bug: SendNotificationToClient not work

[beckwy]

Description

I used the SendNotificationClient method in the example to send a notification to the client. The SendNotification method did not report an error, but the client OnNotification method did not receive the notification.

Code Sample

this is server code

server := server.ServerFromContext(ctx)
	logger.ZapLogger.Info("SendNotificationToClient start")
	err := server.SendNotificationToClient(
		ctx,
		"notifications/progress",
		map[string]any{
			"progress":      10,
			"total":         10,
			"progressToken": 0,
		},
	)
	logger.ZapLogger.Info("SendNotificationToClient end", zap.Any("err", err))
	if err != nil {
		logger.ZapLogger.Info("handleDescribeInstancesTool failed to send notification", zap.Any("err", err))
		return nil, err
	}

this is server log print

{"level":"info","ts":"2025-05-26T15:56:23.100+0800","caller":"tools/describeInstances.go:37","msg":"SendNotificationToClient start"}
{"level":"info","ts":"2025-05-26T15:56:23.100+0800","caller":"tools/describeInstances.go:46","msg":"SendNotificationToClient end","err":null}

this is client code

c = client.NewClient(sseTransport)
	// Set up notification handler
	c.OnNotification(func(notification mcp.JSONRPCNotification) {
		log.Debugf("connectMcpServerWithSSE OnNotification Received notification: %s\n, AITraceId: %v", notification.Method, yReq.AITraceId)
	})
	// Initialize the client
	initRequest := mcp.InitializeRequest{}
	initRequest.Params.ProtocolVersion = mcp.LATEST_PROTOCOL_VERSION
	initRequest.Params.ClientInfo = mcp.Implementation{
		Name:    "MCP-Go Simple Client Example",
		Version: "1.0.0",
	}
	initRequest.Params.Capabilities = mcp.ClientCapabilities{}

client not received notification!

[dugenkui03]

I try the server code you present( call them in ToolHandlerFunc), and it send notification successfully.

there might be two reasons:

1. the place you execute server code could not get client from ctx, could you present more detail about that?
2. perhaps the client program had already closed before successfully receiving the notification

The way I send notification

	mcpServer.AddTool(mcp.NewTool(
		"test-tool",
		mcp.WithDescription("Test tool"),
	), func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
		mcpServer := server.ServerFromContext(ctx)
		_ = mcpServer.SendNotificationToClient(
			ctx,
			"notifications/progress",
			map[string]any{
				"progress":      10,
				"total":         10,
				"progressToken": 0,
			},
		)

		return &mcp.CallToolResult{
			Content: []mcp.Content{
				mcp.TextContent{
					Type: "text",
					Text: "Input parameter: " + request.GetArguments()["parameter-1"].(string),
				},
			},
		}, nil
	})

[beckwy]

I printed the return value of the server.ServerFromContext method

i try to update sdk verison to v0.30.0，use httpstreamable, but it is not work also。

i see this in log

[dugenkui03]

From your code provided in your last comment, I know that you are sending notification in tool function, I think the code is correct.

There is a example example code, you can use to check the difference between your client code and this example. And you can provide more details about your client code.

perhaps the client program had already shut down before successfully receiving the notification

[beckwy]

this is complete client code，in order to verify，i use CallTool add。

	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
	// defer cancel()
	var c *client.Client
	var err error
	log.Debugf("connectMcpServerWithSSE Initializing HTTP client...")
	// Create HTTP transport
	httpTransport, err := transport.NewStreamableHTTP("http://XXX:8080/mcp")
	if err != nil {
		log.Fatalf("connectMcpServerWithSSE Failed to create HTTP transport: %v", err)
	}

	// Create client with the transport
	c = client.NewClient(httpTransport)
if err := c.Start(ctx); err != nil {
		log.Fatalf("connectMcpServerWithSSE Failed to start client: %v, AITraceId: %v", err, yReq.AITraceId)
	}
	// Set up notification handler
	c.OnNotification(func(notification mcp.JSONRPCNotification) {
		log.Debugf("connectMcpServerWithSSE OnNotification Received")
		log.Debugf("connectMcpServerWithSSE OnNotification Received notification: %s\n, AITraceId: %v", notification.Method, yReq.AITraceId)
	})
	// Initialize the client
	initRequest := mcp.InitializeRequest{}
	initRequest.Params.ProtocolVersion = mcp.LATEST_PROTOCOL_VERSION
	initRequest.Params.ClientInfo = mcp.Implementation{
		Name:    "MCP-Go Simple Client Example",
		Version: "1.0.0",
	}
	initRequest.Params.Capabilities = mcp.ClientCapabilities{}

	serverInfo, err := c.Initialize(ctx, initRequest)
	if err != nil {
		log.Fatalf("connectMcpServerWithSSE Failed to initialize: %v,  AITraceId: %v", err, yReq.AITraceId)
	}
	log.Debugf("connectMcpServerWithSSE Connected to server: %s (version %s)\n,  AITraceId: %v",
		serverInfo.ServerInfo.Name,
		serverInfo.ServerInfo.Version,
		yReq.AITraceId)
	log.Debugf("connectMcpServerWithSSE Server capabilities: %+v\n,  AITraceId: %v", serverInfo.Capabilities, yReq.AITraceId)
	log.Debugf("connectMcpServerWithSSE Client initialized successfully. AITraceId: %v", yReq.AITraceId)

	var callToolReq mcp.CallToolRequest
	args := make(map[string]interface{})
	args["AITraceId"] = yReq.AITraceId
	callToolReq.Params.Name = "add"
	callToolReq.Params.Arguments = args
	toolResult, err1 := c.CallTool(ctx, callToolReq)
	log.Debugf("connectMcpServerWithSSE  toolResult1111: %+v,  AITraceId: %+v, err1: %v", toolResult, yReq.AITraceId, err1)

this server tools function

func handleAddTool(
	ctx context.Context,
	request mcp.CallToolRequest,
) (*mcp.CallToolResult, error) {
	server := server.ServerFromContext(ctx)
	logger.ZapLogger.Info("handleAddTool start", zap.Any("server", server))
	err := server.SendNotificationToClient(
		ctx,
		"notifications/progress",
		map[string]any{
			"progress":      10,
			"total":         10,
			"progressToken": 0,
		},
	)
	if err != nil {
		logger.ZapLogger.Info("failed to send notification", zap.Error(err))
		return nil, fmt.Errorf("failed to send notification: %w", err)
	}
	logger.ZapLogger.Info("handleAddTool end", zap.Any("server", server))
	return &mcp.CallToolResult{
		Content: []mcp.Content{
			mcp.TextContent{
				Type: "text",
				Text: "a + b",
			},
		},
	}, nil
}

this is log prove client toolResult is accpte value

2025-05-28 14:17:49.427 DEBUG utils/httpclient.go:216 connectMcpServerWithSSE Initializing HTTP client...
2025-05-28 14:17:49.429 DEBUG utils/httpclient.go:262 connectMcpServerWithSSE Connected to server: bigdata_ai_mcp (version 1.0.0)
2025-05-28 14:17:49.429 DEBUG utils/httpclient.go:266 connectMcpServerWithSSE Server capabilities: {Experimental:map[] Logging:0x20c0e20 Prompts:0xc00023632a Resources:0xc00023632c Tools:0xc00023632e}
2025-05-28 14:17:49.429 DEBUG utils/httpclient.go:267 connectMcpServerWithSSE Client initialized successfully. AITraceId: 6a99baba-e52e-40f5-9c9c-d11eb31ab1ab
2025-05-28 14:17:49.430 DEBUG utils/httpclient.go:276 connectMcpServerWithSSE toolResult1111: &{Result:{Meta:map[]} Content:[{Annotated:{Annotations:} Type:text Text:a + b}] IsError:false}, AITraceId: 6a99baba-e52e-40f5-9c9c-d11eb31ab1ab, err1:

[dugenkui03]

Thank you for providing so many details. I've submitted a PR for this issue.

• The problem http: wrote more than the declared Content-Length is indeed a bug and will be fixed.
• Client can not receive all the notifications: in the httpstreamable server, notification sending occurs in another goroutine, meaning that response may return before the notification is sent. I demonstrate a hook way to ensure client could receive all the notification in the pr.

[beckwy]

Thank you for confirming the problem, looking forward to the follow-up solution

[beckwy]

@dugenkui03 @ezynda3 oh, Today I tested this bug fix with the latest main code. Disappointingly, the client still did not receive the notification. I looked at the code design in detail and found that the code was that the server did not send the notification successfully.

The code is likely to be returned in these two places. There are two main problems now: 1.notification cannot be sent 2.the order of return value and notification cannot be guaranteed.

[dugenkui03]

@beckwy #348 is intended to fix this problem http: wrote more than the declared Content-Length/panic.

Also, as described in #348, the current design do not ensure that client can receive all the notifications from the server.

This PR do not ensure that client could receive all the notification from server, and I think this is as expected, because notification could not block the response. mcp-go maybe should provide sync way to send notification( need more discussion), I provide a hook way to ensure that client could receive the all the notification(as demonstrated in the test code).

[dugenkui03]

there is a hook way to ensure that client could receive all the notifications