MessagesBuilding with Claude

Stop reasons and fallback

Learn what each stop_reason value means and how to handle truncation, tool use, paused turns, and refusals in your application.

Every Messages API response includes a stop_reason field that tells you why Claude stopped generating. Check this field to decide whether to use the response as-is, continue the conversation, retry, or fall back to another model.

For the full response schema, see the Messages API reference.

Quick reference

Value	When it occurs	What to do
`end_turn`	Claude finished its response naturally.	Use the response.
`max_tokens`	The response reached your `max_tokens` limit.	Raise `max_tokens` or continue the response.
`stop_sequence`	Claude emitted one of your `stop_sequences`.	Read `stop_sequence` to see which one fired.
`tool_use`	Claude is calling a tool.	Run the tool and return the result. A server tool call still missing its result block completes in a later response.
`pause_turn`	A server-tool loop reached its iteration limit.	Send the assistant content back to continue.
`refusal`	Claude declined to respond.	Read `stop_details` and retry on a fallback model.
`model_context_window_exceeded`	The response filled the model's context window.	Treat the response as truncated.

The stop_reason field

The stop_reason field is part of every successful Messages API response. Unlike errors, which indicate failures in processing your request, stop_reason tells you why Claude completed its response generation.

Example response

{
  "id": "msg_01234",
  "type": "message",
  "role": "assistant",
  "content"

Quick reference

The stop_reason field

Quick reference

The stop_reason field