diff --git a/quiche/quic/core/crypto/crypto_protocol.h b/quiche/quic/core/crypto/crypto_protocol.h
index 31937bc..99099dd 100644
--- a/quiche/quic/core/crypto/crypto_protocol.h
+++ b/quiche/quic/core/crypto/crypto_protocol.h
@@ -466,6 +466,10 @@
 // Stats collection tags
 const QuicTag kEPID = TAG('E', 'P', 'I', 'D');  // Endpoint identifier.
 
+const QuicTag kMCS1 = TAG('M', 'C', 'S', '1');
+const QuicTag kMCS2 = TAG('M', 'C', 'S', '2');
+const QuicTag kMCS3 = TAG('M', 'C', 'S', '3');
+
 // clang-format on
 
 // These tags have a special form so that they appear either at the beginning