Server Composition

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).

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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.

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Importing vs Mounting

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

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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.

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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). A prefix is added to avoid naming conflicts.

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", weather_mcp)

# 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()

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How Importing Works

When you call await main_mcp.import_server(prefix, subserver):

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 URIs prefixed in the format protocol://{prefix}/path.
   • subserver.resource(uri="data://info") becomes main_mcp.resource(uri="data://{prefix}/info").
3. Resource Templates: Templates are prefixed similarly to resources.
   • subserver.resource(uri="data://{id}") becomes main_mcp.resource(uri="data://{prefix}/{id}").
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.

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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 prefix are delegated to the subserver at runtime.

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", dynamic_mcp)

# 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[0].text)
        # Shows: "Tool Added Dynamically!"

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

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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.

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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", api_server)

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

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

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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", remote_proxy)

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Resource Prefix Formats

When mounting or importing servers, resource URIs are usually prefixed to avoid naming conflicts. FastMCP supports two different formats for resource prefixes:

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Path Format (Default)

In path format, prefixes are added to the path component of the URI:

resource://prefix/path/to/resource

This is the default format since FastMCP 2.4. This format is recommended because it avoids issues with URI protocol restrictions (like underscores not being allowed in protocol names).

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Protocol Format (Legacy)

In protocol format, prefixes are added as part of the protocol:

prefix+resource://path/to/resource

This was the default format in FastMCP before 2.4. While still supported, it’s not recommended for new code as it can cause problems with prefix names that aren’t valid in URI protocols.

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Configuring the Prefix Format

You can configure the prefix format globally in code:

import fastmcp
fastmcp.settings.settings.resource_prefix_format = "protocol"

Or via environment variable:

FASTMCP_RESOURCE_PREFIX_FORMAT=protocol

Or per-server:

from fastmcp import FastMCP

# Create a server that uses legacy protocol format
server = FastMCP("LegacyServer", resource_prefix_format="protocol")

# Create a server that uses new path format
server = FastMCP("NewServer", resource_prefix_format="path")

When mounting or importing servers, the prefix format of the parent server is used.