Nixomatic GitHub Action

A GitHub Action for setting up reproducible Nix environments with packages from specific nixpkgs revisions.

This action uses nixomatic.com to generate Nix flakes on-demand, allowing you to mix and match packages from different nixpkgs revisions in your CI/CD workflows.

Usage

- uses: curriedsoftware/nixomatic-action@main
  with:
    packages: python3@3.13.4 nodejs@24.13.0 jq
    run: python --version

Inputs

Input	Description	Required	Default
packages	Packages to install (space or comma-separated)	Yes	-
install-nix	Whether to install Nix. Set to false if Nix is already available.	No	true
run	Script to run inside the Nix environment. If provided, runs the script and exits. If not provided, adds packages to PATH for subsequent steps.	Yes	-

Package Formats

• package - Latest from nixos-unstable (e.g., jq, ripgrep)
• package@version - Specific version resolved against nixpkgs-multiverse (e.g., python3@3.13.4)
• package:revision - Exact nixpkgs commit hash (e.g., python3:3b93cf5)

Only versions the index actually has can be pinned, and an unknown one fails the workflow at evaluation time. Check what a package can be pinned to before using @version:

$ curl -s 'https://nixomatic.com/versions?p=nodejs'
{"nodejs":["0.10.21","0.12.7","…","24.18.1"]}

The answer is always an object keyed by the spec as it was asked for, whether one package was requested or many, and each list is ordered oldest first — an empty one when the package is not in the index at all:

$ curl -s 'https://nixomatic.com/versions?p=jq,hello'
{"hello":["2.8","…","2.12.3"],"jq":["1.3","…","1.8.2"]}

Asking about a revision instead gives the one version that revision ships, or null when it ships none:

$ curl -s 'https://nixomatic.com/versions?p=nodejs:6a55e4c'
{"nodejs:6a55e4c":"20.15.1"}

Examples

Basic usage

steps:
  - uses: actions/checkout@v4
  - uses: curriedsoftware/nixomatic-action@main
    with:
      packages: cowsay jq ripgrep
      run: |
        cowsay "Hello from nixomatic!"
        jq --version
        rg --version

Specific package versions

steps:
  - uses: actions/checkout@v4
  - uses: curriedsoftware/nixomatic-action@main
    with:
      packages: python3@3.13.4 nodejs@24.13.0
      run: |
        python3 --version
        node --version

Multi-line package list

steps:
  - uses: actions/checkout@v4
  - uses: curriedsoftware/nixomatic-action@main
    with:
      packages: |
        cowsay
        jq
        ripgrep
      run: |
        cowsay "Hello from nixomatic!"
        jq --version
        rg --version

With Nix already installed

steps:
  - uses: actions/checkout@v4
  - uses: cachix/install-nix-action@v31
  - uses: curriedsoftware/nixomatic-action@main
    with:
      packages: hello
      install-nix: false
      run: hello

How It Works

1. Installs Nix using cachix/install-nix-action (unless install-nix: false)
2. Builds a URL with the specified packages pointing to nixomatic.com
3. Runs the provided script inside nix develop (if run is specified)