Would the tests notice?
Made for the earlier edition of this course, which ran model-written code through a course launcher. The current course does not require one.
A passing suite says nothing about the inputs it never tried. Write the oracle from the contract, including the strings it must reject, and never from the code's own output. Run red, let a repair be generated as a proposal, let a run on the unchanged tests decide, and then challenge green: with a property, mutants and a seeded fuzz run, keeping each finding as an exact, replayable input. A survivor shows a change the suite cannot see until a test kills it or a contract reason shows it is equivalent; report every disposition, and check a harness on a known-bad and a repaired target before trusting its silence. None of these checks proves the parser correct; each covers a stated set of inputs or changes.