tool-search-tool changedagents-and-tools/tool-use/tool-search-tool
Nearest release: v2.1.319, published an hour after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+22added
Lines−22removed
From line
67
where the diff opens
First seen
14 Aug 2026
this site's first read of the page
Recorded edits23to this page, all time
The whole hunk
from line 67, old and new numbered
/
from line 67
6767
6868Both tool search variants are available on the following models:
6969
70| Model | Tool versions |
71| ---------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------- |
72| Claude Fable 5.1 (claude-fable-5-1) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
73| Claude Mythos 5.1 (claude-mythos-5-1) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
74| Claude Fable 5 (claude-fable-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
75| Claude Mythos 5 (claude-mythos-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
76| Claude Opus 5.5 (claude-opus-5-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
77| Claude Opus 5 (claude-opus-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
78| Claude Sonnet 5.5 (claude-sonnet-5-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
79| Claude Sonnet 5 (claude-sonnet-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
80| Claude Haiku 5.5 (claude-haiku-5-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
81| Claude Opus 4.8 (claude-opus-4-8) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
82| Claude Opus 4.7 (claude-opus-4-7) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
83| Claude Opus 4.6 (claude-opus-4-6) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
84| Claude Sonnet 4.6 (claude-sonnet-4-6) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
85| Claude Opus 4.5 (claude-opus-4-5-20251101) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
86| Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) ([deprecated](https://platform.claude.com/docs/en/about-claude/model-deprecations)) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
87| Claude Haiku 4.5 (claude-haiku-4-5-20251001) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
70| Model | Tool versions |
71| ---------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------- |
72| Claude Fable 5.1 (claude-fable-5-1) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
73| Claude Mythos 5.1 (claude-mythos-5-1) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
74| Claude Fable 5 (claude-fable-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
75| Claude Mythos 5 (claude-mythos-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
76| Claude Opus 5.5 (claude-opus-5-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
77| Claude Opus 5 (claude-opus-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
78| Claude Sonnet 5.5 (claude-sonnet-5-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
79| Claude Sonnet 5 (claude-sonnet-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
80| Claude Haiku 5.5 (claude-haiku-5-5) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
81| Claude Opus 4.8 (claude-opus-4-8) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
82| Claude Opus 4.7 (claude-opus-4-7) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
83| Claude Opus 4.6 (claude-opus-4-6) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
84| Claude Sonnet 4.6 (claude-sonnet-4-6) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
85| Claude Opus 4.5 (claude-opus-4-5-20251101) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
86| Claude Sonnet 4.5 (claude-sonnet-4-5-20250929) (deprecated; see [Model deprecations](https://platform.claude.com/docs/en/about-claude/model-deprecations)) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
87| Claude Haiku 4.5 (claude-haiku-4-5-20251001) | `tool_search_tool_regex_20251119`, `tool_search_tool_bm25_20251119` |
8888
8989Claude Opus 4.1 and earlier models don’t support the tool search tool.
9090
from line 626
626626
627627Both tool search variants (`regex` and `bm25`) search tool names, descriptions, argument names, and argument descriptions.
628628
629Internally, the API excludes deferred tools from the system-prompt prefix. When Claude discovers a deferred tool through tool search, the API appends a `tool_reference` block inline in the conversation, then expands it into the full tool definition before passing it to Claude. The prefix is untouched, so prompt caching is preserved. The grammar for [strict mode](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) (the rules that constrain tool-call output to match your schemas) builds from the full toolset, so `defer_loading` and strict mode compose without grammar recompilation.
629Internally, the API excludes deferred tools from the system-prompt prefix. When Claude discovers a deferred tool through tool search, the API appends a `tool_reference` block inline in the conversation, then expands it into the full tool definition before passing it to Claude. The grammar for [strict mode](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) (the rules that constrain tool-call output to match your schemas) builds from the full list of tools, so using `defer_loading` with strict mode doesn’t make the API recompile the grammar.
630630
631631## Response format
632632
from line 819
819819
820820## Streaming
821821
822With streaming enabled, you’ll receive tool search events as part of the stream:
822With streaming enabled, you receive tool search events as part of the stream:
823823
824824```sse
825825event: content_block_start
from line 840
840840
841841## Batch requests
842842
843You can include the tool search tool in the [Messages Batches API](https://platform.claude.com/docs/en/build-with-claude/batch-processing).
843You can include the tool search tool in the [Message Batches API](https://platform.claude.com/docs/en/build-with-claude/batch-processing).
844844
845845## Limits and best practices
846846
from line 857
857857
858858* You have 10 or more tools available.
859859* Your tool definitions consume more than 10k tokens.
860* Tool selection accuracy drops as your toolset grows.
860* Tool selection accuracy drops as you add more tools.
861861* You aggregate multiple MCP servers (200+ tools).
862862* Your tool library grows over time.
863863
No line in this hunk matches that.