Nix Flake Checker is a tool from Determinate Systems that performs "health" checks on the flake.lock files in your flake-powered Nix projects. Its goal is to help your Nix projects stay on recent and supported versions of Nixpkgs.

To run the checker in the root of a Nix project:

nix run github:DeterminateSystems/flake-checker

# Or point to an explicit path for flake.lock
nix run github:DeterminateSystems/flake-checker /path/to/flake.lock

Nix Flake Checker looks at your flake.lock's root-level Nixpkgs inputs and checks that:

• Any explicit Nixpkgs Git refs are in this list:
  • nixos-23.11
  • nixos-23.11-small
  • nixos-unstable
  • nixos-unstable-small
  • nixpkgs-23.11-darwin
  • nixpkgs-unstable
• Any Nixpkgs dependencies are less than 30 days old
• Any Nixpkgs dependencies have the NixOS org as the GitHub owner (and thus that the dependency isn't a fork or non-upstream variant)

If you're running it locally, Nix Flake Checker reports any issues via text output in your terminal. But you can also use Nix Flake Checker in CI.

You can automate Nix Flake Checker by adding Determinate Systems' Nix Flake Checker Action to your GitHub Actions workflows:

checks:
  steps:
    - uses: actions/checkout@v4
    - name: Check Nix flake Nixpkgs inputs
      uses: DeterminateSystems/flake-checker-action@main

When run in GitHub Actions, Nix Flake Checker always exits with a status code of 0 by default—and thus never fails your workflows—and reports its findings as a Markdown summary.

The goal of Nix Flake Checker is to help teams stay on recent and supported versions of Nixpkgs. The flake checker collects a little bit of telemetry information to help us make that true.

Here is a table of the telemetry data we collect:

To disable diagnostic reporting, set the diagnostics URL to an empty string by passing --no-telemetry or setting FLAKE_CHECKER_NO_TELEMETRY=true.

You can read the full privacy policy for Determinate Systems, the creators of this tool and the Determinate Nix Installer, here.

The Nix Flake Checker is written in Rust. This repo exposes a parse-flake-lock crate that you can use to parse flake.lock files in your own Rust projects. To add that dependency:

[dependencies]
parse-flake-lock = { git = "https://github.com/DeterminateSystems/flake-checker", branch = "main" }

Here's an example usage:

use std::path::Path;

use parse_flake_lock::{FlakeLock, FlakeLockParseError};

fn main() -> Result<(), FlakeLockParseError> {
    let flake_lock = FlakeLock::new(Path::new("flake.lock"))?;
    println!("flake.lock info:");
    println!("version: {version}", version=flake_lock.version);
    println!("root node: {root:?}", root=flake_lock.root);
    println!("all nodes: {nodes:?}", nodes=flake_lock.nodes);

    Ok(())
}

The parse-flake-lock crate doesn't yet exhaustively parse all input node types, instead using a "fallthrough" mechanism that parses input types that don't yet have explicit struct definitions to a serde_json::value::Value. If you'd like to help make the parser more exhaustive, pull requests are quite welcome.