Checking vLLM API compatibility against the upstream spec

vLLM's OpenAI and Anthropic server surfaces are reimplementations of someone else's spec. vLLM's own code, tests, and docstrings are not evidence of correct behavior — they are the thing under test. Establish what upstream says first, then compare.

Never answer "what should this field do" from memory. Field names, types, and especially defaults drift between spec versions.

1. Pin down the question

Name three things before looking anything up:

• Which surface — OpenAI-compatible or Anthropic-compatible. They are separate code paths in vLLM and separate specs.
• Which endpoint, by path.
• Which field or behavior — a request field, a response field, a streaming event shape, an error code, or a status code.