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New in version 2.9.1 Use this when you need to react to server-side changes like tool list updates or resource modifications. MCP servers can send notifications to inform clients about state changes. The message handler provides a unified way to process these notifications.

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Handling Notifications

The simplest approach is a function that receives all messages and filters for the notifications you care about: 
from fastmcp import Client

async def message_handler(message):
    """Handle MCP notifications from the server."""
    if hasattr(message, 'root'):
        method = message.root.method

        if method == "notifications/tools/list_changed":
            print("Tools have changed - refresh tool cache")
        elif method == "notifications/resources/list_changed":
            print("Resources have changed")
        elif method == "notifications/prompts/list_changed":
            print("Prompts have changed")

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

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MessageHandler Class

For fine-grained targeting, subclass MessageHandler to use 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."""
        print("Tool list changed - refreshing available tools")

    async def on_resource_list_changed(
        self, notification: mcp.types.ResourceListChangedNotification
    ) -> None:
        """Handle resource list changes."""
        print("Resource list changed")

    async def on_prompt_list_changed(
        self, notification: mcp.types.PromptListChangedNotification
    ) -> None:
        """Handle prompt list changes."""
        print("Prompt list changed")

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

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Handler Template

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

class MyMessageHandler(MessageHandler):
    async def on_message(self, message) -> None:
        """Called for ALL messages (requests and notifications)."""
        pass

    async def on_notification(
        self, notification: mcp.types.ServerNotification
    ) -> None:
        """Called for notifications (fire-and-forget)."""
        pass

    async def on_tool_list_changed(
        self, notification: mcp.types.ToolListChangedNotification
    ) -> None:
        """Called when the server's tool list changes."""
        pass

    async def on_resource_list_changed(
        self, notification: mcp.types.ResourceListChangedNotification
    ) -> None:
        """Called when the server's resource list changes."""
        pass

    async def on_prompt_list_changed(
        self, notification: mcp.types.PromptListChangedNotification
    ) -> None:
        """Called when the server's prompt list changes."""
        pass

    async def on_progress(
        self, notification: mcp.types.ProgressNotification
    ) -> None:
        """Called for progress updates during long-running operations."""
        pass

    async def on_logging_message(
        self, notification: mcp.types.LoggingMessageNotification
    ) -> None:
        """Called for log messages from the server."""
        pass

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List Change Notifications

A practical example of maintaining a tool cache that refreshes when tools change: 
from fastmcp import Client
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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Server Requests

While the message handler receives server-initiated requests, you should use dedicated callback parameters for most interactive scenarios: The message handler is primarily for monitoring and handling notifications rather than responding to requests.