停止理由の解説
end_turn、tool_use、max_tokens、stop_sequenceについて学びます
「停止理由の解説」はCoddyKit上の無料Claude Architectレッスンです。 これはレッスン3/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはClaude Architect学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Claude Architectコースには全4レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
Why Stop Reasons Matter
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.
Where to Find It
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.
from anthropic import Anthropic
client = Anthropic()
response = client.messages.create(
model="claude-opus-4-8",
max_tokens=1024,
messages=[{"role": "user", "content": "Hello"}],
)
print(response.stop_reason) # "end_turn"end_turn — Complete
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.
if response.stop_reason == "end_turn":
# Claude is done. Return the answer.
print(response.content[0].text)tool_use — Run a Tool
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.
if response.stop_reason == "tool_use":
tool_call = next(b for b in response.content if b.type == "tool_use")
result = run_tool(tool_call.name, tool_call.input)
# Append the result and loop again (next scenes show how)max_tokens — Truncated
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.
if response.stop_reason == "max_tokens":
# Output was truncated. Retry with a higher max_tokens,
# or use client.messages.stream(...) for long outputs.
print("Response was cut off — increase max_tokens.")stop_sequence — Custom Stop
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".
response = client.messages.create(
model="claude-opus-4-8",
max_tokens=1024,
stop_sequences=["###"],
messages=[{"role": "user", "content": "List three colors, then ###"}],
)
if response.stop_reason == "stop_sequence":
print("Stopped at a custom sequence.")The Four at a Glance
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.
The Agentic Loop
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.
messages = [{"role": "user", "content": user_input}]
while True:
response = client.messages.create(
model="claude-opus-4-8",
max_tokens=4096,
tools=tools,
messages=messages,
)
if response.stop_reason == "end_turn":
break
if response.stop_reason == "tool_use":
messages.append({"role": "assistant", "content": response.content})
results = run_tools(response.content)
messages.append({"role": "user", "content": results})Terminate on the Signal, Not the Text
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.
Iteration Caps Are a Safety Net
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.
MAX_ITERS = 25 # safety net only
for _ in range(MAX_ITERS):
response = client.messages.create(...)
if response.stop_reason == "end_turn":
break # the real, primary stop
# ... handle tool_use ...
else:
log.warning("Hit iteration cap — investigate.")Putting It Together
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.
def handle(response):
sr = response.stop_reason
if sr == "end_turn":
return finish(response)
if sr == "tool_use":
return continue_with_tools(response)
if sr == "max_tokens":
return retry_with_more_tokens(response)
if sr == "stop_sequence":
return handle_boundary(response)Quick Check
An agent loop returns a response with stop_reason == "tool_use". What is the correct next action?
Recap
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.
よくある質問
「停止理由の解説」レッスンは無料ですか？
はい。「停止理由の解説」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Claude Architectコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Claude Architectコースには全4レッスンが含まれています。
「停止理由の解説」で何を学びますか？
end_turn、tool_use、max_tokens、stop_sequenceについて学びます ブラウザで直接実行するハンズオンコードでClaude Architectを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
Claude Architectを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのClaude Architectは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/4です。
「停止理由の解説」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このClaude Architectレッスンでコードを書いて実行できますか？
はい。すべてのClaude Architectレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。