Message Handling - Fastmcp

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

MCP clients can receive various types of messages from servers, including requests that need responses and notifications that don't. The message handler provides a unified way to process all these messages.

Function-Based Handler

The simplest way to handle messages is with a function that receives all messages:

python

from fastmcp import Client

async def message_handler(message):
    """Handle all MCP messages from the server."""
    if hasattr(message, 'root'):
        method = message.root.method
        print(f"Received: {method}")
        
        # Handle specific notifications
        if method == "notifications/tools/list_changed":
            print("Tools have changed - might want to refresh tool cache")
        elif method == "notifications/resources/list_changed":
            print("Resources have changed")

client = Client(
    "my_mcp_server.py",
    message_handler=message_handler,
)

Message Handler Class

For fine-grained targeting, FastMCP provides a MessageHandler class you can subclass to take advantage of specific hooks:

python

from fastmcp import Client
from fastmcp.client.messages import MessageHandler
import mcp.types

class MyMessageHandler(MessageHandler):
    async def on_tool_list_changed(
        self, notification: mcp.types.ToolListChangedNotification
    ) -> None:
        """Handle tool list changes specifically."""
        print("Tool list changed - refreshing available tools")

client = Client(
    "my_mcp_server.py",
    message_handler=MyMessageHandler(),
)

Available Handler Methods

All handler methods receive a single argument - the specific message type:

<Card icon="code" title="Message Handler Methods"> <ResponseField name="on_message(message)" type="Any MCP message"> Called for ALL messages (requests and notifications) </ResponseField> <ResponseField name="on_request(request)" type="mcp.types.ClientRequest"> Called for requests that expect responses </ResponseField> <ResponseField name="on_notification(notification)" type="mcp.types.ServerNotification"> Called for notifications (fire-and-forget) </ResponseField> <ResponseField name="on_tool_list_changed(notification)" type="mcp.types.ToolListChangedNotification"> Called when the server's tool list changes </ResponseField> <ResponseField name="on_resource_list_changed(notification)" type="mcp.types.ResourceListChangedNotification"> Called when the server's resource list changes </ResponseField> <ResponseField name="on_prompt_list_changed(notification)" type="mcp.types.PromptListChangedNotification"> Called when the server's prompt list changes </ResponseField> <ResponseField name="on_progress(notification)" type="mcp.types.ProgressNotification"> Called for progress updates during long-running operations </ResponseField> <ResponseField name="on_logging_message(notification)" type="mcp.types.LoggingMessageNotification"> Called for log messages from the server </ResponseField> </Card>

Example: Handling Tool Changes

Here's a practical example of handling tool list changes:

python

from fastmcp.client.messages import MessageHandler
import mcp.types

class ToolCacheHandler(MessageHandler):
    def __init__(self):
        self.cached_tools = []
    
    async def on_tool_list_changed(
        self, notification: mcp.types.ToolListChangedNotification
    ) -> None:
        """Clear tool cache when tools change."""
        print("Tools changed - clearing cache")
        self.cached_tools = []  # Force refresh on next access

client = Client("server.py", message_handler=ToolCacheHandler())

Handling Requests

While the message handler receives server-initiated requests, for most use cases you should use the dedicated callback parameters instead:

Sampling requests: Use sampling_handler
Progress requests: Use progress_handler
Log requests: Use log_handler

The message handler is primarily for monitoring and handling notifications rather than responding to requests.