diff --git a/packages/linejs/client/features/call/planet/schema.ts b/packages/linejs/client/features/call/planet/schema.ts
index 9f50d87..31c7ff6 100644
--- a/packages/linejs/client/features/call/planet/schema.ts
+++ b/packages/linejs/client/features/call/planet/schema.ts
@@ -1194,6 +1194,25 @@ export function packPlanetScMsgKaReq(inner: Uint8Array): Uint8Array {
 	return finalize(b);
 }
 
+// ─── cc_upd_rsp (mid-call quality report / keepalive) ───────────────────
+
+/**
+ * Pack a flat-format UPD_RSP message.  The relay sends a flood of cc
+ * messages with `{ 1:16, 2:<seq>, 3:300, 4:<blob> }` after `conn_req`.
+ * linejs must echo back an equivalent to avoid T103.
+ *
+ * The flat format bypasses the normal cc_msg oneof wrapper — it is NOT
+ * `wrapCcMsg(CC_MSG.UPD_RSP, ...)`, but a top-level protobuf placed
+ * directly in field 3 (cc) of the outer planet_msg.
+ */
+export function packCcUpdRsp(seq: bigint, intervalMs = 300): Uint8Array {
+	const b: Buf = { bytes: [] };
+	emitUint32(b, 1, CC_MSG.UPD_RSP);
+	emitUint64(b, 2, seq);
+	emitUint32(b, 3, intervalMs);
+	return finalize(b);
+}
+
 // ─── handshake helper — extract rmt_nonce from incoming msg ─────────────
 
 /** Extract the loc_nonce (field 6) from a decrypted incoming planet_msg.
@@ -1348,6 +1367,8 @@ export interface DecodedPlanetCcMsg {
 	bodyTag?: number;
 	bodyName?: string;
 	bodyBytes?: Uint8Array;
+	/** Raw decoded fields for non-standard (flat) cc messages. */
+	rawFields?: DecodedField[];
 }
 
 export interface DecodedPlanetMcMsg {
@@ -1387,6 +1408,15 @@ export function decodePlanetCcMsg(bytes: Uint8Array): DecodedPlanetCcMsg {
 			decoded.bodyBytes = oneof.value;
 		}
 	}
+	if (!decoded.bodyTag && !hdrBytes && !body && fields.length > 0) {
+		const typeVal = asNumberField(fields, 1);
+		if (typeVal !== undefined) {
+			decoded.bodyTag = typeVal;
+			decoded.bodyName = CC_MSG_NAMES[typeVal] ??
+				`cc_flat_${typeVal}`;
+			decoded.rawFields = fields;
+		}
+	}
 	return decoded;
 }
 
diff --git a/packages/linejs/client/features/call/planet/transport.ts b/packages/linejs/client/features/call/planet/transport.ts
index e1386a6..1fb8c73 100644
--- a/packages/linejs/client/features/call/planet/transport.ts
+++ b/packages/linejs/client/features/call/planet/transport.ts
@@ -65,6 +65,7 @@ import {
 	packCcParticipateReq,
 	packCcRelReq,
 	packCcSetupReq,
+	packCcUpdRsp,
 	packKeepaliveReq,
 	packMcDataReq,
 	packMcDataRsp,
@@ -1274,6 +1275,9 @@ export class PlanetTransport implements CallTransport {
 						},
 					).catch(() => {});
 				}
+				if (msg.cc?.rawFields && msg.cc.bodyTag === CC_MSG.UPD_RSP) {
+					void this.#handleFlatCcUpd(msg.cc.rawFields).catch(() => {});
+				}
 			} catch {
 				// Keep the raw reply flowing even if a newer message type is unknown.
 			}
@@ -2308,6 +2312,53 @@ export class PlanetTransport implements CallTransport {
 		);
 	}
 
+	async #handleFlatCcUpd(rawFields: DecodedField[]): Promise<void> {
+		const seq = rawFields.find((f) => f.tag === 2)?.value;
+		const seqBig = typeof seq === "bigint" ? seq : 0n;
+		const interval = fieldNumber(rawFields, 3) ?? 300;
+		if (!this.#updRspActive) {
+			this.#updRspActive = true;
+			this.#updRspIntervalMs = interval;
+			this.#startCcUpdTimer();
+		}
+		this.#lastUpdSeq = seqBig;
+	}
+
+	#updRspActive = false;
+	#updRspIntervalMs = 300;
+	#lastUpdSeq = 0n;
+	#ccUpdTimer: ReturnType<typeof setTimeout> | undefined;
+	#ccUpdSeq = 0n;
+
+	#startCcUpdTimer() {
+		this.#clearCcUpdTimer();
+		const tick = () => {
+			if (this.#closed) return;
+			void this.#sendCcUpdRsp().catch(() => {}).finally(() => {
+				if (!this.#closed && this.#updRspActive) {
+					this.#ccUpdTimer = setTimeout(tick, this.#updRspIntervalMs);
+				}
+			});
+		};
+		this.#ccUpdTimer = setTimeout(tick, this.#updRspIntervalMs);
+	}
+
+	#clearCcUpdTimer() {
+		if (this.#ccUpdTimer !== undefined) {
+			clearTimeout(this.#ccUpdTimer);
+			this.#ccUpdTimer = undefined;
+		}
+	}
+
+	async #sendCcUpdRsp(): Promise<void> {
+		this.#ccUpdSeq++;
+		const updBytes = packCcUpdRsp(this.#ccUpdSeq, this.#updRspIntervalMs);
+		await this.#sendEnvelope(
+			{ kind: "cc", data: updBytes },
+			{ msgId: ccMsgId(CC_MSG.UPD_RSP) },
+		);
+	}
+
 	#clearKeepalive() {
 		if (this.#keepaliveTimer !== undefined) {
 			clearTimeout(this.#keepaliveTimer);
@@ -2347,6 +2398,7 @@ export class PlanetTransport implements CallTransport {
 	async close(): Promise<void> {
 		this.#closed = true;
 		this.#clearKeepalive();
+		this.#clearCcUpdTimer();
 		try {
 			if (
 				this.#setupSent && this.#route && (this.#sock || this.#opts.wireSend)