Mounting Servers

Compose servers by mounting one inside another

New in version 2.2.0

Mounting lets you combine multiple FastMCP servers into one. When you mount a server, all its components become available through the parent. Under the hood, FastMCP uses FastMCPProvider (v3.0.0+) to source components from the mounted server.

Why Mount Servers

Large applications benefit from modular organization. Rather than defining all components in one massive file, create focused servers for specific domains and combine them:

• Modularity: Break down applications into smaller, focused servers
• Reusability: Create utility servers and mount them wherever needed
• Teamwork: Different teams can work on separate servers
• Organization: Keep related functionality grouped together

Basic Mounting

Use mount() to add another server’s components to your server:

from fastmcp import FastMCP

# Create focused subservers
weather_server = FastMCP("Weather")

@weather_server.tool
def get_forecast(city: str) -> str:
    """Get weather forecast for a city."""
    return f"Sunny in {city}"

@weather_server.resource("data://cities")
def list_cities() -> list[str]:
    """List supported cities."""
    return ["London", "Paris", "Tokyo"]

# Create main server and mount the subserver
main = FastMCP("MainApp")
main.mount(weather_server)

# Now main has access to 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.

Mounting vs Importing

FastMCP offers two ways to combine servers:

Feature	mount()	import_server()
Link Type	Live (dynamic)	One-time copy (static)
Updates	Changes reflected immediately	Changes not reflected
Performance	Runtime delegation	Faster - no delegation
Use Case	Modular runtime composition	Bundling finalized components

Live Mounting

With mount(), changes to the subserver are immediately reflected:

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!"

# This works because mount() creates a live link

Static Importing

With import_server(), components are copied once at import time:

main = FastMCP("Main")

async def setup():
    await main.import_server(static_server, namespace="static")

# Changes to static_server after this point are NOT reflected in main

Direct vs Proxy Mounting

New in version 2.2.7

FastMCP supports two mounting modes:

Direct Mounting (Default)

The parent server directly accesses the mounted server’s objects in memory:

main.mount(subserver, namespace="api")

• No client lifecycle events on mounted server
• Mounted server’s lifespan is not executed
• Communication via direct method calls

Proxy Mounting

The as_proxy parameter is deprecated. Mounted servers now always have their lifespan and middleware invoked. To create a proxy server explicitly, use create_proxy() from fastmcp.server.

Previously, the parent server could treat the mounted server as a separate entity with its own lifecycle. This behavior is now the default for all mounted servers:

• Full client lifecycle events on mounted server
• Mounted server’s lifespan is executed
• Communication via in-memory Client transport

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

When using live mounting, operations like list_tools() on the parent server 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:

• Using import_server() for static composition
• Implementing caching strategies
• 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)