MCP Test MCP

# Message Handling > Handle MCP messages, requests, and notifications with custom message handlers. export const VersionBadge = ({version}) => { return `<code className="version-badge-container">` `<p className="version-badge">` `<span className="version-badge-label">`New in version:  `<code className="version-badge-version">`{version}`</code>` `</p>` `</code>`; }; <VersionBadge 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. ## 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`](/clients/sampling) * **Progress requests**: Use [`progress_handler`](/clients/progress) * **Log requests**: Use [`log_handler`](/clients/logging) The message handler is primarily for monitoring and handling notifications rather than responding to requests.