feat: Add ToolSearchTool and defer_loading for dynamic tool discovery #3620
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Summary
Adds support for dynamic tool discovery without loading all tool definitions upfront, reducing token usage for agents with many tools.
ToolSearchToolbuiltin tool for on-demand tool discoverysearch_typeof 'regex' or 'bm25' (defaults toNone= provider's default)defer_loadingfield toToolDefinitionandToolclassToolSearchToolwhen any tool hasdefer_loading=Truedefer_loadingto APIadvanced-tool-use-2025-11-20beta automatically when features are usedSupported by Anthropic models.
See https://docs.anthropic.com/en/docs/agents-and-tools/tool-use/tool-search-tool
Test plan
tests/models/anthropic/test_tool_search.pyRelated
Split from #3550 per @DouweM's recommendation to separate the features into individual PRs.
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