPI Specification 3.0.3
3.0.2     | 2018-10-08 | Patch release of the OpenAPI Specification 3.0.2
3.0.1     | 2017-12-06 | Patch release of the OpenAPI Specification 3.0.1
3.0.0     | 2017-07-26 | Release of the OpenAPI Specification 3.0.0
3.0.0-rc2 | 2017-06-16 | rc2 of the 3.0 specification
3.0.0-rc1 | 2017-04-27 | rc1 of the 3.0 specification
3.0.0-rc0 | 2017-02-28 | Implementer's Draft of the 3.0 specification
2.0       | 2015-12-31 | Donation of Swagger 2.0 to the OpenAPI Initiative
2.0       | 2014-09-08 | Release of Swagger 2.0
1.2       | 2014-03-14 | Initial release of the formal document.
1.1       | 2012-08-22 | Release of Swagger 1.1
1.0       | 2011-08-10 | First release of the Swagger Specification

## <a name="dataTypeConversion"></a>Appendix B: Data Type Conversion

Serializing typed data to plain text, which can occur in `text/plain` message bodies or `multipart` parts, as well as in the `application/x-www-form-urlencoded` format in either URL query strings or message bodies, involves significant implementation- or application-defined behavior.

Schema Objects validate data based on the [JSON Schema data model](https://datatracker.ietf.org/doc/html/draft-wright-json-schema-00#section-4.2), which only recognizes four primitive data types: strings (which are [only broadly interoperable as UTF-8](https://datatracker.ietf.org/doc/html/rfc7159#section-8.1)), numbers, booleans, and `null`.
Notably, integers are not a distinct type from other numbers, with `type: integer` being a convenience defined mathematically, rather than based on the presence or absence of a decimal point in any string representation.

The [Parameter Object](#parameterObject) and [Encoding Object](#encodingObject) offer features to control how to arrange values from array or object types.
They can also be used to control how strings are further encoded to avoid reserved or illegal characters.
However, there is no general-purpose specification for converting schema-validated non-UTF-8 primitive data types (or entire arrays or objects) to strings.

Two cases do offer standards-based guidance:

* [RFC3987 §3.1](https://datatracker.ietf.org/doc/html/rfc3987#section-3.1) provides guidance for converting non-Unicode strings to UTF-8, particularly in the context of URIs (and by extension, the form media types which use the same encoding rules)
* [RFC6570 §2.3](https://www.rfc-editor.org/rfc/rfc6570#section-2.3) specifies which values, including but not limited to `null`, are considered _undefined_ and therefore treated specially in the expansion process when serializing based on that specification

Implementations of RFC6570 often have their own conventions for converting non-string values, but these are implementation-specific and not defined by the RFC itself.
This is one reason for the OpenAPI Specification to leave these conversions as implementation-defined:  It allows using RFC6570 implementations regardless of how they choose to perform the conversions.

To control the serialization of numbers, booleans, and `null` (or other values RFC6570 deems to be undefined) more precisely, schemas can be defined as `type: string` and constrained using `pattern`, `enum`, `format`, and other keywords to communicate how applications must pre-convert their data prior to schema validation.
The resulting strings would not require any further type conversion.

The `format` keyword can assist in serialization.
Some formats (such as `date-time` or `byte`) are unambiguous, while others (such as [`decimal`](https://spec.openapis.org/registry/format/decimal.html) in the [Format Registry](https://spec.openapis.org/registry/format/)) are less clear.
However, care must be taken with `format` to ensure that the specific formats are supported by all relevant tools as unrecognized formats are ignored.

Requiring input as pre-formatted, schema-validated strings also improves round-trip interoperability as not all programming languages and environments support the same data types.

## <a name="usingRFC6570Implementations"></a>Appendix C: Using RFC6570 Implementations

Serialization is defined in terms of RFC6570 URI Templates in two scenarios:

Object | Condition
------ | ---------
[Parameter Object](#parameterObject) | When `schema` is present
[Encoding Object](#encodingObject) | When encoding for `application/x-www-form-urlencoded` and any of `style`, `explode`, or `allowReserved` are used

Implementations of this specification MAY use an implementation of RFC6570 to perform variable expansion, however, some caveats apply.

Note that when using `style: form` RFC6570 expansion to produce an `application/x-www-form-urlencoded` HTTP message body, it is necessary to remove the `?` prefix that is produced to satisfy the URI query string syntax.

Note also that not all RFC6570 implementations support all four levels of operators, all of which are needed to fully support the OpenAPI Specification's usage.
Using an implementation with a lower level of support will require additional manual construction of URI Templates to work around the limitations.

### Equivalences Between Fields and RFC6570 Operators

Certain field values translate to RFC6570 operators (or lack thereof):

field | value | equivalent
----- | ----- | ----------
style | simple | _n/a_
style | matrix | `;` prefix operator
style | label | `.` prefix operator
style | form | `?` prefix operator
allowReserved | `false` | _n/a_
allowReserved | `true` | `+` prefix operator
explode | `false` | _n/a_
explode | `true` | `*` modifier suffix

Multiple `style: form` parameters are equivalent to a single RFC6570 [variable list](https://www.rfc-editor.org/rfc/rfc6570#section-2.2) using the `?` prefix operator:

```YAML
parameters:
- name: foo
  in: query
  schema:
    type: object
  explode: true
- name: bar
  in: query
  schema:
    type: string
```

This example is equivalent to RFC6570's `{?foo*,bar}`, and ***NOT*** `{?foo*}{&bar}`, which is problematic because if `foo` is not defined, the result will be an invalid URI.
The `&` prefix operator has no equivalent in the Parameter Object.

Note that RFC6570 does not specify behavior for compound values beyond the single level addressed by `explode`.  The results of using objects or arrays where no behavior is clearly specified for them is implementation-defined.

### Non-RFC6570 Field Values and Combinations

Configurations with no direct RFC6570 equivalent SHOULD also be handled according to RFC6570.
Implementations MAY create a properly delimited URI Template with variables for individual names and values using RFC6570 regular or reserved expansion (based on `allowReserved`).

This includes:
  * the styles `pipeDelimited`, `spaceDelimited`, and `deepObject`, which have no equivalents at all
  * the combination of the style `form` with `allowReserved: true`, which is not allowed because only one prefix operator can be used at a time
  * any parameter name that is not a legal RFC6570 variable name

The Parameter Object's `name` field has a much more permissive syntax than [RFC6570 variable name syntax](https://www.rfc-editor.org/rfc/rfc6570#section-2.3).
A parameter name that includes characters outside of the allowed RFC6570 variable character set MUST be percent-encoded before it can be used in a URI Template.

### Examples

Let's say we want to use the following data in a form query string, where `formulas` is exploded, and `words` is not:

```YAML
formulas:
  a: x+y
  b: x/y
  c: x^y
words:
- math
- is
- fun
```

#### RFC6570-Equivalent Expansion

This array of parameter objects uses regular `style: form` expansion, fully supported by RFC6570:

```YAML
parameters:
- name: formulas
  in: query
  schema:
    type: object
    additionalProperties:
      type: string
  explode: true
- name: words
  in: query
  schema:
    type: array
    items:
      type: string
```

This translates to the following URI Template:

```urlencoded
{?formulas*,words}
```

when expanded with the data given earlier, we get:

```urlencoded
?a=x%2By&b=x%2Fy&c=x%5Ey&words=math,is,fun
```

#### Expansion With Non-RFC6570-Supported Options

But now let's say that (for some reason), we really want that `/` in the `b` formula to show up as-is in the query string, and we want our words to be space-separated like in a written phrase.
To do that, we'll add `allowReserved: true` to `formulas`, and change to `style: spaceDelimited` for `words`:

```YAML
parameters:
- name: formulas
  in: query
  schema:
    type: object
    additionalProperties:
      type: string
  explode: true
  allowReserved: true
- name: words
  in: query
  style: spaceDelimited
  schema:
    type: array
    items:
      type: string
```

We can't combine the `?` and `+` RFC6570 prefixes, and there's no way with RFC6570 to replace the `,` separator with a space character.
So we need to restructure the data to fit a manually constructed URI Template that passes all of the pieces through the right sort of expansion.

Here is one such template, using a made-up convention of `words.0` for the first entry in the words value, `words.1` for the second.

```urlencoded
?a={+a}&b={+b}&c={+c}&words={words.0} {words.1} {words.2}
```

RFC6570 [mentions](https://www.rfc-editor.org/rfc/rfc6570.html#section-2.4.2) the use of `.` "to indicate name hierarchy in substructures," but does not define any specific naming convention or behavior for it.
Since the `.` usage is not automatic, we'll need to construct an appropriate input structure for this new template.

We'll also need to pre-process the values for `formulas` because while `/` and most other reserved characters are allowed in the query string by RFC3986, `[`, `]`, and `#` [are not](https://datatracker.ietf.org/doc/html/rfc3986#appendix-A), and `&`, `=`, and `+` all have [special behavior](https://www.rfc-editor.org/rfc/rfc1866#section-8.2.1) in the `application/x-www-form-urlencoded` format, which is what we are using in the query string.

Setting `allowReserved: true` does _not_ make reserved characters that are not allowed in URIs allowed, it just allows them to be _passed through expansion unchanged._
Therefore, any tooling still needs to percent-encode those characters because reserved expansion will not do it, but it _will_ leave the percent-encoded triples unchanged.
See also [Appendix E](#to indicate name hierarchy in substructures) for further guidance on percent-encoding and form media types, including guidance on handling the delimiter characters for `spaceDelimited`, `pipeDelimited`, and `deepObject` in parameter names and values.

So here is our data structure that arranges the names and values to suit the template above, where values for `formulas` have `[]#&=+` pre-percent encoded (although only `+` appears in this example):

```YAML
a: x%2By
b: x/y
c: x^y
words.0: math
words.1: is
words.2: fun
```

Expanding our manually assembled template with our restructured data yields the following query string:

```urlencoded
?a=x%2By&b=x/y&c=x%5Ey&words=math%20is%20fun
```
The `/` and the pre-percent-encoded `%2B` have been left alone, but the disallowed `^` character (inside a value) and space characters (in the template but outside of the expanded variables) were percent-encoded.

#### Undefined Values and Manual URI Template Construction

Care must be taken when manually constructing templates to handle the values that [RFC6570 considers to be _undefined_](https://datatracker.ietf.org/doc/html/rfc6570#section-2.3) correctly:

```YAML
formulas: {}
words:
- hello
- world
```

Using this data with our original RFC6570-friendly URI Template, `{?formulas*,words}`, produces the following:


```urlencoded
?words=hello,world
```

This means that the manually constructed URI Template and restructured data need to leave out the `formulas` object entirely so that the `words` parameter is the first and only parameter in the query string.

Restructured data:

```YAML
words.0: hello
words.1: world
```

Manually constructed URI Template:

```urlencoded
?words={words.0} {words.1}
```

Result:

```urlencoded
?words=hello%20world
```

#### Illegal Variable Names as Parameter Names
In this example, the heart emoji is not legal in URI Template names (or URIs):

```YAML
parameters:
- name: ❤️
  in: query
  schema:
    type: string
```

We can't just pass `❤️: love!` to an RFC6570 implementation.
Instead, we have to pre-percent-encode the name (which is a six-octet UTF-8 sequence) in both the data and the URI Template:

```YAML
"%E2%9D%A4%EF%B8%8F": love!
```

```urlencoded
{?%E2%9D%A4%EF%B8%8F}
```

This will expand to the result:

```urlencoded
?%E2%9D%A4%EF%B8%8F=love%21
```

## <a name="serializingHeadersAndCookies"></a>Appendix D: Serializing Headers and Cookies

RFC6570's percent-encoding behavior is not always appropriate for `in: header` and `in: cookie` parameters.
In many cases, it is more appropriate to use `content` with a media type such as `text/plain` and require the application to assemble the correct string.

For both cookies ([RFC6265](https://www.rfc-editor.org/rfc/rfc6265)) and HTTP headers using the structured fields ([RFC8941](https://www.rfc-editor.org/rfc/rfc8941)) syntax, non-ASCII content is handled using base64 encoding (`format: byte`).
Note that the standard base64 encoding alphabet includes non-URL-safe characters that are percent-encoded by RFC6570 expansion; serializing values through both encodings is NOT RECOMMENDED.

Most HTTP headers predate the structured field syntax, and a comprehensive assessment of their syntax and encoding rules is well beyond the scope of this specification.
While [RFC8187](https://www.rfc-editor.org/rfc/rfc8187) recommends percent-encoding HTTP field (header or trailer) parameters, these parameters appear after a `;` character.
With `style: simple`, that delimiter would itself be percent-encoded, violating the general HTTP field syntax.

Using `style: form` with `in: cookie` is ambiguous for a single value, and incorrect for multiple values.
This is true whether the multiple values are the result of using `explode: true` or not.

This style is specified to be equivalent to RFC6570 form expansion which includes the `?` character (see Appendix C for more details), which is not part of the cookie syntax.
However, examples of this style in past versions of this specification have not included the `?` prefix, suggesting that the comparison is not exact.
Because implementations that rely on an RFC6570 implementation and those that perform custom serialization based on the style example will produce different results, it is implementation-defined as to which of the two results is correct.

For multiple values, `style: form` is always incorrect as name=value pairs in cookies are delimited by `; ` (a semicolon followed by a space character) rather than `&`.

## <a name="percentEncodingAndFormMediaTypes"></a>Appendix E: Percent-Encoding and Form Media Types

_**NOTE:** In this section, the `application/x-www-form-urlencoded` and `multipart/form-data` media types are abbreviated as `form-urlencoded` and `form-data`, respectively, for readability._

Percent-encoding is used in URIs and media types that derive their syntax from URIs.
This process is concerned with three sets of characters, the names of which vary among specifications but are defined as follows for the purposes of this section:

* _unreserved_ characters do not need to be percent-encoded; while it is safe to percent-encode them, doing so produces a URI that is [not normalized](https://datatracker.ietf.org/doc/html/rfc3986#section-6.2.2.2)
* _reserved_ characters either have special behavior in the URI syntax (such as delimiting components) or are reserved for other specifications that need to define special behavior (e.g. `form-urlencoded` defines special behavior for `=`, `&`, and `+`)
* _unsafe_ characters are known to cause problems when parsing URIs in certain environments

Unless otherwise specified, this section uses RFC3986's definition of [reserved](https://datatracker.ietf.org/doc/html/rfc3986#section-2.2) and [unreserved](https://datatracker.ietf.org/doc/html/rfc3986#section-2.3), and defines the unsafe set as all characters not included in either of those sets.

### Percent-Encoding and `form-urlencoded`

Each URI component (such as the query string) considers some of the reserved characters to be unsafe, either because they serve as delimiters between the components (e.g. `#`), or (in the case of `[` and `]`) were historically considered globally unsafe but were later given reserved status for limited purposes.

Reserved characters with no special meaning defined within a component can be left un-percent encoded.
However, other specifications can define special meanings, requiring percent-encoding for those characters outside of the additional special meanings.

The `form-urlencoded` media type defines special meanings for `=` and `&` as delimiters, and `+` as the replacement for the space character (instead of its percent-encoded form of `%20`).
This means that while these three characters are reserved-but-allowed in query strings by RFC3986, they must be percent-encoded in `form-urlencoded` query strings except when used for their `form-urlencoded` purposes; see [Appendix C](#usingRFC6570Implementations) for an example of handling `+` in form values.

### Percent-Encoding and `form-data`

[RFC7578 §2](https://datatracker.ietf.org/doc/html/rfc7578#section-2) suggests RFC3986-based percent-encoding as a mechanism to keep text-based per-part header data such as file names within the ASCII character set.
This suggestion was not part of older (pre-2015) specifications for `form-data`, so care must be taken to ensure interoperability.

The `form-data` media type allows arbitrary text or binary data in its parts, so percent-encoding is not needed and is likely to cause interoperability problems unless the `Content-Type` of the part is defined to require it.

### Generating and Validating URIs and `form-urlencoded` Strings

URI percent encoding and the `form-urlencoded` media type have complex specification histories spanning multiple revisions and, in some cases, conflicting claims of ownership by different standards bodies.
Unfortunately, these specifications each define slightly different percent-encoding rules, which need to be taken into account if the URIs or `form-urlencoded` message bodies will be subject to strict validation.
(Note that many URI parsers do not perform validation by default.)

This specification normatively cites the following relevant standards:

Specification | Date | OAS Usage | Percent-Encoding | Notes
------------- | ---- | --------- | ----- | -----
[RFC3986 URI&nbsp;Generic&nbsp;Syntax](https://datatracker.ietf.org/doc/html/rfc3986) | 01/2005 | URI/URL syntax | RFC3986 | obsoletes RFC1738, RFC2396
[RFC6570 URI&nbsp;Template](https://datatracker.ietf.org/doc/html/rfc6570) | 03/2012 | style-based serialization | RFC3986 | does not use `+` for <code>form&#8209;urlencoded</code>
[RFC1866&nbsp;§8.2.1 form&#8209;urlencoded](https://datatracker.ietf.org/doc/html/rfc1866#section-8.2.1) | 11/1995 | content-based serialization | RFC1738 | obsoleted by [HTML&nbsp;4.01&nbsp;§17.13.4.1](https://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1), [WHATWG&nbsp;URL&nbsp;§5](https://url.spec.whatwg.org/#urlencoded-serializing)

Style-based serialization is used in the [Parameter Object](#parameterObject) when `schema` is present, and in the [Encoding Object](#encodingObject) when at least one of `style`, `explode`, or `allowReserved` is present.
See [Appendix C](#usingRFC6570Implementations) for more details of RFC6570's two different approaches to percent-encoding, including an example involving `+`.

Content-based serialization is defined by the [Media Type Object](#mediaTypeObject), and used with the [Parameter Object](#parameterObject) when the `content` field is present, and with the [Encoding Object](#encodingObject) based on the `contentType` field when the style fields listed in the previous section are absent.
Each part is encoded based on the media type (e.g. `text/plain` or `application/json`), and must then be percent-encoded for use in a `form-urlencoded` string.

Note that content-based serialization for `form-data` does not expect or require percent-encoding in the data, only in per-part header values.

#### Interoperability with Historical Specifications

In most cases, generating query strings in strict compliance with RFC3986 is sufficient to pass validation (including JSON Schema's `format: uri` and `format: uri-reference`), but some `form-urlencoded` implementations still expect the slightly more restrictive RFC1738 rules to be used.

Since all RFC1738-compliant URIs are compliant with RFC3986, applications needing to ensure historical interoperability SHOULD use RFC1738's rules.

#### Interoperability with Web Browser Environments

WHATWG is a [web browser-oriented](https://whatwg.org/faq#what-is-the-whatwg-working-on) standards group that has defined a "URL Living Standard" for parsing and serializing URLs in a browser context, including parsing and serializing `form-urlencoded` data.
WHATWG's percent-encoding rules for query strings are different depending on whether the query string is [being treated as `form-urlencoded`](https://url.spec.whatwg.org/#application-x-www-form-urlencoded-percent-encode-set) (where it requires more percent-encoding than RFC1738) or [as part of the generic syntax](https://url.spec.whatwg.org/#query-percent-encode-set), where it allows characters that RFC3986 forbids.

Implementations needing maximum compatibility with web browsers SHOULD use WHATWG's `form-urlencoded` percent-encoding rules.
However, they SHOULD NOT rely on WHATWG's less stringent generic query string rules, as the resulting URLs would fail RFC3986 validation, including JSON Schema's `format: uri` and `format: uri-reference`.

### Decoding URIs and `form-urlencoded` Strings

The percent-decoding algorithm does not care which characters were or were not percent-decoded, which means that URIs percent-encoded according to any specification will be decoded correctly.

Similarly, all `form-urlencoded` decoding algorithms simply add `+`-for-space handling to the percent-decoding algorithm, and will work regardless of the encoding specification used.

However, care must be taken to use `form-urlencoded` decoding if `+` represents a space, and to use regular percent-decoding if `+` represents itself as a literal value.

### Percent-Encoding and Illegal or Reserved Delimiters

The `[` and `]`, `|`, and space characters, which are used as delimiters for the `deepObject`, `pipeDelimited`, and `spaceDelimited` styles, respectively, all MUST be percent-encoded to comply with RFC3986.
This requires users to pre-encode the character(s) in some other way in parameter names and values to distinguish them from the delimiter usage when using one of these styles.

The space character is always illegal and encoded in some way by all implementations of all versions of the relevant standards.
While one could use the `form-urlencoded` convention of `+` to distinguish spaces in parameter names and values from `spaceDelimited` delimiters encoded as `%20`, the specifications define the decoding as a single pass, making it impossible to distinguish the different usages in the decoded result.

Some environments use `[`, `]`, and possibly `|` unencoded in query strings without apparent difficulties, and WHATWG's generic query string rules do not require percent-encoding them.
Code that relies on leaving these delimiters unencoded, while using regular percent-encoding for them within names and values, is not guaranteed to be interoperable across all implementations.

For maximum interoperability, it is RECOMMENDED to either define and document an additional escape convention while percent-encoding the delimiters for these styles, or to avoid these styles entirely.
The exact method of additional encoding/escaping is left to the API designer, and is expected to be performed before serialization and encoding described in this specification, and reversed after this specification's encoding and serialization steps are reversed.
This keeps it outside of the processes governed by this specification.