fix(jsonrpc2): respond to requests with empty method names

Decode empty-method JSON-RPC messages as requests instead of
responses so stdio servers return -32601 instead of dropping them.

Fixes #976

Author: Piyush Bag

internal/jsonrpc2/messages.go

@@ -183,8 +183,10 @@ func DecodeMessage(data []byte) (Message, error) {
 	if err != nil {
 		return nil, err
 	}
-	if msg.Method != "" {
-		// has a method, must be a call
+	if msg.Method != "" || (msg.Result == nil && msg.Error == nil && (msg.Params != nil || id.IsValid())) {
+		// Requests carry a method (including the empty string, which is invalid
+		// but must still be dispatched so the peer receives a JSON-RPC error).
+		// Responses carry result or error without a method.
 		return &Request{
 			Method: msg.Method,
 			ID:     id,

internal/jsonrpc2/wire_test.go

@@ -102,6 +102,39 @@ func newResponse(id any, result any, rerr error) jsonrpc2.Message {
 	return msg
 }
 
+// go-sdk#976: empty method must decode as a request (encode omits empty method).
+func TestDecodeEmptyMethodRequest(t *testing.T) {
+	encoded := []byte(`{"jsonrpc":"2.0","id":5,"method":"","params":{}}`)
+	msg, err := jsonrpc2.DecodeMessage(encoded)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req, ok := msg.(*jsonrpc2.Request)
+	if !ok {
+		t.Fatalf("message type = %T, want *jsonrpc2.Request", msg)
+	}
+	if req.Method != "" {
+		t.Errorf("Method = %q, want empty string", req.Method)
+	}
+	if req.ID != jsonrpc2.Int64ID(5) {
+		t.Errorf("ID = %v, want 5", req.ID.Raw())
+	}
+	if !req.IsCall() {
+		t.Error("empty method with id=5 should be a call")
+	}
+}
+
+func TestDecodeResponseUnchanged(t *testing.T) {
+	encoded := []byte(`{"jsonrpc":"2.0","id":2,"result":{}}`)
+	msg, err := jsonrpc2.DecodeMessage(encoded)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := msg.(*jsonrpc2.Response); !ok {
+		t.Fatalf("message type = %T, want *jsonrpc2.Response", msg)
+	}
+}
+
 func checkJSON(t *testing.T, got, want []byte) {
 	// compare the compact form, to allow for formatting differences
 	g := &bytes.Buffer{}

mcp/transport_test.go

@@ -124,3 +124,26 @@ func TestIOConnRead(t *testing.T) {
 		})
 	}
 }
+
+// go-sdk#976: stdio must surface empty-method calls as requests, not responses.
+func TestIOConnRead_EmptyMethod(t *testing.T) {
+	tr := newIOConn(rwc{
+		rc: io.NopCloser(strings.NewReader(`{"jsonrpc":"2.0","id":5,"method":"","params":{}}`)),
+	})
+	t.Cleanup(func() { tr.Close() })
+
+	msg, err := tr.Read(context.Background())
+	if err != nil {
+		t.Fatalf("ioConn.Read() error = %v", err)
+	}
+	req, ok := msg.(*jsonrpc.Request)
+	if !ok {
+		t.Fatalf("message type = %T, want *jsonrpc.Request", msg)
+	}
+	if req.Method != "" {
+		t.Errorf("Method = %q, want empty string", req.Method)
+	}
+	if req.ID != jsonrpc2.Int64ID(5) {
+		t.Errorf("ID = %v, want 5", req.ID.Raw())
+	}
+}