Free.
The Corund plugin is free and MIT-licensed. It runs on your machine, on your tree, and uploads nothing.
The plugin
A Claude Code Stop hook. When the agent says it is done, Corund reverts the non-test changes, re-runs the tests the agent just wrote, and refuses the stop if they still pass. There is no account, no server, and nothing to pay for.
/plugin marketplace add Grade-Inc/corund-plugin /plugin install corund
Python 3.9+, git, and a JS test runner. Jest is live; Vitest is built on the same engine. No third-party packages.
Also on the pull request. The same red-on-revert engine runs as a GitHub Action in your own CI, where it works on Python as well: it reverts the pull request's non-test diff, runs the new and changed tests, requires them to fail, restores the diff, requires them to pass, and posts both runs on a receipt. It is open source and it runs on your own Actions minutes. Observe by default — it reports rather than blocks unless you opt a check in. corund-action on GitHub
C1 runs a pull request's changed tests two or three more times, so a gated pull request uses more of your Actions minutes than an ungated one.