Python: Keep pending Anthropic server tool calls as server_tool_use

[Shivam Sharma (ShivamSharma43)]

Motivation & Context

When Claude calls a local tool and a server tool (for example get_web_search_tool()) in the same turn, the response ends with a local tool_use plus a pending server_tool_use. The parser keeps the hosted call as an informational function_call with hosted=True, but _prepare_message_for_anthropic sends every function_call back as tool_use. Anthropic then expects a local tool_result for the server call and rejects the next request with HTTP 400 (tool_use ids were found without tool_result blocks immediately after: srvtoolu_...). Per Anthropic's server-tool docs, the client returns the local result and the API runs the pending server tool on the next request, which needs the block to stay server_tool_use.

Description & Review Guide

• What are the major changes? _prepare_messages_for_anthropic collects the hosted calls that have no function_result anywhere in the conversation and passes them down to _prepare_message_for_anthropic, which sends those calls as server_tool_use instead of tool_use. The hosted call is found from the call's own hosted flag, which survives to_dict/from_dict, so a reloaded session behaves the same.
• What is the impact of these changes? The parallel local plus server tool turn from the issue now produces assistant [tool_use(local), server_tool_use(srv)] followed by user [tool_result(local)]. Server tool calls that already have a result in the conversation keep today's tool_use/tool_result pairing. I left those alone on purpose: the parsed web_search_tool_result content is stored stringified on the Content, so a native result block (with encrypted_content) can't be rebuilt from history. Switching only the call type would leave the result without a matching tool_use.
• What do you want reviewers to focus on? Whether keeping completed server tool calls on the existing pairing is the right scope, or whether you'd rather round-trip them natively in a follow-up (which would need the structured result kept on the content).

Tests: test_prepare_messages_preserves_parallel_server_tool_use (in memory and after a to_dict/from_dict reload, from real BetaToolUseBlock/BetaServerToolUseBlock parsing) fails on main and passes with the change. test_prepare_messages_keeps_completed_server_tool_call_paired_with_its_result pins the unchanged pairing once the search result arrives. The anthropic package unit tests pass (204), ruff check/format are clean, and mypy/pyright report no new errors in agent_framework_anthropic compared with main. I did not run it against the live Anthropic API.

Related Issue

Fixes #9478

The issue was reproduced by the repository's triage workflow (reproduced label, assigned to Eduard van Valkenburg (@eavanvalkenburg)). It has no maintainer comment on the direction yet, and no open PR references it. Happy to adjust or close if you'd rather handle it differently.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: implementation, tests and this PR description were drafted with Claude Code and reviewed before submitting.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

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