Migrating live software is standard engineering; migrating a live LLM system carrying enterprise traffic is terrifying because nobody can tell you what "the same answers" means.
When building Olympus, we needed to migrate two active production agents onto the platform SDK. A big-bang rewrite or hopeful cutover was out of the question. To make the cutover provable rather than hopeful, we built a golden-set dual-run regression harness (`olympus_evalkit`).
The harness records real request/response cassettes from production workflows, executes both the legacy and platform code paths against identical inputs, and systematically diffs the structured outputs, reasoning trees, and citations.
Gating this harness in CI ensures that any commit that alters output behavior fails the build before reaching production. For AI systems, the test you actually need is not "is the output good", but "is the output the same as the thing we already trust".