diff --git a/internal/tuic/server_test.go b/internal/tuic/server_test.go
index 5d7f7316a..baf505d79 100644
--- a/internal/tuic/server_test.go
+++ b/internal/tuic/server_test.go
@@ -196,12 +196,30 @@ func testServerTCPConnectE2E(t *testing.T, controller string) {
 		t.Fatalf("expected active email alice@example.com, got %v", activeEmails)
 	}
 
-	deltas := server.CollectClientTraffic()
-	if len(deltas) == 0 {
-		t.Fatalf("expected traffic deltas, got none")
-	}
-	if deltas[0].Email != "alice@example.com" || deltas[0].Up < int64(len(testMsg)) || deltas[0].Down < int64(len(testMsg)) {
-		t.Fatalf("unexpected traffic deltas: %+v", deltas[0])
+	waitForClientTraffic(t, server, "alice@example.com", int64(len(testMsg)))
+}
+
+// waitForClientTraffic accumulates drained deltas because the up and down counters are
+// bumped on different relay goroutines, so the echo can arrive before the upload is counted.
+func waitForClientTraffic(t *testing.T, server *Server, email string, minBytes int64) {
+	t.Helper()
+	var up, down int64
+	deadline := time.Now().Add(4 * time.Second)
+	for {
+		for _, delta := range server.CollectClientTraffic() {
+			if delta.Email != email {
+				t.Fatalf("unexpected traffic delta for %q: %+v", delta.Email, delta)
+			}
+			up += delta.Up
+			down += delta.Down
+		}
+		if up >= minBytes && down >= minBytes {
+			return
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("traffic for %s = up %d, down %d; want both >= %d", email, up, down, minBytes)
+		}
+		time.Sleep(5 * time.Millisecond)
 	}
 }
 
@@ -356,13 +374,7 @@ func testServerUDPDatagramE2E(t *testing.T, controller string) {
 	}
 
 	// 4. Verify traffic
-	deltas := server.CollectClientTraffic()
-	if len(deltas) == 0 {
-		t.Fatalf("expected traffic deltas, got none")
-	}
-	if deltas[0].Email != "bob@example.com" || deltas[0].Up < int64(len(udpMsg)) || deltas[0].Down < int64(len(udpMsg)) {
-		t.Fatalf("unexpected traffic deltas: %+v", deltas[0])
-	}
+	waitForClientTraffic(t, server, "bob@example.com", int64(len(udpMsg)))
 }
 
 func TestServerUDPStreamE2E(t *testing.T) {