pause_turn and server-side tools

Server-side tools run their own sampling loop. When it hits its limit you get a pause, not an error.

3 min read · Lesson 2 of 12 in this domain

Server-side tools such as web search run their own loop inside a single API call — Claude searches, reads, searches again, all before you get a response. That inner loop has an iteration cap so a runaway search cannot spin forever on Anthropic's side. When it hits the cap the response comes back mid-thought, and the signal for that is pause_turn. It is not an error and nothing has gone wrong; the work is simply incomplete and the server is offering to carry on.

• Server-side tools (web search, web fetch, code execution) run on Anthropic infrastructure — you declare them and results arrive as content blocks in the same response.
• That server-side loop has a default iteration limit. Reaching it returns stop_reason: "pause_turn".
• To resume: re-send the user message plus the paused assistant response. The server detects the trailing server-tool block and continues.
• Do not append a "Continue." user message — it is unnecessary and pollutes the conversation.
• Set a max_continuations ceiling (e.g. 5) so a pathological turn cannot loop forever.
• Note the SDK tool runners do not auto-resume a pause — a paused turn ends the runner and is returned as the final message, silently truncated.

if response.stop_reason == "pause_turn":
    messages = [
        {"role": "user", "content": user_query},
        {"role": "assistant", "content": response.content},
    ]
    response = client.messages.create(
        model="claude-opus-5", messages=messages, tools=tools
    )

Source: Claude Docs — Handling stop reasons · Independent study aid, not affiliated with or endorsed by Anthropic.