diff --git a/protocol.go b/protocol.go
index 24eaac3..b666057 100644
--- a/protocol.go
+++ b/protocol.go
@@ -6,7 +6,6 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/gorilla/websocket"
 	"github.com/vmihailenco/msgpack/v5"
 )
 
@@ -99,16 +98,15 @@ type ErrorInfo struct {
 }
 
 // ParseProtocolMessage decodes a WebSocket frame into a ProtocolMessage.
-// For text frames (JSON) and binary frames (msgpack).
+// The format is taken from the connection's "format" query parameter:
+// "msgpack" is decoded as msgpack, everything else (including "json" and an
+// absent value) is decoded as JSON.
 // Returns the parsed message. On failure, returns a message with Action=-1.
-func ParseProtocolMessage(data []byte, messageType int) ProtocolMessage {
-	if messageType == websocket.TextMessage {
-		return parseJSON(data)
-	}
-	if messageType == websocket.BinaryMessage {
+func ParseProtocolMessage(data []byte, format string) ProtocolMessage {
+	if format == "msgpack" {
 		return parseMsgpack(data)
 	}
-	return ProtocolMessage{Action: -1}
+	return parseJSON(data)
 }
 
 func parseJSON(data []byte) ProtocolMessage {
diff --git a/proxy_test.go b/proxy_test.go
index c3cd439..2181312 100644
--- a/proxy_test.go
+++ b/proxy_test.go
@@ -315,7 +315,7 @@ func TestWsPassthrough(t *testing.T) {
 	defer deleteSession(t, controlURL, session.SessionID)
 
 	// Connect through proxy
-	conn := connectWs(t, fmt.Sprintf("localhost:%d", port), "key=test.key:secret")
+	conn := connectWs(t, fmt.Sprintf("localhost:%d", port), "format=json", "key=test.key:secret")
 	defer conn.Close()
 
 	// Should receive CONNECTED from mock upstream
@@ -467,7 +467,7 @@ func TestWsFrameSuppression(t *testing.T) {
 	})
 	defer deleteSession(t, controlURL, session.SessionID)
 
-	conn := connectWs(t, fmt.Sprintf("localhost:%d", port))
+	conn := connectWs(t, fmt.Sprintf("localhost:%d", port), "format=json")
 	defer conn.Close()
 
 	// Read CONNECTED
@@ -515,7 +515,7 @@ func TestWsInjectAndClose(t *testing.T) {
 	})
 	defer deleteSession(t, controlURL, session.SessionID)
 
-	conn := connectWs(t, fmt.Sprintf("localhost:%d", port))
+	conn := connectWs(t, fmt.Sprintf("localhost:%d", port), "format=json")
 	defer conn.Close()
 
 	// Should receive the injected DISCONNECTED message
@@ -672,7 +672,7 @@ func TestWsSuppressOnwards(t *testing.T) {
 	})
 	defer deleteSession(t, controlURL, session.SessionID)
 
-	conn := connectWs(t, fmt.Sprintf("localhost:%d", port))
+	conn := connectWs(t, fmt.Sprintf("localhost:%d", port), "format=json")
 	defer conn.Close()
 
 	// Read CONNECTED (arrives before suppress_onwards fires)
@@ -854,7 +854,7 @@ func TestRuleTimesLimit(t *testing.T) {
 
 func TestProtocolParseJSON(t *testing.T) {
 	msg := `{"action":10,"channel":"test-channel"}`
-	pm := ParseProtocolMessage([]byte(msg), websocket.TextMessage)
+	pm := ParseProtocolMessage([]byte(msg), "json")
 	if pm.Action != ActionAttach {
 		t.Fatalf("expected action %d, got %d", ActionAttach, pm.Action)
 	}
@@ -865,7 +865,7 @@ func TestProtocolParseJSON(t *testing.T) {
 
 func TestProtocolParseJSONWithError(t *testing.T) {
 	msg := `{"action":9,"error":{"code":40142,"statusCode":401,"message":"Token expired"}}`
-	pm := ParseProtocolMessage([]byte(msg), websocket.TextMessage)
+	pm := ParseProtocolMessage([]byte(msg), "json")
 	if pm.Action != ActionError {
 		t.Fatalf("expected action %d, got %d", ActionError, pm.Action)
 	}
@@ -885,7 +885,7 @@ func TestProtocolParseMsgpack(t *testing.T) {
 		"channel": "test",
 	}
 	data := mustMarshalMsgpack(t, raw)
-	pm := ParseProtocolMessage(data, websocket.BinaryMessage)
+	pm := ParseProtocolMessage(data, "msgpack")
 	if pm.Action != ActionAttach {
 		t.Fatalf("expected action %d, got %d", ActionAttach, pm.Action)
 	}
diff --git a/ws_proxy.go b/ws_proxy.go
index 344a848..dae222e 100644
--- a/ws_proxy.go
+++ b/ws_proxy.go
@@ -28,6 +28,10 @@ func HandleWsProxy(session *Session, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// Protocol format ("json" or "msgpack") is declared by the SDK via the
+	// "format" query param and is fixed for the lifetime of the connection.
+	format := queryParams["format"]
+
 	// Create WsConnection and register it
 	wc := NewWsConnection(0)
 	session.AddWsConn(wc)
@@ -142,13 +146,13 @@ func HandleWsProxy(session *Session, w http.ResponseWriter, r *http.Request) {
 	// server → client relay
 	go func() {
 		defer wg.Done()
-		relayFrames(session, wc, serverConn, clientConn, "server_to_client", "ws_frame_to_client")
+		relayFrames(session, wc, serverConn, clientConn, "server_to_client", "ws_frame_to_client", format)
 	}()
 
 	// client → server relay
 	go func() {
 		defer wg.Done()
-		relayFrames(session, wc, clientConn, serverConn, "client_to_server", "ws_frame_to_server")
+		relayFrames(session, wc, clientConn, serverConn, "client_to_server", "ws_frame_to_server", format)
 	}()
 
 	wg.Wait()
@@ -163,7 +167,7 @@ func HandleWsProxy(session *Session, w http.ResponseWriter, r *http.Request) {
 }
 
 // relayFrames reads frames from src and writes to dst, applying rules.
-func relayFrames(session *Session, wc *WsConnection, src, dst *websocket.Conn, direction, matchType string) {
+func relayFrames(session *Session, wc *WsConnection, src, dst *websocket.Conn, direction, matchType, format string) {
 	for {
 		if wc.IsClosed() {
 			return
@@ -202,11 +206,11 @@ func relayFrames(session *Session, wc *WsConnection, src, dst *websocket.Conn, d
 		}
 
 		// Parse protocol message for rule matching and logging
-		pm := ParseProtocolMessage(data, msgType)
+		pm := ParseProtocolMessage(data, format)
 
 		// Log the frame (as JSON for readability, even if binary)
 		var logMsg json.RawMessage
-		if msgType == websocket.TextMessage {
+		if format != "msgpack" {
 			logMsg = json.RawMessage(data)
 		} else {
 			// For binary frames, log the parsed summary