URL: https://gofastmcp.com/servers/transforms/transforms
Title: Transforms Overview - FastMCP

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Mental Model
Built-in Transforms
Server vs Provider Transforms
Provider-Level Transforms
Server-Level Transforms
Transform Order
Custom Transforms
Working with Tools
Transforms Overview
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Modify components as they flow through your server
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New in version
3.0.0
Transforms modify components as they flow from providers to clients. When a client asks “what tools do you have?”, the request passes through each transform in the chain. Each transform can modify the components before passing them along.
​
Mental Model
Think of transforms as filters in a pipeline. Components flow from providers through transforms to reach clients:
Provider → [Transform A] → [Transform B] → Client
When listing components, transforms receive sequences and return transformed sequences—a pure function pattern. When getting a specific component by name, transforms use a middleware pattern with
call_next
, working in reverse: mapping the client’s requested name back to the original, then transforming the result.
​
Built-in Transforms
FastMCP provides several transforms for common use cases:
Namespace
- Prefix component names to prevent conflicts when composing servers
Tool Transformation
- Rename tools, modify descriptions, reshape arguments
Enabled
- Control which components are visible at runtime
Tool Search
- Replace large tool catalogs with on-demand search
Resources as Tools
- Expose resources to tool-only clients
Prompts as Tools
- Expose prompts to tool-only clients
Code Mode (Experimental)
- Replace many tools with programmable
search
+
execute
​
Server vs Provider Transforms
Transforms can be added at two levels, each serving different purposes.
​
Provider-Level Transforms
Provider transforms apply to components from a specific provider. They run first, modifying components before they reach the server level.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
providers
import
FastMCPProvider
from
fastmcp
.
server
.
transforms
import
Namespace
,
ToolTransform
from
fastmcp
.
tools
.
tool_transform
import
ToolTransformConfig
sub_server
=
FastMCP
(
"
Sub
"
)
@
sub_server
.
tool
def
process
(
data
:
str
)
->
str
:
return
f
"Processed:
{
data
}
"
# Create provider and add transforms
provider
=
FastMCPProvider
(
sub_server
)
provider
.
add_transform
(
Namespace
(
"
api
"
))
provider
.
add_transform
(
ToolTransform
({
"
api_process
"
:
ToolTransformConfig
(
description
=
"
Process data through the API
"
),
}))
main
=
FastMCP
(
"
Main
"
,
providers
=[
provider
])
# Tool is now: api_process with updated description
When using
mount()
, the returned provider reference lets you add transforms directly.
main
=
FastMCP
(
"
Main
"
)
mount
=
main
.
mount
(
sub_server
,
namespace
=
"
api
"
)
mount
.
add_transform
(
ToolTransform
({
...
}))
​
Server-Level Transforms
Server transforms apply to all components from all providers. They run after provider transforms, seeing the already-transformed names.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
transforms
import
Namespace
mcp
=
FastMCP
(
"
Server
"
,
transforms
=[
Namespace
(
"
v1
"
)])
@
mcp
.
tool
def
greet
(
name
:
str
)
->
str
:
return
f
"Hello,
{
name
}
!"
# All tools become v1_toolname
Server-level transforms are useful for API versioning or applying consistent naming across your entire server.
​
Transform Order
Transforms stack in the order they’re added. The first transform added is innermost (closest to the provider), and subsequent transforms wrap it.
from
fastmcp
.
server
.
providers
import
FastMCPProvider
from
fastmcp
.
server
.
transforms
import
Namespace
,
ToolTransform
from
fastmcp
.
tools
.
tool_transform
import
ToolTransformConfig
provider
=
FastMCPProvider
(
server
)
provider
.
add_transform
(
Namespace
(
"
api
"
))
# Applied first
provider
.
add_transform
(
ToolTransform
({
# Sees namespaced names
"
api_verbose_name
"
:
ToolTransformConfig
(
name
=
"
short
"
),
}))
# Flow: "verbose_name" -> "api_verbose_name" -> "short"
When a client requests “short”, the transforms reverse the mapping: ToolTransform maps “short” to “api_verbose_name”, then Namespace strips the prefix to find “verbose_name” in the provider.
​
Custom Transforms
Create custom transforms by subclassing
Transform
and overriding the methods you need.
from
collections
.
abc
import
Sequence
from
fastmcp
.
server
.
transforms
import
Transform
,
GetToolNext
from
fastmcp
.
tools
.
tool
import
Tool
class
TagFilter
(
Transform
):
"""
Filter tools to only those with specific tags.
"""
def
__init__
(
self
,
required_tags
:
set
[
str
]):
self
.
required_tags
=
required_tags
async
def
list_tools
(
self
,
tools
:
Sequence
[
Tool
])
->
Sequence
[
Tool
]:
return
[
t
for
t
in
tools
if
t
.
tags
&
self
.
required_tags
]
async
def
get_tool
(
self
,
name
:
str
,
call_next
:
GetToolNext
)
->
Tool
|
None
:
tool
=
await
call_next
(
name
)
if
tool
and
tool
.
tags
&
self
.
required_tags
:
return
tool
return
None
The
Transform
base class provides default implementations that pass through unchanged. Override only the methods relevant to your transform.
Each component type has two methods with different patterns:
Method
Pattern
Purpose
list_tools(tools)
Pure function
Transform the sequence of tools
get_tool(name, call_next)
Middleware
Transform lookup by name
list_resources(resources)
Pure function
Transform the sequence of resources
get_resource(uri, call_next)
Middleware
Transform lookup by URI
list_resource_templates(templates)
Pure function
Transform the sequence of templates
get_resource_template(uri, call_next)
Middleware
Transform template lookup by URI
list_prompts(prompts)
Pure function
Transform the sequence of prompts
get_prompt(name, call_next)
Middleware
Transform lookup by name
List methods receive sequences directly and return transformed sequences. Get methods use
call_next
for routing flexibility—when a client requests “new_name”, your transform maps it back to “original_name” before calling
call_next()
.
class
PrefixTransform
(
Transform
):
def
__init__
(
self
,
prefix
:
str
):
self
.
prefix
=
prefix
async
def
list_tools
(
self
,
tools
:
Sequence
[
Tool
])
->
Sequence
[
Tool
]:
return
[
t
.
model_copy
(
update
={
"
name
"
:
f
"
{self
.
prefix
}
_
{
t
.
name
}
"
})
for
t
in
tools
]
async
def
get_tool
(
self
,
name
:
str
,
call_next
:
GetToolNext
)
->
Tool
|
None
:
# Reverse the prefix to find the original
if
not
name
.
startswith
(
f
"
{self
.
prefix
}
_"
):
return
None
original
=
name
[
len
(
self
.
prefix
)
+
1
:]
tool
=
await
call_next
(
original
)
if
tool
:
return
tool
.
model_copy
(
update
={
"
name
"
:
name
})
return
None
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