---
title: base
sidebarTitle: base
---
# `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: ...
# Clients now see only ``search_tools`` and ``call_tool``.
# The original tools are discoverable via search.
mcp.add_transform(RegexSearchTransform())
## Functions
### `serialize_tools_for_output_json`
```python
serialize_tools_for_output_json(tools: Sequence[Tool]) -> list[dict[str, Any]]
```
Serialize tools to the same dict format as ``list_tools`` output.
### `serialize_tools_for_output_markdown`
```python
serialize_tools_for_output_markdown(tools: Sequence[Tool]) -> str
```
Serialize tools to compact markdown, using ~65-70% fewer tokens than JSON.
## Classes
### `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:**
#### `transform_tools`
```python
transform_tools(self, tools: Sequence[Tool]) -> Sequence[Tool]
```
Replace the catalog with pinned + synthetic search/call tools.
#### `get_tool`
```python
get_tool(self, name: str, call_next: GetToolNext) -> Tool | None
```
Intercept synthetic tool names; delegate everything else.