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HTTP 400 Bad Request

The server cannot process the request due to a client error — malformed JSON, invalid parameters, or schema violations.

4xx · Client error✗ not retryable — fix the request instead

In AI APIs specifically

The most common AI-API 400s: max_tokens exceeding the model limit, invalid model IDs, malformed tool schemas, and context-window overflow (Anthropic returns 400 with an invalid_request_error when the prompt is too long).

Fix checklist

  • Read the error body — providers return a specific message and param pointer.
  • Validate JSON and tool schemas before sending.
  • Check token counts against the model's context window.
  • Pin SDK versions — request shapes change across API versions.

Spec: RFC reference

Related status codes

401 Unauthorized

Authentication is missing or invalid for this request..

402 Payment Required

The request requires payment — billing is not set up or a balance is exhausted..

403 Forbidden

The server understood the request but refuses to authorize it — valid credentials, insufficient permission..

404 Not Found

The requested resource does not exist at this URL..

← All HTTP status codes