mutate-go

Run the mutate.sh script with the target file: mutate.sh path/to/file.go. It discovers the Go module, narrows gremlins to that one file, runs the package's test suite once per mutant, and prints survivors grouped by the action each one calls for.

Mutation testing with gremlins breaks the code one operator at a time and checks whether any test fails, while coverage only records that a line executed. A test asserting only require.NoError gives full coverage but no protection against wrong behavior.

This usually means the package's test files sit behind build tags like //go:build unit, so go test finds no test files and gremlins gathers empty coverage. The script detects this and tells you which tags to pass with --tags.

Gremlins sizes each mutant's timeout from the coverage run duration, so a sub-second suite makes every mutant report TIMED OUT. The script retries once with a large timeout coefficient, or you can set --timeout-coefficient manually.

Avoid it as a CI gate with a score threshold, on IO glue or generated code where mutants are mostly equivalent, or for chasing a number. It also cannot detect behavior never implemented, such as a missing validation or unhandled error.

Yes, pass the --package flag to mutate every file in the package rather than just the named file. You can also add --all-mutators to enable gremlins' opt-in logical, bitwise, and loop-control mutants for a wider pass.