Hydra
What it is
Hydra is a continuous integration service for Nix-based projects. It checks out source code on a schedule or continuously, builds it, runs tests, and produces reports. Work is organized into jobsets: their inputs can trigger builds when they change, and one jobset can depend on another. Flake jobsets are supported when their outputs expose a hydraJobs attribute set. Jobs can produce packages, ISO images, or tarballs, and inputs can use Git checkout sources, including documented GitHub examples. Hydra also provides a JSON API described by the repository’s hydra-api.yaml specification. The free, open-source software is licensed under GPL-3.0 and costs nothing. It is intended for Nix and NixOS package developers, not simply for people using NixOS. Running Hydra is currently supported only on NixOS, with the NixOS module recommended for production. Restricted flake evaluation blocks access to files outside the Nix store. Some state remains in the database, so backups are needed; the wiki also notes that hydra-queue-runner can leave queued builds unscheduled.
For and against
For
- Automates builds, tests, and reports.
- Jobsets can respond to changed inputs.
- Supports Flake jobsets and multiple build products.
- Provides a JSON API.
- Free and licensed under GPL-3.0.
Against
- Running Hydra is currently supported only on NixOS.
- Some database state requires backups.
- Queued builds can remain unscheduled.
The verdict
Hydra provides jobset-based continuous integration for Nix projects, with support for Flakes and a JSON API. Its NixOS-only deployment support and need for database backups are important operational considerations.
Who it suits
Hydra suits Nix and NixOS package developers who need to build and test projects. It is not needed just to use NixOS.
Plans and prices
Hydra
GPL-3.0 licensed; running Hydra is currently only supported on NixOS
Read 4 Oct 2026 · github.com
On the record
- API
- The repository includes a JSON API specification in hydra-api.yaml and points users to Swagger Editor for exploration.github.com · read 14 Oct 2026
- API surface
- The hydra-api.yaml file contains 1,267 lines, including 1,220 lines of code.github.com · read 14 Oct 2026
- Binary cache
- Successful builds become substitutable store paths that can feed a binary cache.zemdregon.github.io · read 14 Oct 2026
- Build model
- Hydra evaluates Nix expressions into derivations and builds them on configured machines.zemdregon.github.io · read 14 Oct 2026
- Build outputs
- A Hydra build is an instantiated job such as a package, ISO image, or tarball.wiki.nixos.org · read 14 Oct 2026
- Build products
- Hydra can build jobs such as packages, ISO images, and tarballs as part of a jobset.wiki.nixos.org · read 4 Oct 2026
- Builds and tests
- Hydra periodically or continuously checks out project source, builds it, runs tests, and produces reports.wiki.nixos.org · read 4 Oct 2026
- Checks distinction
- Hydra does not use a flake's checks output as its job tree; checks are generally local or forge-CI gates.zemdregon.github.io · read 14 Oct 2026
- Database dependency
- A self-hosted deployment uses PostgreSQL for persistent Hydra state.zemdregon.github.io · read 14 Oct 2026
- Declarative configuration
- Hydra supports both imperative and declarative project and jobset configuration.wiki.nixos.org · read 14 Oct 2026
- Deployment
- The repository says running Hydra is currently only supported on NixOS and recommends the NixOS module for production.github.com · read 4 Oct 2026
- Documentation
- The project links to a Hydra User's Guide, NixOS Wiki material, hydra-cli, and a build-farm setup presentation.github.com · read 14 Oct 2026
- Evaluations
- An evaluation interprets Nix code into derivation files that define the build jobs.wiki.nixos.org · read 14 Oct 2026
- Flakes
- Hydra supports Flake jobsets whose outputs expose a hydraJobs attribute set.wiki.nixos.org · read 4 Oct 2026
- Flake support
- For flakes, Hydra consumes the hydraJobs output attribute as its job tree.zemdregon.github.io · read 14 Oct 2026
- Foundation operations
- The NixOS Foundation says its Hydra build farm has hundreds of macOS, x86-64 Linux, and aarch64 Linux build cores and executes more than 350,000 builds weekly.nixos.org · read 14 Oct 2026
- Independent review coverage
- G2's Hydra page says it does not have enough reviews to provide buying insight; the listing is for a project-management product rather than NixOS Hydra.g2.com · read 14 Oct 2026
- Independent technical assessment
- An independent Nix documentation site describes Hydra as heavyweight for self-hosting and suggests lightweight forge CI for most application repositories.zemdregon.github.io · read 14 Oct 2026
