LethAL Mutation Testing for AL

LethAL tests your tests. It breaks your AL code in small deliberate ways (flip a boundary, remove a SetRange, drop a CalcFields, turn Modify(true) into Modify(false)) and runs your suite against every broken copy. A mutant your tests catch is killed. One they miss survived, and every survivor marks a line where a real bug would ship straight past your suite.

Mutation testing never existed in AL because the naive loop is compile, publish, test, repeat, for every single mutant. A few hundred mutants at BC deploy speeds is a long weekend. LethAL compiles once: every mutant ships inside the same build, and switching to the next one is a table write, not a recompile. The bundled credit limit demo scores 32 mutants in 16 seconds, publish and baseline included.

It is also built to be driven by agents. Reports and progress streams are JSON with published schemas, doctor --json names exactly which pre-flight check failed, exit codes say whether a result is usable, and if LethAL cannot prove the server was in a sane state, it quarantines the run instead of reporting it. Every survivor is a small, provable task: write the missing test, rerun, watch the verdict flip. Pre-alpha, sandbox and dev containers only.