Server Composition

import { VersionBadge } from '/snippets/version-badge.mdx'

As your MCP applications grow, you might want to organize your tools, resources, and prompts into logical modules or reuse existing server components. FastMCP supports composition through two methods:

• import_server: For a one-time copy of components with prefixing (static composition).
• mount: For creating a live link where the main server delegates requests to the subserver (dynamic composition).

Why Compose Servers?

• Modularity: Break down large applications into smaller, focused servers (e.g., a WeatherServer, a DatabaseServer, a CalendarServer).
• Reusability: Create common utility servers (e.g., a TextProcessingServer) and mount them wherever needed.
• Teamwork: Different teams can work on separate FastMCP servers that are later combined.
• Organization: Keep related functionality grouped together logically.

Importing vs Mounting

The choice of importing or mounting depends on your use case and requirements.

Proxy Servers

FastMCP supports MCP proxying, which allows you to mirror a local or remote server in a local FastMCP instance. Proxies are fully compatible with both importing and mounting.

You can also create proxies from configuration dictionaries that follow the MCPConfig schema, which is useful for quickly connecting to one or more remote servers. See the Proxy Servers documentation for details on configuration-based proxying. Note that MCPConfig follows an emerging standard and its format may evolve over time.

Prefixing rules for tools, prompts, resources, and templates are identical across importing, mounting, and proxies. When prefixes are used, resource URIs are prefixed using path format (since 2.4.0): resource://prefix/path/to/resource.

Importing (Static Composition)

The import_server() method copies all components (tools, resources, templates, prompts) from one FastMCP instance (the subserver) into another (the main server). An optional prefix can be provided to avoid naming conflicts. If no prefix is provided, components are imported without modification. When multiple servers are imported with the same prefix (or no prefix), the most recently imported server's components take precedence.

from fastmcp import FastMCP
import asyncio

# Define subservers
weather_mcp = FastMCP(name="WeatherService")

@weather_mcp.tool
def get_forecast(city: str) -> dict:
    """Get weather forecast."""
    return {"city": city, "forecast": "Sunny"}

@weather_mcp.resource("data://cities/supported")
def list_supported_cities() -> list[str]:
    """List cities with weather support."""
    return ["London", "Paris", "Tokyo"]

# Define main server
main_mcp = FastMCP(name="MainApp")

# Import subserver
async def setup():
    await main_mcp.import_server(weather_mcp, prefix="weather")

# Result: main_mcp now contains prefixed components:
# - Tool: "weather_get_forecast"
# - Resource: "data://weather/cities/supported" 

if __name__ == "__main__":
    asyncio.run(setup())
    main_mcp.run()

How Importing Works

When you call await main_mcp.import_server(subserver, prefix={whatever}):

1. Tools: All tools from subserver are added to main_mcp with names prefixed using {prefix}_.
   • subserver.tool(name="my_tool") becomes main_mcp.tool(name="{prefix}_my_tool").
2. Resources: All resources are added with both URIs and names prefixed.
   • URI: subserver.resource(uri="data://info") becomes main_mcp.resource(uri="data://{prefix}/info").
   • Name: resource.name becomes "{prefix}_{resource.name}".
3. Resource Templates: Templates are prefixed similarly to resources.
   • URI: subserver.resource(uri="data://{id}") becomes main_mcp.resource(uri="data://{prefix}/{id}").
   • Name: template.name becomes "{prefix}_{template.name}".
4. Prompts: All prompts are added with names prefixed using {prefix}_.
   • subserver.prompt(name="my_prompt") becomes main_mcp.prompt(name="{prefix}_my_prompt").

Note that import_server performs a one-time copy of components. Changes made to the subserver after importing will not be reflected in main_mcp. The subserver's lifespan context is also not executed by the main server.

Importing Without Prefixes

You can also import servers without specifying a prefix, which copies components using their original names:

from fastmcp import FastMCP
import asyncio

# Define subservers
weather_mcp = FastMCP(name="WeatherService")

@weather_mcp.tool
def get_forecast(city: str) -> dict:
    """Get weather forecast."""
    return {"city": city, "forecast": "Sunny"}

@weather_mcp.resource("data://cities/supported")
def list_supported_cities() -> list[str]:
    """List cities with weather support."""
    return ["London", "Paris", "Tokyo"]

# Define main server
main_mcp = FastMCP(name="MainApp")

# Import subserver
async def setup():
    # Import without prefix - components keep original names
    await main_mcp.import_server(weather_mcp)

# Result: main_mcp now contains:
# - Tool: "get_forecast" (original name preserved)
# - Resource: "data://cities/supported" (original URI preserved)

if __name__ == "__main__":
    asyncio.run(setup())
    main_mcp.run()

Conflict Resolution

When importing multiple servers with the same prefix, or no prefix, components from the most recently imported server take precedence.

Mounting (Live Linking)

The mount() method creates a live link between the main_mcp server and the subserver. Instead of copying components, requests for components matching the optional prefix are delegated to the subserver at runtime. If no prefix is provided, the subserver's components are accessible without prefixing. When multiple servers are mounted with the same prefix (or no prefix), the most recently mounted server takes precedence for conflicting component names.

import asyncio
from fastmcp import FastMCP, Client

# Define subserver
dynamic_mcp = FastMCP(name="DynamicService")

@dynamic_mcp.tool
def initial_tool():
    """Initial tool demonstration."""
    return "Initial Tool Exists"

# Mount subserver (synchronous operation)
main_mcp = FastMCP(name="MainAppLive")
main_mcp.mount(dynamic_mcp, prefix="dynamic")

# Add a tool AFTER mounting - it will be accessible through main_mcp
@dynamic_mcp.tool
def added_later():
    """Tool added after mounting."""
    return "Tool Added Dynamically!"

# Testing access to mounted tools
async def test_dynamic_mount():
    tools = await main_mcp.get_tools()
    print("Available tools:", list(tools.keys()))
    # Shows: ['dynamic_initial_tool', 'dynamic_added_later']
    
    async with Client(main_mcp) as client:
        result = await client.call_tool("dynamic_added_later")
        print("Result:", result.data)
        # Shows: "Tool Added Dynamically!"

if __name__ == "__main__":
    asyncio.run(test_dynamic_mount())

How Mounting Works

When mounting is configured:

1. Live Link: The parent server establishes a connection to the mounted server.
2. Dynamic Updates: Changes to the mounted server are immediately reflected when accessed through the parent.
3. Prefixed Access: The parent server uses prefixes to route requests to the mounted server.
4. Delegation: Requests for components matching the prefix are delegated to the mounted server at runtime.

The same prefixing rules apply as with import_server for naming tools, resources, templates, and prompts. This includes prefixing both the URIs/keys and the names of resources and templates for better identification in multi-server configurations.

Performance Considerations

Due to the "live link", operations like list_tools() on the parent server will be impacted by the speed of the slowest mounted server. In particular, HTTP-based mounted servers can introduce significant latency (300-400ms vs 1-2ms for local tools), and this slowdown affects the whole server, not just interactions with the HTTP-proxied tools. If performance is important, importing tools via import_server() may be a more appropriate solution as it copies components once at startup rather than delegating requests at runtime.

Mounting Without Prefixes

You can also mount servers without specifying a prefix, which makes components accessible without prefixing. This works identically to importing without prefixes, including conflict resolution.

Direct vs. Proxy Mounting

FastMCP supports two mounting modes:

1. Direct Mounting (default): The parent server directly accesses the mounted server's objects in memory.

   • No client lifecycle events occur on the mounted server
   • The mounted server's lifespan context is not executed
   • Communication is handled through direct method calls

2. Proxy Mounting: The parent server treats the mounted server as a separate entity and communicates with it through a client interface.

   • Full client lifecycle events occur on the mounted server
   • The mounted server's lifespan is executed when a client connects
   • Communication happens via an in-memory Client transport

# Direct mounting (default when no custom lifespan)
main_mcp.mount(api_server, prefix="api")

# Proxy mounting (preserves full client lifecycle)
main_mcp.mount(api_server, prefix="api", as_proxy=True)

# Mounting without a prefix (components accessible without prefixing)
main_mcp.mount(api_server)

FastMCP automatically uses proxy mounting when the mounted server has a custom lifespan, but you can override this behavior with the as_proxy parameter.

Interaction with Proxy Servers

When using FastMCP.as_proxy() to create a proxy server, mounting that server will always use proxy mounting:

# Create a proxy for a remote server
remote_proxy = FastMCP.as_proxy(Client("http://example.com/mcp"))

# Mount the proxy (always uses proxy mounting)
main_server.mount(remote_proxy, prefix="remote")

Tag Filtering with Composition

When using include_tags or exclude_tags on a parent server, these filters apply recursively to all components, including those from mounted or imported servers. This allows you to control which components are exposed at the parent level, regardless of how your application is composed.

import asyncio
from fastmcp import FastMCP, Client

# Create a subserver with tools tagged for different environments
api_server = FastMCP(name="APIServer")

@api_server.tool(tags={"production"})
def prod_endpoint() -> str:
    """Production-ready endpoint."""
    return "Production data"

@api_server.tool(tags={"development"})
def dev_endpoint() -> str:
    """Development-only endpoint."""
    return "Debug data"

# Mount the subserver with production tag filtering at parent level
prod_app = FastMCP(name="ProductionApp", include_tags={"production"})
prod_app.mount(api_server, prefix="api")

# Test the filtering
async def test_filtering():
    async with Client(prod_app) as client:
        tools = await client.list_tools()
        print("Available tools:", [t.name for t in tools])
        # Shows: ['api_prod_endpoint']
        # The 'api_dev_endpoint' is filtered out

        # Calling the filtered tool raises an error
        try:
            await client.call_tool("api_dev_endpoint")
        except Exception as e:
            print(f"Filtered tool not accessible: {e}")

if __name__ == "__main__":
    asyncio.run(test_filtering())

How Recursive Filtering Works

Tag filters apply in the following order:

1. Child Server Filters: Each mounted/imported server first applies its own include_tags/exclude_tags to its components.
2. Parent Server Filters: The parent server then applies its own include_tags/exclude_tags to all components, including those from child servers.

This ensures that parent server tag policies act as a global policy for everything the parent server exposes, no matter how your application is composed.