Executable API compatibility and migration evidence

This required 20–30-hour package supplies the compatibility core of guided Lab 3 and sharpens Labs 1–2: lessons/predictions 5–7 hours, implementation and consumer fixtures 9–15, assessment/repair 6–8. Reuse outputs in existing labs; estimates await semester reconciliation and learner pilots.

Prerequisite bridge

Review set inclusion, Cartesian products, absence versus null, and request/response roles. Revisit Semester 06 retry uncertainty and Module 03 translation versus business acceptance. A schema defines possible values; it does not fully define business meaning.

Sequence

1. Compatibility direction.
2. Counterexample contracts.
3. Migration and retry evidence.
4. Implement api-compatibility-contracts.zip using Python 3 standard library. Run python3 -B check.py starter, retaining predictions before reference calibration.
5. Complete AC1–AC6.

The harness checks 625 schema pairs against a structural inclusion oracle, 200 payload cases, empty-object witnesses, determinism, input preservation and invalid schemas. It is a finite closed-object model, not an OpenAPI validator or proof of semantic compatibility. Preserve the original actual HTTP API, durable retry/concurrency/expiry harness, old/new consumer migration, rollback and independent consumer review.

Semester integration

The reconciled workload now includes this package, original module work, required strand and assessment. Its allocation supersedes earlier provisional timing notes; do not count reused artifacts twice.