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Both clients work. Which endpoint you use is a question of which SDK you already have, not which models you can reach — they reach the same ones.

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From an OpenAI client

Two lines:
/v1/chat/completions, /v1/completions, /v1/models and /v1/models/{model} all behave as the SDK expects. The model name is the one thing that must change — ours are not OpenAI’s. See Models and resource names.

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From an Anthropic client

Give the SDK the origin, not the /v1 base — it appends /v1/messages itself:
x-api-key is accepted, so the SDK’s auth code is untouched. Claude Code works by changing base_url alone. Two differences from Anthropic’s own API:
  • No claude-* aliasing. Model names stay ours.
  • max_tokens is optional. Anthropic requires it; here, omitting it falls back to the model’s default rather than failing the call.

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Ignored, or rejected

A parameter this API does not implement is ignored, not rejected — your existing request bodies will not start failing because they carry something extra. The exceptions are the handful that would change the answer if ignored. Those return INVALID_REQUEST:
The reasoning is the same one behind tool calling and structured outputs: silently ignoring a parameter that changes the shape of the answer produces a response that looks right and is not. An error you cannot miss is the better failure.
A few more are accepted and then deliberately dropped — top_k on /v1/messages is accepted so Anthropic clients do not break, and not passed to the model. The per-parameter truth is on each endpoint page in the API reference.

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What to check after switching

Model names

The only change that is not configuration. Look them up rather than translating.

Retry logic

Branch on error.code, not on the status — three codes share 429 and one never succeeds.

Usage parsing

Streaming carries usage on the last event without include_usage.

Provider assumptions

Nothing in a response names who served it. Code that branched on that has nothing to branch on.