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When you call the Messages endpoint, each response includes a stop_reason that explains why the model stopped. Handling this field correctly is key to building reliable apps. For details about stop_reason in the response schema, see Messages.

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What is stop_reason?

stop_reason is present on every successful response. It doesn’t mean there was an error—it tells you how the model completed its response. Example response (truncated): 
{
  "id": "msg_abf981cf3f1c449eacb7a85c064ca505",
  "type": "message",
  "role": "assistant",
  "content": [
    { "type": "text", "text": "Here’s a concise update..." }
  ],
  "stop_reason": "end_turn",
  "stop_sequence": null
}

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Stop reason values

When a request fails, the stop reason matches one of the error values below and the response is returned as an error envelope.
stop_reasonWhy it happensWhat to do
end_turnThe model completed its response normally. Sometimes you’ll see an empty response after tool execution if you send extra user text with a tool_result.Treat as complete. If empty, send only the tool_result block, or add a new user message like “Please continue.”
max_tokensThe response hit your max_tokens limit.Increase max_tokens or ask the model to continue.
stop_sequenceThe model generated one of your custom stop sequences.Treat output as complete at the boundary or adjust/remove the sequence.
tool_useThe model wants you to run a tool.Execute the tool and return a tool_result block.
pause_turnThe model intentionally paused.Resume the conversation when ready.
refusalThe model refused for safety or policy reasons.Surface the refusal or re‑prompt with allowed content.
invalid_request_errorThe request inputs are invalid (e.g., artifacts not fully ingested, missing tool config, invalid IDs).Fix inputs and retry.
permission_errorThe token lacks access to the referenced resource.Check org/project membership and artifact visibility.
request_too_largeInput size or token count exceeds model limits.Reduce input, chunk content, or lower context size.
api_errorAn internal server error occurred.Retry with exponential backoff.
overloaded_errorThe system is temporarily overloaded.Retry with backoff and jitter.