fix: accept HTTP 204 No Content in SendNotification

client/transport/streamable_http.go

@@ -573,26 +573,24 @@ func (c *StreamableHTTP) SendNotification(ctx context.Context, notification mcp.
 	}
 	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusAccepted {
-		// Handle unauthorized error
-		if resp.StatusCode == http.StatusUnauthorized {
-			if c.oauthHandler != nil {
-				return &OAuthAuthorizationRequiredError{
-					Handler: c.oauthHandler,
-				}
+	switch resp.StatusCode {
+	case http.StatusOK, http.StatusAccepted, http.StatusNoContent:
+		return nil
+	case http.StatusUnauthorized:
+		if c.oauthHandler != nil {
+			return &OAuthAuthorizationRequiredError{
+				Handler: c.oauthHandler,
 			}
-			return ErrUnauthorized
 		}
-
+		return ErrUnauthorized
+	default:
 		body, _ := io.ReadAll(resp.Body)
 		return fmt.Errorf(
 			"notification failed with status %d: %s",
 			resp.StatusCode,
 			body,
 		)
 	}
-
-	return nil
 }
 
 func (c *StreamableHTTP) SetNotificationHandler(handler func(mcp.JSONRPCNotification)) {

client/transport/streamable_http_test.go

@@ -988,6 +988,36 @@ func TestStreamableHTTP_SendNotification_Unauthorized_StaticToken(t *testing.T)
 	}
 }
 
+// TestStreamableHTTP_SendNotification_Accepts204NoContent verifies that SendNotification
+// treats HTTP 204 No Content as a success response per RFC 7231.
+// See: https://github.com/mark3labs/mcp-go/issues/700
+func TestStreamableHTTP_SendNotification_Accepts204NoContent(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	}))
+	defer server.Close()
+
+	transport, err := NewStreamableHTTP(server.URL)
+	if err != nil {
+		t.Fatalf("Failed to create StreamableHTTP: %v", err)
+	}
+
+	if err := transport.Start(context.Background()); err != nil {
+		t.Fatalf("Failed to start transport: %v", err)
+	}
+
+	err = transport.SendNotification(context.Background(), mcp.JSONRPCNotification{
+		JSONRPC: "2.0",
+		Notification: mcp.Notification{
+			Method: "notifications/initialized",
+		},
+	})
+
+	if err != nil {
+		t.Fatalf("SendNotification should accept 204 No Content, got error: %v", err)
+	}
+}
+
 // TestStreamableHTTPHostOverride tests Host header override for StreamableHTTP transport
 func TestStreamableHTTPHostOverride(t *testing.T) {
 	// Create a test server that captures the Host header