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.

2. Read vLLM's implementation

In a vLLM checkout, the request/response models live beside each endpoint:

Each directory holds protocol.py (the Pydantic models), serving.py (behavior), and api_router.py (routing and status codes). Read the field declaration and the code that consumes it — a field can be declared faithfully and then ignored, which is still an incompatibility.

3. Fetch the upstream spec

Prefer a source that can actually be read as text, in this order.

OpenAI

1. The OpenAPI spec — authoritative, machine-readable, and fetches cleanly: https://raw.githubusercontent.com/openai/openai-openapi/master/openapi.yaml
2. The installed openai Python SDK's typed params, which track the spec closely. Locate them with python -c "import openai,os;print(os.path.dirname(openai.__file__))", then read e.g. types/chat/completion_create_params.py, types/chat/chat_completion.py, types/chat/chat_completion_chunk.py, types/responses/. Note the SDK version — an old SDK is an old spec.
3. The prose reference at https://platform.openai.com/docs/api-reference/... for semantics the types do not carry. It is script-heavy and often fetches poorly; treat a failed fetch as a fetch failure, not as absence of a field.

Anthropic

1. https://docs.claude.com/en/api/messages and https://docs.claude.com/en/api/messages-streaming — these render as clean text and are the primary source for both fields and streaming event order.
2. The installed anthropic SDK's types/message_create_params.py, types/message.py, types/raw_message_stream_event.py.

Say which source and which version answered the question, so the finding can be re-checked later.

4. Compare on all four axes

A field is compatible only when name, type, default, and semantics all match. Most real bugs hide in the last two: a field that exists with the right type but the wrong default, or one that is accepted and silently dropped.

Also check, when relevant:

• Streaming — chunk shape, the order of events, and the terminal event (data: [DONE] for Chat Completions and Completions; response.completed, response.failed, or response.incomplete with no [DONE] for Responses; message_stop for Anthropic).
• Errors — HTTP status and the error body's type/code strings.
• Omitted vs null — upstream often distinguishes an absent field from an explicit null. Pydantic defaults erase that distinction unless the model is written to preserve it.

5. Classify before calling it a bug

Three outcomes, and they are not the same:

• Matches upstream — no action.
• vLLM extension — a field upstream has no name for at all (vllm_xargs, chat_template_kwargs, sampling knobs like min_p). These are intentional and allowed. Do not report them as violations. Check per surface: top_k is an extension on OpenAI but a real upstream field on Anthropic Messages.
• Incompatibility — a field that shares a name with upstream but differs in type, default, or behavior, or an upstream field that is missing, ignored, or malformed. This is the reportable case.

An unsupported-but-declared field is its own subcase: decide whether vLLM rejects it explicitly or accepts and ignores it, and report which, since silently ignoring a documented field is the more damaging failure.

6. Report

State: the endpoint and field, what upstream specifies (with the source), what vLLM does (with file:line), which of the three outcomes it is, and the smallest fix. When behavior turns on a spec detail that is easy to disbelieve, quote the upstream line rather than paraphrasing it.