URL: https://gofastmcp.com/servers/transforms/tool-search
Title: Tool Search - FastMCP

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Tool Search
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Quickstart
Servers
Overview
Core Components
Working with Tools
Overview
Tool Transformation
Code Mode
NEW
Tool Search
NEW
Namespace
Visibility
NEW
Resources as Tools
NEW
Prompts as Tools
NEW
Tool Fingerprinting
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Operations
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Auth
Web Frameworks
AI Assistants
AI SDKs
MCP.json
More
Settings
CLI
Upgrading
Development
What's New
FAQ
On this page
How It Works
Search Strategies
Regex Search
BM25 Search
Which to Choose
Configuration
Limiting Results
Pinning Tools
Custom Tool Names
The call_tool Proxy
Auth and Visibility
Working with Tools
Tool Search
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Replace large tool catalogs with on-demand search
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New in version
3.1.0
When a server exposes hundreds or thousands of tools, sending the full catalog to an LLM wastes tokens and degrades tool selection accuracy. Search transforms solve this by replacing the tool listing with a search interface — the LLM discovers tools on demand instead of receiving everything upfront.
​
How It Works
When you add a search transform,
list_tools()
returns just two synthetic tools instead of the full catalog:
search_tools
finds tools matching a query and returns their full definitions
call_tool
executes a discovered tool by name
The original tools are still callable. They’re hidden from the listing but remain fully functional — the search transform controls
discovery
, not
access
.
Both synthetic tools search across tool names, descriptions, parameter names, and parameter descriptions. A search for
"email"
would match a tool named
send_email
, a tool with “email” in its description, or a tool with an
email_address
parameter.
Search results are returned in the same JSON format as
list_tools
, including the full input schema, so the LLM can construct valid calls immediately without a second round-trip.
​
Search Strategies
FastMCP provides two search transforms. They share the same interface — two synthetic tools, same configuration options — but differ in how they match queries to tools.
​
Regex Search
RegexSearchTransform
matches tools against a regex pattern using case-insensitive
re.search
. It has zero overhead and no index to build, making it a good default when the LLM knows roughly what it’s looking for.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
transforms
.
search
import
RegexSearchTransform
mcp
=
FastMCP
(
"
My Server
"
,
transforms
=[
RegexSearchTransform
()])
@
mcp
.
tool
def
search_database
(
query
:
str
,
limit
:
int
=
10
)
->
list
[
dict
]:
"""
Search the database for records matching the query.
"""
...
@
mcp
.
tool
def
delete_record
(
record_id
:
str
)
->
bool
:
"""
Delete a record from the database by its ID.
"""
...
@
mcp
.
tool
def
send_email
(
to
:
str
,
subject
:
str
,
body
:
str
)
->
bool
:
"""
Send an email to the given recipient.
"""
...
The LLM’s
search_tools
call takes a
pattern
parameter — a regex string:
# Exact substring match
result
=
await
client
.
call_tool
(
"
search_tools
"
,
{
"
pattern
"
:
"
database
"
})
# Returns: search_database, delete_record
# Regex pattern
result
=
await
client
.
call_tool
(
"
search_tools
"
,
{
"
pattern
"
:
"
send.*email|notify
"
})
# Returns: send_email
Results are returned in catalog order. If the pattern is invalid regex, the search returns an empty list rather than raising an error.
​
BM25 Search
BM25SearchTransform
ranks tools by relevance using the
BM25 Okapi
algorithm. It’s better for natural language queries because it scores each tool based on term frequency and document rarity, returning results ranked by relevance rather than filtering by match/no-match.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
transforms
.
search
import
BM25SearchTransform
mcp
=
FastMCP
(
"
My Server
"
,
transforms
=[
BM25SearchTransform
()])
# ... define tools ...
The LLM’s
search_tools
call takes a
query
parameter — natural language:
result
=
await
client
.
call_tool
(
"
search_tools
"
,
{
"
query
"
:
"
tools for deleting things from the database
"
})
# Returns: delete_record ranked first, search_database second
BM25 builds an in-memory index from the searchable text of all tools. The index is created lazily on the first search and automatically rebuilt whenever the tool catalog changes — for example, when tools are added, removed, or have their descriptions updated. The staleness check is based on a hash of all searchable text, so description changes are detected even when tool names stay the same.
​
Which to Choose
Use
regex
when your LLM is good at constructing targeted patterns and you want deterministic, predictable results. Regex is also simpler to debug — you can see exactly what pattern was sent.
Use
BM25
when your LLM tends to describe what it needs in natural language, or when your tool catalog has nuanced descriptions where relevance ranking adds value. BM25 handles partial matches and synonyms better because it scores on individual terms rather than requiring a single pattern to match.
​
Configuration
Both search transforms accept the same configuration options.
​
Limiting Results
By default, search returns at most 5 tools. Adjust
max_results
based on your catalog size and how much context you want the LLM to receive per search:
mcp
.
add_transform
(
RegexSearchTransform
(
max_results
=
10
))
mcp
.
add_transform
(
BM25SearchTransform
(
max_results
=
3
))
With regex, results stop as soon as the limit is reached (first N matches in catalog order). With BM25, all tools are scored and the top N by relevance are returned.
​
Pinning Tools
Some tools should always be visible regardless of search. Use
always_visible
to pin them in the listing alongside the synthetic tools:
mcp
.
add_transform
(
RegexSearchTransform
(
always_visible
=[
"
help
"
,
"
status
"
],
))
# list_tools returns: help, status, search_tools, call_tool
Pinned tools appear directly in
list_tools
so the LLM can call them without searching. They’re excluded from search results to avoid duplication.
​
Custom Tool Names
The default names
search_tools
and
call_tool
can be changed to avoid conflicts with real tools:
mcp
.
add_transform
(
RegexSearchTransform
(
search_tool_name
=
"
find_tools
"
,
call_tool_name
=
"
run_tool
"
,
))
​
The
call_tool
Proxy
The
call_tool
proxy forwards calls to the real tool. When a client calls
call_tool(name="search_database", arguments={...})
, the proxy resolves
search_database
through the server’s normal tool pipeline — including transforms and middleware — and executes it.
The proxy rejects attempts to call the synthetic tools themselves.
call_tool(name="call_tool")
raises an error rather than recursing.
Tools discovered through search can also be called directly via
client.call_tool("search_database", {...})
without going through the proxy. The proxy exists for LLMs that only know about the tools returned by
list_tools
and need a way to invoke discovered tools through a tool they can see.
​
Auth and Visibility
Search results respect the full authorization pipeline. Tools filtered by middleware, visibility transforms, or component-level auth checks won’t appear in search results.
The search tool queries
list_tools()
through the complete pipeline at search time, so the same filtering that controls what a client sees in the listing also controls what they can discover through search.
from
fastmcp
.
server
.
transforms
import
Visibility
from
fastmcp
.
server
.
transforms
.
search
import
RegexSearchTransform
mcp
=
FastMCP
(
"
My Server
"
)
# ... define tools ...
# Disable admin tools globally
mcp
.
add_transform
(
Visibility
(
False
,
tags
={
"
admin
"
}))
# Add search — admin tools won't appear in results
mcp
.
add_transform
(
RegexSearchTransform
())
Session-level visibility changes (via
ctx.disable_components()
) are also reflected immediately in search results.
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