diff --git a/src/validation.h b/src/validation.h
--- a/src/validation.h
+++ b/src/validation.h
@@ -190,6 +190,9 @@
 extern bool fCheckBlockIndex;
 extern bool fCheckpointsEnabled;
 extern size_t nCoinCacheUsage;
+/** True if and only if chainActive.Tip()'s medianTimePast >= UAHFStartTime.
+ * Updated in validation.cpp :: UpdateTip() */
+extern std::atomic_bool fActiveChainTipIsUAHF;
 
 /** A fee rate smaller than this is considered zero fee (for relaying, mining
  * and transaction creation) */
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -74,6 +74,8 @@
 bool fRequireStandard = true;
 bool fCheckBlockIndex = false;
 bool fCheckpointsEnabled = DEFAULT_CHECKPOINTS_ENABLED;
+std::atomic_bool fActiveChainTipIsUAHF(false);
+
 size_t nCoinCacheUsage = 5000 * 300;
 uint64_t nPruneTarget = 0;
 int64_t nMaxTipAge = DEFAULT_MAX_TIP_AGE;
@@ -2236,6 +2238,10 @@
 
     chainActive.SetTip(pindexNew);
 
+    // other code that cares about being conditional on UAHF
+    // might need this flag
+    fActiveChainTipIsUAHF = IsUAHFenabledForCurrentBlock(config);
+
     // New best block
     mempool.AddTransactionsUpdated(1);