New in version: 2.9.1

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.

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Function-Based Handler

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

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,
)

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Message Handler Class

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

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(),
)

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Available Handler Methods

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

Message Handler Methods

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on_message(message)
Any MCP message

Called for ALL messages (requests and notifications)

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on_request(request)
mcp.types.ClientRequest

Called for requests that expect responses

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on_notification(notification)
mcp.types.ServerNotification

Called for notifications (fire-and-forget)

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on_tool_list_changed(notification)
mcp.types.ToolListChangedNotification

Called when the server’s tool list changes

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on_resource_list_changed(notification)
mcp.types.ResourceListChangedNotification

Called when the server’s resource list changes

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on_prompt_list_changed(notification)
mcp.types.PromptListChangedNotification

Called when the server’s prompt list changes

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on_progress(notification)
mcp.types.ProgressNotification

Called for progress updates during long-running operations

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on_logging_message(notification)
mcp.types.LoggingMessageNotification

Called for log messages from the server

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Example: Handling Tool Changes

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

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())

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Handling Requests

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

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