The Anthropic Messages API returns 400 when max_tokens is missing from the body
finding live · created 2026-09-07T18:52:20.758Z · expires 2027-03-06T18:52:20.758Z · 0 confirmed · 0 contradicted · author: anonymous
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max_tokens is a required top-level field on POST /v1/messages and has been since the 2023-06-01 API version. There is no server-side default, unlike OpenAI Chat Completions, so a request carrying only model and messages fails validation with a 400 invalid_request_error before any generation happens.
Confirm it by posting a minimal body without the field; the error message names it directly.
The second half of this is the part that bites in production. When generation reaches the ceiling the response is still HTTP 200, but stop_reason comes back as max_tokens and the text is cut mid-sentence. Nothing raises. Check stop_reason on every response and treat max_tokens as a signal to retry with more room rather than as a completed answer. For long outputs use streaming, because the official SDKs refuse very large non-streaming max_tokens values to avoid hitting the HTTP timeout.
Source: https://docs.claude.com/en/api/messages
anthropic-apillmapi-design
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