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fastmcp.server.transforms.search.base

Base class for search transforms. Search transforms replace list_tools() output with a small set of synthetic tools — a search tool and a call-tool proxy — so LLMs can discover tools on demand instead of receiving the full catalog. All concrete search transforms (RegexSearchTransform, BM25SearchTransform, etc.) inherit from BaseSearchTransform and implement _make_search_tool() and _search() to provide their specific search strategy. Example:: from fastmcp import FastMCP from fastmcp.server.transforms.search import RegexSearchTransform mcp = FastMCP(“Server”) @mcp.tool def add(a: int, b: int) -> int: … @mcp.tool def multiply(x: float, y: float) -> float: …

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Clients now see only search_tools and call_tool.

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The original tools are discoverable via search.

mcp.add_transform(RegexSearchTransform())

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Functions

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serialize_tools_for_output_json 

serialize_tools_for_output_json(tools: Sequence[Tool]) -> list[dict[str, Any]]
Serialize tools to the same dict format as list_tools output.

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serialize_tools_for_output_markdown 

serialize_tools_for_output_markdown(tools: Sequence[Tool]) -> str
Serialize tools to compact markdown, using ~65-70% fewer tokens than JSON.

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Classes

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BaseSearchTransform

Replace the tool listing with a search interface. When this transform is active, list_tools() returns only:
  • Any tools listed in always_visible (pinned).
  • A search tool that finds tools matching a query.
  • A call_tool proxy that executes tools discovered via search.
Hidden tools remain callable — get_tool() delegates unknown names downstream, so direct calls and the call-tool proxy both work. Search results respect the full auth pipeline: middleware, visibility transforms, and component-level auth checks all apply. Args:
  • max_results: Maximum number of tools returned per search.
  • always_visible: Tool names that stay in the list_tools output alongside the synthetic search/call tools.
  • search_tool_name: Name of the generated search tool.
  • call_tool_name: Name of the generated call-tool proxy.
Methods:

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transform_tools 

transform_tools(self, tools: Sequence[Tool]) -> Sequence[Tool]
Replace the catalog with pinned + synthetic search/call tools.

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get_tool 

get_tool(self, name: str, call_next: GetToolNext) -> Tool | None
Intercept synthetic tool names; delegate everything else.