Mutation testing

Mutation testing checks that focused tests detect small behavioral changes in critical code. It complements line coverage: a line can be executed without being meaningfully asserted.

The gate enforces seven reviewed operators in mutation policy and keeps scope narrowly constrained to one operatorized regression path per run.

Current targets

Area	Production file	Focused test
Combat wire format	packages/aonw_core/lib/game/domain/combat/combat_serialization.dart	packages/aonw_core/test/game/combat_serialization_test.dart
Unit command validation	lib/game/domain/reducer/unit/unit_command_validator.dart	test/game/domain/reducer/unit_command_validator_test.dart
Authentication input	server/lib/src/auth/auth_input_validator.dart	server/test/auth/auth_input_validator_test.dart

Each mutant runs alone in an isolated workspace. Only a real test failure counts as a kill. Analysis errors, crashes, timeouts, malformed output, or filesystem side effects fail the mutation tool itself.

The committed baseline records the exact target/operator census and all survivors. The accepted baseline has no survivors; there is no percentage waiver or inline suppression mechanism.

Commands

make mutation

After an intentional target or assertion change:

make mutation-snapshot
diff -u tool/mutation_baseline.json /tmp/aonw-mutation-baseline.json

Review the complete census. Do not edit mutant identities or counts by hand.

The gate runs on POSIX hosts. Native Windows is not supported until equivalent process isolation is implemented.