temperature, top_p and top_k return a 400 in API version 2026-10-21

Q: How do I fix this when moving to claude-*?

Remove temperature, top_p and top_k from every request, on every model, when you send anthropic-version: 2026-10-21.

Returns 400 Applies to claude-* on the Anthropic Messages API with anthropic-version: 2026-10-21.

Anthropic's docs describe this error but don't quote the exact message, so we don't either.

How to fix it

Remove temperature, top_p and top_k from every request, on every model, when you send anthropic-version: 2026-10-21.

Before

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "claude-opus-4-8",
  "temperature": 1
}

After (Claude Opus 4.8 (API version rules only))

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "claude-opus-4-8"
}

Check your whole request in the linter