diff --git a/backend/internal/handler/ws/handler.go b/backend/internal/handler/ws/handler.go
index 5b548f4e..1e58252f 100644
--- a/backend/internal/handler/ws/handler.go
+++ b/backend/internal/handler/ws/handler.go
@@ -231,7 +231,7 @@ func (h *Handler) writePump(client *Client) {
 			// keep the connection alive (staleThreshold = 6s by default).
 			pingMsg, err := json.Marshal(PingFrame{
 				Type:    ServerPing,
-				Message: time.Now().UTC().Format(time.RFC3339),
+				Message: time.Now().Unix(),
 			})
 			if err != nil {
 				logger.L().Errorf("ws: marshal ActionCable ping for user=%d: %v", client.UserID, err)
@@ -370,7 +370,7 @@ func (h *Handler) handleUnsubscribe(client *Client, cmd CommandFrame) {
 func (h *Handler) handlePing(client *Client) {
 	pingData, _ := json.Marshal(PingFrame{
 		Type:    ServerPing,
-		Message: time.Now().UTC().Format(time.RFC3339),
+		Message: time.Now().Unix(),
 	})
 	client.Send <- pingData
 }
diff --git a/backend/internal/handler/ws/protocol.go b/backend/internal/handler/ws/protocol.go
index 0f15c9f1..0b72fa06 100644
--- a/backend/internal/handler/ws/protocol.go
+++ b/backend/internal/handler/ws/protocol.go
@@ -12,7 +12,7 @@ package ws
 //   {"type":"event","event":"message.created","payload":{...},"identifier":"{\"channel\":\"AccountChannel\",\"account_id\":1}"}
 //   {"type":"confirm_subscribe","identifier":"..."}
 //   {"type":"confirm_unsubscribe","identifier":"..."}
-//   {"type":"ping","message":"2026-05-23T10:00:00Z"}
+//   {"type":"ping","message":1779520800}
 //   {"type":"reject_subscribe","identifier":"...","reason":"..."}
 
 // CommandType — client→server action types
@@ -97,7 +97,7 @@ type RejectFrame struct {
 // PingFrame is a heartbeat pong response.
 type PingFrame struct {
 	Type    ServerMessageType `json:"type"`
-	Message string            `json:"message"` // timestamp string
+	Message int64             `json:"message"` // Unix seconds, matching ActionCable's Time.now.to_i
 }
 
 // WelcomeFrame is sent upon successful WebSocket connection.
@@ -160,6 +160,6 @@ const (
 // --- Ping/pong Configuration ---
 
 const (
-	// PingInterval is how often the server sends ping frames to detect dead connections.
-	PingInterval = 5 // seconds — must be < ActionCable staleThreshold (6s) to avoid reconnect loops
+	// PingInterval matches ActionCable::Server::Connections::BEAT_INTERVAL.
+	PingInterval = 3 // seconds — two missed beats reach ActionCable's 6s stale threshold
 )
diff --git a/backend/internal/handler/ws/ws_test.go b/backend/internal/handler/ws/ws_test.go
index 1395b0d6..f653872f 100644
--- a/backend/internal/handler/ws/ws_test.go
+++ b/backend/internal/handler/ws/ws_test.go
@@ -141,18 +141,43 @@ func TestRejectFrame_JSONRoundTrip(t *testing.T) {
 	assert.Equal(t, rf.Reason, decoded.Reason)
 }
 
-func TestPingFrame_JSONRoundTrip(t *testing.T) {
-	pf := PingFrame{
-		Type:    ServerPing,
-		Message: "2026-01-01T00:00:00Z",
-	}
-	data, err := json.Marshal(pf)
-	require.NoError(t, err)
+func TestWritePump_ActionCableHeartbeatCompatibility(t *testing.T) {
+	serverConn, clientConn := newTestWSConn(t)
+	defer clientConn.Close()
 
-	var decoded PingFrame
-	err = json.Unmarshal(data, &decoded)
-	require.NoError(t, err)
-	assert.Equal(t, pf.Message, decoded.Message)
+	client := NewClient(1, 10, serverConn, NewHubSimple())
+	done := make(chan struct{})
+	go func() {
+		(&Handler{}).writePump(client)
+		close(done)
+	}()
+	defer func() {
+		close(client.Send)
+		<-done
+	}()
+
+	require.NoError(t, clientConn.SetReadDeadline(time.Now().Add(8*time.Second)))
+	receivedAt := make([]time.Time, 2)
+	for i := range receivedAt {
+		messageType, data, err := clientConn.ReadMessage()
+		receivedAt[i] = time.Now()
+		require.NoError(t, err)
+		require.Equal(t, websocket.TextMessage, messageType)
+
+		var frame struct {
+			Type    ServerMessageType `json:"type"`
+			Message json.RawMessage   `json:"message"`
+		}
+		require.NoError(t, json.Unmarshal(data, &frame))
+		require.Equal(t, ServerPing, frame.Type)
+		require.Regexp(t, `^[0-9]+$`, string(frame.Message), "message must be an unquoted Unix timestamp")
+
+		var unixSeconds int64
+		require.NoError(t, json.Unmarshal(frame.Message, &unixSeconds))
+		assert.InDelta(t, receivedAt[i].Unix(), unixSeconds, 1)
+	}
+
+	assert.WithinDuration(t, receivedAt[0].Add(3*time.Second), receivedAt[1], time.Second)
 }
 
 func TestWelcomeFrame_JSONRoundTrip(t *testing.T) {
@@ -182,10 +207,6 @@ func TestDisconnectFrame_JSONRoundTrip(t *testing.T) {
 	assert.Equal(t, df.Reconnect, decoded.Reconnect)
 }
 
-func TestPingInterval_Constant(t *testing.T) {
-	assert.Equal(t, 5, PingInterval)
-}
-
 // --- Hub Tests ---
 
 func TestNewHubSimple(t *testing.T) {