diff --git a/server/session.go b/server/session.go
index 0ded99fb..48fd52d7 100644
--- a/server/session.go
+++ b/server/session.go
@@ -171,6 +171,9 @@ func (s *MCPServer) SendLogMessageToClient(ctx context.Context, notification mcp
 func (s *MCPServer) sendNotificationToAllClients(notification mcp.JSONRPCNotification) {
 	s.sessions.Range(func(k, v any) bool {
 		if session, ok := v.(ClientSession); ok && session.Initialized() {
+			if sessionWithStreamableHTTPConfig, ok := session.(SessionWithStreamableHTTPConfig); ok {
+				sessionWithStreamableHTTPConfig.UpgradeToSSEWhenReceiveNotification()
+			}
 			select {
 			case session.NotificationChannel() <- notification:
 				// Successfully sent notification
diff --git a/server/streamable_http_test.go b/server/streamable_http_test.go
index 9e444c53..32dccc3a 100644
--- a/server/streamable_http_test.go
+++ b/server/streamable_http_test.go
@@ -2251,3 +2251,91 @@ func TestStreamableHTTP_SendNotificationToSpecificClient(t *testing.T) {
 		}
 	})
 }
+
+// TestStreamableHTTP_AddToolDuringToolCall tests that adding a tool while a tool call
+// is in progress doesn't break the client's response.
+// This is a regression test for issue #638 where notifications sent via
+// sendNotificationToAllClients during an in-progress request would cause
+// the response to fail with "unexpected nil response".
+func TestStreamableHTTP_AddToolDuringToolCall(t *testing.T) {
+	mcpServer := NewMCPServer("test-mcp-server", "1.0",
+		WithToolCapabilities(true), // Enable tool list change notifications
+	)
+	// Add a tool that takes some time to complete
+	mcpServer.AddTool(mcp.NewTool("slow_tool",
+		mcp.WithDescription("A tool that takes time to complete"),
+	), func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+		// Simulate work that takes some time
+		time.Sleep(100 * time.Millisecond)
+		return mcp.NewToolResultText("done"), nil
+	})
+	server := NewTestStreamableHTTPServer(mcpServer, WithStateful(true))
+	defer server.Close()
+	// Initialize to get session
+	resp, err := postJSON(server.URL, initRequest)
+	if err != nil {
+		t.Fatalf("Failed to initialize: %v", err)
+	}
+	sessionID := resp.Header.Get(HeaderKeySessionID)
+	resp.Body.Close()
+	if sessionID == "" {
+		t.Fatal("Expected session ID in response header")
+	}
+	// Start the tool call in a goroutine
+	resultChan := make(chan struct {
+		statusCode int
+		body       string
+		err        error
+	})
+	go func() {
+		toolRequest := map[string]any{
+			"jsonrpc": "2.0",
+			"id":      1,
+			"method":  "tools/call",
+			"params": map[string]any{
+				"name": "slow_tool",
+			},
+		}
+		toolBody, _ := json.Marshal(toolRequest)
+		req, _ := http.NewRequest("POST", server.URL, bytes.NewReader(toolBody))
+		req.Header.Set("Content-Type", "application/json")
+		req.Header.Set(HeaderKeySessionID, sessionID)
+		resp, err := server.Client().Do(req)
+		if err != nil {
+			resultChan <- struct {
+				statusCode int
+				body       string
+				err        error
+			}{0, "", err}
+			return
+		}
+		defer resp.Body.Close()
+		body, _ := io.ReadAll(resp.Body)
+		resultChan <- struct {
+			statusCode int
+			body       string
+			err        error
+		}{resp.StatusCode, string(body), nil}
+	}()
+	// Wait a bit then add a new tool while the slow_tool is executing
+	// This triggers sendNotificationToAllClients
+	time.Sleep(50 * time.Millisecond)
+	mcpServer.AddTool(mcp.NewTool("new_tool",
+		mcp.WithDescription("A new tool added during execution"),
+	), func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+		return mcp.NewToolResultText("new tool result"), nil
+	})
+	// Wait for the tool call to complete
+	result := <-resultChan
+	if result.err != nil {
+		t.Fatalf("Tool call failed with error: %v", result.err)
+	}
+	if result.statusCode != http.StatusOK {
+		t.Errorf("Expected status 200, got %d. Body: %s", result.statusCode, result.body)
+	}
+	// The response should contain the tool result
+	// It may be SSE format (text/event-stream) due to the notification upgrade
+	if !strings.Contains(result.body, "done") {
+		t.Errorf("Expected response to contain 'done', got: %s", result.body)
+	}
+}