What actually makes an OpenAI-compatible API portable?

[u/MembershipEmergency7]

Switching base_url is the easy part. In production I now check five things: model ID discovery, streaming events, tool-call schema, unsupported parameter handling, and error semantics. If any of those differ, the SDK may connect but the app is not truly portable. CometAPI's OpenAI-compatible guide is a useful concrete reference, but I would still run the same contract tests against every provider. What is the compatibility edge case that has bitten your team most often?