URL: https://gofastmcp.com/servers/composition
Title: Composing Servers - FastMCP

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Mounting External Servers
Mounting npm/uvx Packages
Namespacing
How Namespacing Works
Dynamic Composition
Tag Filtering
Performance Considerations
Custom Routes
Conflict Resolution
MCP Providers
Composing Servers
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Combine multiple servers into one
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New in version
2.2.0
As your application grows, you’ll want to split it into focused servers — one for weather, one for calendar, one for admin — and combine them into a single server that clients connect to. That’s what
mount()
does.
When you mount a server, all its tools, resources, and prompts become available through the parent. The connection is live: add a tool to the child after mounting, and it’s immediately visible through the parent.
from
fastmcp
import
FastMCP
weather
=
FastMCP
(
"
Weather
"
)
@
weather
.
tool
def
get_forecast
(
city
:
str
)
->
str
:
"""
Get weather forecast for a city.
"""
return
f
"Sunny in
{
city
}
"
@
weather
.
resource
(
"
data://cities
"
)
def
list_cities
()
->
list
[
str
]:
"""
List supported cities.
"""
return
[
"
London
"
,
"
Paris
"
,
"
Tokyo
"
]
main
=
FastMCP
(
"
MainApp
"
)
main
.
mount
(
weather
)
# main now serves get_forecast and data://cities
​
Mounting External Servers
Mount remote HTTP servers or subprocess-based MCP servers using
create_proxy()
:
from
fastmcp
import
FastMCP
from
fastmcp
.
server
import
create_proxy
mcp
=
FastMCP
(
"
Orchestrator
"
)
# Mount a remote HTTP server (URLs work directly)
mcp
.
mount
(
create_proxy
(
"
http://api.example.com/mcp
"
),
namespace
=
"
api
"
)
# Mount local Python scripts (file paths work directly)
mcp
.
mount
(
create_proxy
(
"
./my_server.py
"
),
namespace
=
"
local
"
)
​
Mounting npm/uvx Packages
For npm packages or Python tools, use the config dict format:
from
fastmcp
import
FastMCP
from
fastmcp
.
server
import
create_proxy
mcp
=
FastMCP
(
"
Orchestrator
"
)
# Mount npm package via config
github_config
=
{
"
mcpServers
"
:
{
"
default
"
:
{
"
command
"
:
"
npx
"
,
"
args
"
:
[
"
-y
"
,
"
@modelcontextprotocol/server-github
"
]
}
}
}
mcp
.
mount
(
create_proxy
(
github_config
),
namespace
=
"
github
"
)
# Mount Python tool via config
sqlite_config
=
{
"
mcpServers
"
:
{
"
default
"
:
{
"
command
"
:
"
uvx
"
,
"
args
"
:
[
"
mcp-server-sqlite
"
,
"
--db
"
,
"
data.db
"
]
}
}
}
mcp
.
mount
(
create_proxy
(
sqlite_config
),
namespace
=
"
db
"
)
Or use explicit transport classes:
from
fastmcp
import
FastMCP
from
fastmcp
.
server
import
create_proxy
from
fastmcp
.
client
.
transports
import
NpxStdioTransport
,
UvxStdioTransport
mcp
=
FastMCP
(
"
Orchestrator
"
)
mcp
.
mount
(
create_proxy
(
NpxStdioTransport
(
package
=
"
@modelcontextprotocol/server-github
"
)),
namespace
=
"
github
"
)
mcp
.
mount
(
create_proxy
(
UvxStdioTransport
(
tool_name
=
"
mcp-server-sqlite
"
,
tool_args
=[
"
--db
"
,
"
data.db
"
])),
namespace
=
"
db
"
)
For advanced configuration, see
Proxying
.
​
Namespacing
New in version
3.0.0
When mounting multiple servers, use namespaces to avoid naming conflicts:
weather
=
FastMCP
(
"
Weather
"
)
calendar
=
FastMCP
(
"
Calendar
"
)
@
weather
.
tool
def
get_data
()
->
str
:
return
"
Weather data
"
@
calendar
.
tool
def
get_data
()
->
str
:
return
"
Calendar data
"
main
=
FastMCP
(
"
Main
"
)
main
.
mount
(
weather
,
namespace
=
"
weather
"
)
main
.
mount
(
calendar
,
namespace
=
"
calendar
"
)
# Tools are now:
# - weather_get_data
# - calendar_get_data
​
How Namespacing Works
Component Type
Without Namespace
With
namespace="api"
Tool
my_tool
api_my_tool
Prompt
my_prompt
api_my_prompt
Resource
data://info
data://api/info
Template
data://{id}
data://api/{id}
Namespacing uses
transforms
under the hood.
​
Dynamic Composition
Because
mount()
creates a live link, you can add components to a child server after mounting and they’ll be immediately available through the parent:
main
=
FastMCP
(
"
Main
"
)
main
.
mount
(
dynamic_server
,
namespace
=
"
dynamic
"
)
# Add a tool AFTER mounting - it's accessible through main
@
dynamic_server
.
tool
def
added_later
()
->
str
:
return
"
Added after mounting!
"
​
Tag Filtering
New in version
3.0.0
Parent server tag filters apply recursively to mounted servers:
api_server
=
FastMCP
(
"
API
"
)
@
api_server
.
tool
(
tags
={
"
production
"
})
def
prod_endpoint
()
->
str
:
return
"
Production data
"
@
api_server
.
tool
(
tags
={
"
development
"
})
def
dev_endpoint
()
->
str
:
return
"
Debug data
"
# Mount with production filter
prod_app
=
FastMCP
(
"
Production
"
)
prod_app
.
mount
(
api_server
,
namespace
=
"
api
"
)
prod_app
.
enable
(
tags
={
"
production
"
},
only
=
True
)
# Only prod_endpoint (namespaced as api_prod_endpoint) is visible
​
Performance Considerations
Operations like
list_tools()
on the parent are affected by the performance of all mounted servers. This is particularly noticeable with:
HTTP-based mounted servers (300-400ms vs 1-2ms for local tools)
Mounted servers with slow initialization
Deep mounting hierarchies
If low latency is critical, consider implementing caching strategies or limiting mounting depth.
​
Custom Routes
New in version
2.4.0
Custom HTTP routes defined with
@server.custom_route()
are also forwarded when mounting:
subserver
=
FastMCP
(
"
Sub
"
)
@
subserver
.
custom_route
(
"
/health
"
,
methods
=[
"
GET
"
])
async
def
health_check
():
return
{
"
status
"
:
"
ok
"
}
main
=
FastMCP
(
"
Main
"
)
main
.
mount
(
subserver
,
namespace
=
"
sub
"
)
# /health is now accessible through main's HTTP app
​
Conflict Resolution
New in version
3.0.0
When mounting multiple servers with the same namespace (or no namespace), the
most recently mounted
server takes precedence for conflicting component names:
server_a
=
FastMCP
(
"
A
"
)
server_b
=
FastMCP
(
"
B
"
)
@
server_a
.
tool
def
shared_tool
()
->
str
:
return
"
From A
"
@
server_b
.
tool
def
shared_tool
()
->
str
:
return
"
From B
"
main
=
FastMCP
(
"
Main
"
)
main
.
mount
(
server_a
)
main
.
mount
(
server_b
)
# shared_tool returns "From B" (most recently mounted)
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