Tool runner exits early when response contains only server tool use blocks (e.g. web_search, web_fetch) and stop reason: pause_turn

[carlymr]

Description

When using client.beta.messages.tool_runner() with server-side tools like web_search or web_fetch alongside client-side @beta_tool functions, the runner exits prematurely when the API responds with a server_tool_use block and stop_reason: "pause_turn".

Root cause

In _beta_runner.py, the __run__ loop calls generate_tool_call_response() after each iteration. That method only looks for blocks with type == "tool_use":

tool_use_blocks = [block for block in content if block.type == "tool_use"]
if not tool_use_blocks:
    return None

When a response contains only server_tool_use blocks (which have type == "server_tool_use"), this returns None, causing the loop to exit:

response = self.generate_tool_call_response()
if response is None:
    log.debug("Tool call was not requested, exiting from tool runner loop.")
    return

The runner should instead recognize that pause_turn means the server is still processing and continue the loop without requiring a client-side tool result.

Reproduction

import anthropic

client = anthropic.Anthropic()

@anthropic.beta_tool
def save_recipe(title: str, ingredients: list[str], steps: list[str], servings: float):
    with open(title + ".txt", "w") as f:
        f.write(title + "\n")

runner = client.beta.messages.tool_runner(
    model="claude-sonnet-4-5-20250929",
    max_tokens=1024,
    betas=["web-fetch-2025-09-10"],
    tools=[
        save_recipe,
        {"type": "web_search_20250305", "name": "web_search", "max_uses": 5},
        {"type": "web_fetch_20250910", "name": "web_fetch", "max_uses": 5},
    ],
    messages=[{"role": "user", "content": "Search for a brownie recipe and save it"}],
)

final = runner.until_done()
# final.content[0] is a BetaServerToolUseBlock, not text —
# the runner stopped before the server tool was resolved
print(final.content[0].type)  # "server_tool_use"

Expected behavior

The tool runner should continue looping when it receives a pause_turn stop reason with server tool use blocks, sending the response back as a continuation until the server tools are resolved and Claude produces a final text response.

Actual behavior

The runner exits the loop and until_done() returns a message whose content is a BetaServerToolUseBlock, causing an AttributeError if you access .content[0].text.

SDK version

0.79.0 (latest as of 2025-02-07)