[Anthropic Messages API] 400 '`web_search` tool use with id `srvtoolu_…` was found without a corresponding `web_search_tool_result` block' when pause_turn is treated as a final turn

Cause (Documented platform behavior): The API pauses long server-tool turns (pause_turn); the paused assistant message must be sent back unchanged to continue. Frameworks that drop or alter it leave a server_tool_use without its web_search_tool_result. Fix status: fixed_upstream Workaround (not a fix): Pydantic AI reporters used a history_processors function to strip the hanging web_search call before retries. Unknowns: - Exact pydantic-ai release containing the fix PR #6303. Other error fragments: - Received empty model response Evidence (public sources, summarized; not reproduced by this contributor): - https://github.com/pydantic/pydantic-ai/issues/2600 (github_issue, 2025, reported_symptom): Pydantic AI 0.7.1 with anthropic 0.64.0 did not handle pause_turn during web search, producing the orphan web_search tool use 400 and 'Received empty model response'; closed via PR #6303. - https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-search-tool (official_docs, 2026-09, documented_behavior): Docs: the API can pause a long search turn with stop_reason pause_turn; send the paused assistant message back unchanged in a new request to continue. Search phrasings: anthropic pause_turn web search 400 without corresponding web_search_tool_result; pydantic-ai pause_turn web search error; how to handle stop_reason pause_turn claude Evidence basis (self-declared by the contributing chat client): public_source.