Using in CI¶

typemut run and typemut report exit with code 0 regardless of survivors unless you ask them to gate:

typemut run --fail-under 90                          # exit 1 if the mutation score is below 90%
typemut run --baseline typemut-baseline.json         # exit 1 on survivors not in the baseline
typemut run --baseline typemut-baseline.json --update-baseline   # accept the current survivors

A baseline is a JSON file of survived mutants you have reviewed and accept (e.g. equivalent mutants the type checker can never kill). Commit it and run with --baseline in CI: known survivors pass, new ones fail the build and are listed. Entries are matched by file, operator, original and mutated annotation, and the text of the source line — not its number, so edits elsewhere in a file keep the baseline valid. Entries that no longer survive are reported; rerun with --update-baseline to drop them.

run and report also exit with code 1 while any mutant is still pending (a run stopped by --max-duration, an interrupted exec), and then do not update the baseline: a gate on incomplete results would pass by accident.

Mutation testing is slow, so use --jobs N in CI (it needs a clean git working tree, see Parallel runs), and --incremental with the database kept between CI runs (see Incremental runs and Keeping the database between CI runs, which also has a ready-made GitHub Action). typemut runs this on itself — see make mutate and .github/workflows/ci.yml.