The obvious lesson is that you need some kind of test/harness, even for normal development. Your spec will always leave room for interpretation in a sense that is undesired.

Obviously matching functions byte-byte is not it, but I think with enough creativity, you can have guardrails and oracles almost everywhere.

E.g. before diving into feature development, you can have agents generate a test suite based on your implementation first. Then, when developing features, this test suite shows what broke and agents can double check if that was intended.

How that test suite looks like obviously highly depends on the environment.