diff --git a/kv/coordinator.go b/kv/coordinator.go
index 15db4b0c..157b8f5e 100644
--- a/kv/coordinator.go
+++ b/kv/coordinator.go
@@ -57,7 +57,12 @@ func (c *Coordinate) Dispatch(ctx context.Context, reqs *OperationGroup[OP]) (*C
 
 	if reqs.IsTxn && reqs.StartTS == 0 {
 		// Leader-only timestamp issuance to avoid divergence across shards.
+		// When the leader assigns StartTS, also clear any caller-provided
+		// CommitTS so dispatchTxn generates both timestamps consistently.
+		// A caller-supplied CommitTS without a matching StartTS could produce
+		// CommitTS <= StartTS (an invalid transaction).
 		reqs.StartTS = c.nextStartTS()
+		reqs.CommitTS = 0
 	}
 
 	if reqs.IsTxn {
diff --git a/kv/sharded_coordinator.go b/kv/sharded_coordinator.go
index 7ecc58ca..8f7a2ed9 100644
--- a/kv/sharded_coordinator.go
+++ b/kv/sharded_coordinator.go
@@ -70,6 +70,11 @@ func (c *ShardedCoordinator) Dispatch(ctx context.Context, reqs *OperationGroup[
 			return nil, err
 		}
 		reqs.StartTS = startTS
+		// When the coordinator assigns StartTS, also clear any caller-provided
+		// CommitTS so dispatchTxn generates both timestamps consistently.
+		// A caller-supplied CommitTS without a matching StartTS could produce
+		// CommitTS <= StartTS (an invalid transaction).
+		reqs.CommitTS = 0
 	}
 
 	if reqs.IsTxn {