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Every time you call the Claude API, the response comes back with a stop_reason field. It tells you why the model stopped generating.

This one field drives your whole control flow. A reliable agent inspects stop_reason after each turn and decides what to do next based on it.

There are four values you must know: end_turn, tool_use, max_tokens, and stop_sequence. Let's learn each one.

The stop_reason lives on the response object returned by messages.create.

Read it directly. Do not scan the text output for words like "done" or "finished" to decide what happened. The model controls stop_reason; text is just content.

end_turn means Claude finished its response naturally. It said everything it wanted to say.

This is the normal completion signal. In an agent loop, end_turn is your cue to stop looping and return the answer to the user.

tool_use means Claude wants to call one of the tools you gave it. The response is not finished — Claude is waiting for a tool result.

Your job: run the requested tool, append the result to the message history, and call the API again so Claude can continue.

max_tokens means the response was cut off because it hit the max_tokens limit you set in the request.

The output is incomplete — it stopped mid-thought, not because Claude was done. The fix is to raise max_tokens, or stream the response for very long outputs.

Never treat max_tokens as a successful completion.

stop_sequence means Claude hit a custom stop string that you defined in your request via stop_sequences.

Generation halts the moment that string is produced. This is useful when you want the model to stop at a known boundary, such as "###" or "END".

Here is the full set for this lesson:

• end_turn — Claude finished naturally. Stop the loop.
• tool_use — Claude wants a tool. Run it, append the result, call again.
• max_tokens — Output truncated. Raise the limit or stream.
• stop_sequence — Hit a custom stop string you defined.

Two of these (end_turn, stop_sequence) mean the turn is complete. One (tool_use) means continue. One (max_tokens) means something went wrong with your limit.

An agent is just a loop driven by stop_reason:

Send a request, inspect stop_reason. If it is tool_use, run the tools, append the results to the history, and repeat. Keep going until stop_reason is end_turn.

The model keeps no state between calls, so you must send the full message history every turn.

The single most important rule: terminate the loop on stop_reason, never by parsing text for words like "done", "finished", or "complete".

Text-matching is fragile — the model might say "I'm done thinking, let me check one more file" and your code would stop too early. The stop_reason is the model's structured, reliable signal.

You may add a maximum iteration count to your loop so a runaway agent cannot loop forever. That is good practice.

But an iteration cap is a safety net, never the primary way you stop. The primary stop is always end_turn. Decisions about when work is done are model-driven; the cap only catches the rare case where something goes wrong.

A robust handler branches on every stop reason explicitly:

• end_turn → return the result.
• tool_use → execute tools, append results, continue.
• max_tokens → the output is truncated; raise the limit and retry rather than using a partial answer.
• stop_sequence → handle the known boundary you defined.

Handling all branches is what separates a reliable agent from one that silently breaks on edge cases.

An agent loop returns a response with stop_reason == "tool_use". What is the correct next action?

Key takeaways:

• end_turn — complete; stop the loop and return.
• tool_use — run the tool, append the result, call again.
• max_tokens — truncated; raise the limit or stream, don't use the partial output.
• stop_sequence — hit a custom stop string you defined.

Always drive your control flow from stop_reason, never from parsing text. Terminate on end_turn; keep iteration caps as a safety net only. Master this and the agentic loop becomes simple and reliable.