Overview

Hydra checks out project source periodically or continuously, builds it, runs tests and produces reports. Jobsets connect inputs to build work, can trigger when inputs change, and can depend on other jobsets. This model fits teams whose CI work is already organized around Nix rather than teams that simply use NixOS and want a general-purpose CI service.

Key features

Jobsets, Flakes and build products

Hydra supports Flake jobsets when their outputs expose a hydraJobs attribute set. Jobsets can produce packages, ISO images and tarballs, making the system useful beyond package compilation when those outputs are part of a Nix project. Inputs can use Git checkout sources, and documented examples use GitHub repositories.

Restricted-mode Flake evaluation prevents access to files outside the Nix store, including fetched Flake inputs. That security boundary matters when designing builds: projects that expect evaluation to read external files must fit their setup to this restriction.

API and build infrastructure

A JSON API is defined by the repository's hydra-api.yaml specification, which gives teams an API surface to integrate with. Hydra also supports self-hosted runners and an artifact or dependency cache, aligning it with teams that want to keep build infrastructure under their own control.

Pricing

Hydra's sole plan, Hydra, costs 0.00 USD per free and is GPL-3.0 licensed. There are no paid tiers or seat and quota caps to weigh from the stated plan terms. The trade-off is that this is self-hosted software, not a hosted service: teams supply and maintain the infrastructure, and running Hydra is currently supported only on NixOS. There is no hosted option.

Platforms

Hydra is a Linux, web and API product deployed in a self-hosted environment. In practice, its supported deployment is narrower than that platform list suggests: the repository says running Hydra is currently supported only on NixOS and recommends the NixOS module for production.

Who it's for

Hydra is best suited to Nix and NixOS package developers who need CI for jobset-based builds and can operate NixOS infrastructure. It is not necessary simply to use NixOS. Teams wanting managed CI, or unable to take responsibility for a NixOS deployment, should look elsewhere.

Pros and cons

Pro: Jobsets, Flake support and several build product types suit projects that express CI work in Nix.
Pro: A JSON API, self-hosted runners and artifact or dependency cache support an infrastructure-owning workflow.
Pro: The GPL-3.0 licensed plan costs 0.00 USD per free.
Con: NixOS-only supported deployment limits teams running other server environments.
Con: Hydra stores some state in its database, so a complete stateless configuration is not currently possible and backups are needed.
Con: The wiki reports that hydra-queue-runner can sometimes leave builds queued but unscheduled, a reliability concern for operators to account for.

Alternatives

For a broader CI shortlist, see Continuous Integration Software. Hydra's alternatives in the directory are Nintendo Switch emulators, not CI products: NeXium, Eden, Ruzu, Citron NEO, Nuzu, Ryubing, Ryujinx Classic and Sumi. They are emulator options rather than like-for-like replacements for Nix CI.

Verdict

Choose Hydra if your team builds Nix projects, wants jobset- and Flake-oriented CI, and can run and back up NixOS infrastructure. Its main reason to choose is that focused Nix workflow support comes without a software fee. Look elsewhere if you need hosted CI, deployment beyond NixOS, or a service that does not require database-state backups and active queue operations.

	CruiseControl.NET	Hydra
Free plan	No	Yes
Free trial	No	No
Paid from	—	Free
Open source	No	No
Platforms	Windows, Linux	api, Linux, self-hosted, Web
Free plan	Yes	—
Hosted option	No	No
Self-hosted runners	Yes	Yes
Artifact or dependency cache	—	Yes

CruiseControl.NET vs Hydra

CruiseControl.NET

Hydra