fix invalid response to MCP initialized notification

apps/language_server/lib/language_server/mcp/request_handler.ex

@@ -160,6 +160,11 @@ defmodule ElixirLS.LanguageServer.MCP.RequestHandler do
           "id" => id
         }
 
+      %{"method" => _} ->
+        # JSON-RPC notification (no "id" field). Per spec, the server must
+        # not reply to notifications, even for unknown methods.
+        nil
+
       _ ->
         %{
           "jsonrpc" => "2.0",

apps/language_server/test/mcp/request_handler_test.exs

@@ -374,6 +374,27 @@ defmodule ElixirLS.LanguageServer.MCP.RequestHandlerTest do
 
       assert response == nil
     end
+
+    test "initialized notification does not get response" do
+      # MCP clients (e.g. Codex) send this notification after initialize.
+      # Per JSON-RPC 2.0 spec, notifications (no "id") must not receive a reply.
+      request = %{
+        "jsonrpc" => "2.0",
+        "method" => "initialized"
+      }
+
+      assert RequestHandler.handle_request(request) == nil
+    end
+
+    test "unknown notification (no id) does not get response" do
+      request = %{
+        "jsonrpc" => "2.0",
+        "method" => "notifications/something_unknown",
+        "params" => %{}
+      }
+
+      assert RequestHandler.handle_request(request) == nil
+    end
   end
 
   describe "integration with actual modules" do