Hydra
Hydra is a continuous integration service for projects built with Nix.
What Hydra is
Hydra is a continuous integration service for projects built with Nix. It checks out project source on a schedule or continuously, builds it, runs tests, and produces reports. Developers organize work into jobsets: changes to their inputs can trigger builds, and one jobset may rely on another. Hydra supports Flake jobsets when their outputs provide a hydraJobs attribute set. A jobset can produce packages, ISO images, or tarballs. Git checkout sources can be used as inputs, and the project’s JSON API is defined by its hydra-api.yaml specification. Hydra is aimed at Nix and NixOS package developers; it is not required just to use NixOS. The software is open source under GPL-3.0 and costs nothing. It is self-hosted, with no hosted option listed, and running it is currently supported only on NixOS; the repository recommends its NixOS module for production. Flake evaluation is restricted from accessing files outside the Nix store. Hydra also stores some state in its database, so backups are needed.
Good
- Builds projects, runs tests, and produces reports.
- Jobsets can trigger builds when inputs change.
- Supports Flake jobsets and several build products.
- Provides a JSON API.
- Self-hosted runners and artifact or dependency cache are listed.
Watch out
- Running Hydra is currently supported only on NixOS.
- No hosted option is listed.
- A fully stateless configuration is not currently possible.
- The wiki reports queue runner scheduling issues.
Who it suits
Hydra suits Nix and NixOS package developers who need to build and test projects through jobsets. It is not needed by people who only want to use NixOS.
Verdict
Hydra offers Nix-focused build and test automation with jobsets, Flake support, and a JSON API. Its NixOS-only deployment support and database state mean operators need to account for deployment constraints and backups.
Plans and prices
From Hydra's own pricing page, checked 4 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| Hydra | Free | GPL-3.0 licensed; running Hydra is currently only supported on NixOS |
The facts
Each one read from Hydra's own pages, with the date it was checked.
What it does
- API
- The repository includes a JSON API specification in hydra-api.yaml and points users to Swagger Editor for exploration.checked 14 Oct 2026
- API surface
- The hydra-api.yaml file contains 1,267 lines, including 1,220 lines of code.checked 14 Oct 2026
- Binary cache
- Successful builds become substitutable store paths that can feed a binary cache.checked 14 Oct 2026
- Build model
- Hydra evaluates Nix expressions into derivations and builds them on configured machines.checked 14 Oct 2026
- Build outputs
- A Hydra build is an instantiated job such as a package, ISO image, or tarball.checked 14 Oct 2026
- Build products
- Hydra can build jobs such as packages, ISO images, and tarballs as part of a jobset.checked 4 Oct 2026
- Builds and tests
- Hydra periodically or continuously checks out project source, builds it, runs tests, and produces reports.checked 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.checked 14 Oct 2026
- Database dependency
- A self-hosted deployment uses PostgreSQL for persistent Hydra state.checked 14 Oct 2026
- Declarative configuration
- Hydra supports both imperative and declarative project and jobset configuration.checked 14 Oct 2026
- Deployment
- The repository says running Hydra is currently only supported on NixOS and recommends the NixOS module for production.checked 4 Oct 2026
- Documentation
- The project links to a Hydra User's Guide, NixOS Wiki material, hydra-cli, and a build-farm setup presentation.checked 14 Oct 2026
- Evaluations
- An evaluation interprets Nix code into derivation files that define the build jobs.checked 14 Oct 2026
- Flakes
- Hydra supports Flake jobsets whose outputs expose a hydraJobs attribute set.checked 4 Oct 2026
- Flake support
- For flakes, Hydra consumes the hydraJobs output attribute as its job tree.checked 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.checked 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.checked 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.checked 14 Oct 2026
- Integrations
- Hydra jobset inputs can use Git checkout sources, and the documented examples use GitHub repositories.checked 4 Oct 2026
- Intended users
- Hydra is intended for Nix and NixOS package developers and is not needed simply to use NixOS.checked 4 Oct 2026
- Jobsets
- Jobsets combine inputs with a Nix expression and can re-evaluate when inputs change.checked 14 Oct 2026
- Known issue
- The wiki reports that hydra-queue-runner can sometimes get stuck while builds remain queued and unscheduled.checked 4 Oct 2026
- License
- Hydra is distributed under the GNU General Public License version 3.checked 14 Oct 2026
- Limit: database contention
- A Hydra issue reports that evaluating very large jobsets can lock the builds table and delay queue-runner updates for around 30 minutes.checked 14 Oct 2026
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