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nix/doc/manual/rl-next/faster-nix-flake-check.md
Seth Flynn ecdda5798c nix flake check: Skip substitutable derivations 
Since `nix flake check` doesn't produce a `result` symlink, it doesn't
actually need to build/substitute derivations that are already known
to have succeeded, i.e. that are substitutable.

This can speed up CI jobs in cases where the derivations have already
been built by other jobs. For instance, a command like

  nix flake check github:NixOS/hydra/aa62c7f7db31753f0cde690f8654dd1907fc0ce2

should no longer build anything because the outputs are already in
cache.nixos.org.

Based-on: https://github.com/DeterminateSystems/nix-src/pull/134
Based-on: https://gerrit.lix.systems/c/lix/+/3841
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
2025-09-15 21:31:03 +02:00

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synopsis prs
`nix flake check` now skips derivations that can be substituted
13574

Previously, nix flake check would evaluate and build/substitute all derivations. Now, it will skip downloading derivations that can be substituted. This can drastically decrease the time invocations take in environments where checks may already be cached (like in CI).