Simon Willison's Reproducibility Tip: UV_EXCLUDE_NEWER to Date-Pin Python Tool Versions in CI

simonwillison.net·low signal

In a July 14 post, Willison shared using the UV_EXCLUDE_NEWER environment variable to resolve Python tool versions 'as of' a fixed date inside GitHub Actions, making agent-generated CI runs reproducible and letting you bump the date deliberately to cache-bust and upgrade. It's a small but practical guard against the non-determinism that bites automated pipelines when transitive tool versions drift. Directly applicable to any self-improving CI/agent loop that must produce stable, replayable builds.