Checkpoint: Advanced Testing

The exam question: CI is red, but rerunning the suite turns it green. What do you do first — rerun again, or hunt the nondeterminism?

The checkpoint: a test-record audit plus a deterministic re-implementation. First classify which tests are untrustworthy and why (the flaky causes), then rewrite the risky ones around injected time, seeded randomness, and isolated state — the exact moves from this module.