diff --git a/internal/services/datatransfer_test.go b/internal/services/datatransfer_test.go
index eacd87b..2984b36 100644
--- a/internal/services/datatransfer_test.go
+++ b/internal/services/datatransfer_test.go
@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"git.saqut.com/saqut/mwse/internal/testutil"
+	"git.saqut.com/saqut/mwse/internal/ws"
 )
 
 // findReply scans the captured frames for a [payload, id] frame whose id slot is
@@ -100,6 +101,50 @@ func TestTunnelLargePayloadIntact(t *testing.T) {
 	}
 }
 
+// TestWebRTCSignalingRelay is the #31 parity test. WebRTC signaling is not a
+// distinct engine concept: the SDK tunnels offer/answer/ICE as opaque
+// {type:":rtcpack:", payload:{...}} packs over pack/to. The engine must carry
+// them through unchanged, in both directions, without inspecting the RTC payload.
+func TestWebRTCSignalingRelay(t *testing.T) {
+	hub := newHub()
+	a, fa := connect(hub, "a")
+	b, fb := connect(hub, "b")
+
+	signal := func(from *ws.Client, to string, rtc map[string]any) {
+		hub.Handle(from, msg("pack/to", "to", to,
+			"pack", map[string]any{"type": ":rtcpack:", "payload": rtc}))
+	}
+
+	// A → B: SDP offer.
+	signal(a, "b", map[string]any{"type": "offer", "value": map[string]any{"sdp": "v=0...offer"}})
+	offerPack := asMap(t, waitSignal(t, fb, "pack")["pack"])
+	if offerPack["type"] != ":rtcpack:" {
+		t.Fatalf("offer relayed as %v, want :rtcpack:", offerPack["type"])
+	}
+	if rtc := asMap(t, offerPack["payload"]); rtc["type"] != "offer" {
+		t.Fatalf("inner signaling type = %v, want offer", rtc["type"])
+	}
+
+	// B → A: SDP answer.
+	signal(b, "a", map[string]any{"type": "answer", "value": map[string]any{"sdp": "v=0...answer"}})
+	answerPack := asMap(t, waitSignal(t, fa, "pack")["pack"])
+	if rtc := asMap(t, answerPack["payload"]); rtc["type"] != "answer" {
+		t.Fatalf("inner signaling type = %v, want answer", rtc["type"])
+	}
+
+	// A → B: ICE candidate — payload carried verbatim.
+	cand := map[string]any{"candidate": "candidate:842163049 1 udp ...", "sdpMLineIndex": float64(0)}
+	signal(a, "b", map[string]any{"type": "icecandidate", "value": cand})
+	waitFor(t, func() bool {
+		for _, raw := range fb.Writes() {
+			if strings.Contains(string(raw), "icecandidate") {
+				return true
+			}
+		}
+		return false
+	})
+}
+
 // assertNoAddressLeak fails if a relayed payload exposes anything resembling the
 // sender's real network identity.
 func assertNoAddressLeak(t *testing.T, payload map[string]any) {