diff --git a/clients/go/cmd/mxgw-go/main.go b/clients/go/cmd/mxgw-go/main.go
index 768ea8c..3604d40 100644
--- a/clients/go/cmd/mxgw-go/main.go
+++ b/clients/go/cmd/mxgw-go/main.go
@@ -263,6 +263,11 @@ func runPing(ctx context.Context, args []string, stdout, stderr io.Writer) error
 			Reply:   mustMarshalProto(reply),
 		})
 	}
+	// DiagnosticMessage carries the echoed ping text set by the gateway.
+	// Fall back to the kind string when the gateway returns an empty message
+	// (forward-compat guard for future gateway versions). writeCommandOutput
+	// is not reused here because it would print the opaque Kind enum rather
+	// than the human-readable echo.
 	echo := reply.GetDiagnosticMessage()
 	if echo == "" {
 		echo = reply.GetKind().String()
diff --git a/clients/go/cmd/mxgw-go/main_test.go b/clients/go/cmd/mxgw-go/main_test.go
index 4e02a39..efbe9c4 100644
--- a/clients/go/cmd/mxgw-go/main_test.go
+++ b/clients/go/cmd/mxgw-go/main_test.go
@@ -255,6 +255,13 @@ func TestRunPingJSON(t *testing.T) {
 	if out.Command != "ping" {
 		t.Fatalf("command = %q, want %q", out.Command, "ping")
 	}
+	// The fake gateway echoes "pong:<message>" in diagnostic_message; verify the
+	// echo appears in the serialised reply so a future regression that wired
+	// PingRaw to the wrong proto field would be caught here.
+	replyStr := string(out.Reply)
+	if !strings.Contains(replyStr, "pong:hello") {
+		t.Fatalf("ping JSON reply missing echoed message %q; reply = %s", "pong:hello", replyStr)
+	}
 }
 
 // TestRunPingRequiresSessionID verifies the ping subcommand rejects missing session-id.