AT9 — Correctness/Performance: More tests at lower pyramid layers give better correctness assurance. A unit test that runs in one millisecond provides the same correctness signal for one function as an end-to-end test that runs in one minute — but the unit test enables immediate feedback. The performance (CI speed) of the test suite determines how often it is run, which determines how much it actually catches.
AT3 — Simplicity/Flexibility: Contract tests add process complexity: consumers must publish their contracts, producers must run consumer contracts, both parties must agree on versioning. For a system with one consumer and one producer, this overhead is not justified. For a system with ten consumers, contract tests prevent the coordination overhead of manually verifying every interface on every change.