fix: handle notifications/initialized and ignore unknown notifications

Codex (via rmcp 0.15.0) sends "notifications/initialized" rather than
"initialized" after the handshake. The server was returning a -32601
error for this unknown method, which caused rmcp to close the transport
before tools/list could complete.

Also silently ignore any other unknown notifications (messages without
an id) per the MCP spec, instead of returning an error response.

Author: tmsdnl

internal/mcp/server.go

@@ -179,7 +179,7 @@ func (s *server) handleMessage(data []byte) *jsonrpcResponse {
 	switch req.Method {
 	case "initialize":
 		return s.handleInitialize(req)
-	case "initialized":
+	case "initialized", "notifications/initialized":
 		return nil // notification, no response
 	case "ping":
 		return resultResponse(req.ID, map[string]any{})
@@ -188,6 +188,10 @@ func (s *server) handleMessage(data []byte) *jsonrpcResponse {
 	case "tools/call":
 		return s.handleToolsCall(req)
 	default:
+		// Notifications have no id; silently ignore unknown ones per spec.
+		if req.ID == nil {
+			return nil
+		}
 		return errorResponse(req.ID, -32601, "method not found: "+req.Method)
 	}
 }

internal/mcp/server_test.go

@@ -124,9 +124,21 @@ func TestHandleInitialize_VersionNegotiation(t *testing.T) {
 
 func TestHandleInitialized_NoResponse(t *testing.T) {
 	s := testServer(t)
-	resp := s.handleMessage([]byte(`{"jsonrpc":"2.0","method":"initialized"}`))
+	for _, method := range []string{"initialized", "notifications/initialized"} {
+		msg := `{"jsonrpc":"2.0","method":"` + method + `"}`
+		resp := s.handleMessage([]byte(msg))
+		if resp != nil {
+			t.Errorf("notification %q should return nil, got %v", method, resp)
+		}
+	}
+}
+
+func TestUnknownNotification_NoResponse(t *testing.T) {
+	s := testServer(t)
+	// Unknown notifications (no id) must be silently ignored per MCP spec.
+	resp := s.handleMessage([]byte(`{"jsonrpc":"2.0","method":"notifications/unknown"}`))
 	if resp != nil {
-		t.Error("initialized notification should return nil")
+		t.Errorf("unknown notification should return nil, got %v", resp)
 	}
 }