diff --git a/docs/adr/0005-automatic-hybrid-media-routing.md b/docs/adr/0005-automatic-hybrid-media-routing.md
index 03c3892..aa3abdb 100644
--- a/docs/adr/0005-automatic-hybrid-media-routing.md
+++ b/docs/adr/0005-automatic-hybrid-media-routing.md
@@ -479,10 +479,15 @@ viewers cannot attach behind any healthy root, a later reviewed config may
 admit additional direct server-fed edges under a separate explicit egress cap;
 it never creates a third endpoint edge or unbounded whole-room fanout.
 
-Sub-second failure recovery is a target after failure detection. The current
-30-second control heartbeat cannot meet it for silent partitions, so the media
-plane needs a small 100-200 ms liveness/queue signal or an equivalent native
-transport event. This signal must be measured before its interval is fixed.
+Sub-second media recovery remains a target after media failure detection. A
+separate browser application challenge detects a silently partitioned signaling
+WebSocket on a bounded 5-second baseline plus two exact 2-second misses, then
+reauthenticates on a new socket without asking ordinary P2P or duplicate
+same-revision SFU media to close and without reporting `route-failed`. Existing
+`peer-selected` TURN authority remains session-bound and follows its bounded
+recovery path after session replacement. Control-plane recovery is not media
+evidence and does not replace a measured media liveness or native transport
+event for sub-second route recovery.
 
 ## Selected-Edge TURN After SFU
 
diff --git a/docs/research/peer-assisted-media.md b/docs/research/peer-assisted-media.md
index 384218c..a0e5eb2 100644
--- a/docs/research/peer-assisted-media.md
+++ b/docs/research/peer-assisted-media.md
@@ -491,10 +491,54 @@ media and broader audio/A-V verification remain open. The
 native host shared-encode sender is a separate planned phase regardless of this
 experiment's result and is not implemented here.
 
-The recovery gate above does not cover silent network partitions. With the
-default 30-second heartbeat, server detection can take 30 to 60 seconds before
-the same 5-second grace begins; that case remains unverified and must be reported
-separately.
+## Browser Signaling Silent-Partition Detection
+
+The recovery gate above does not cover a browser WebSocket that remains open
+locally after its network path has silently failed. The server's default
+30-second protocol heartbeat can take 30 to 60 seconds to observe that failure,
+and browser script cannot observe WebSocket Ping or Pong control frames. The
+[WHATWG WebSockets Living Standard](https://websockets.spec.whatwg.org/)
+(accessed 2026-08-22) exposes messages, generic error/close events, and
+`readyState`; it also specifies that constructing a WebSocket immediately
+starts a new connection while `close()` starts a closing handshake whose close
+event arrives later. The application therefore needs a small text-message
+challenge rather than waiting for either protocol Ping/Pong or the old socket's
+close event.
+
+The retained compatibility seam is response-only opt-in under `screener-v5`:
+the Web client sends `signaling-challenge { sequence }`, and the server sends
+the exact `signaling-challenge-response { sequence }` only to that requesting
+socket. The Native sender's strict decoder rejects unknown server message
+types, so the server must never send this response unsolicited. Existing Web
+clients and Native senders do not opt in and therefore never receive the new
+message. A new server remains compatible with those old clients, but new Web
+assets and the signaling server still need an ordered, atomic deployment
+because an old server rejects the new client message. Supporting that reverse
+version skew would require explicit capability negotiation or a protocol bump.
+
+The Web watchdog is eligible only while the current socket is authenticated,
+the share is online, and the endpoint has an active authoritative route (the
+Host itself, an ordinary Viewer with an online Host, or a peer-assisted Viewer
+whose active upstream is not `none`). After five seconds it sends one
+challenge. An exact two-second miss enters an internal suspect state and sends
+one fresh confirm challenge; an exact second two-second miss logically retires
+that socket generation and immediately constructs a replacement without
+waiting for the old closing handshake. Responses from another sequence or
+socket generation are ignored, and the server revalidates the current
+authenticated session before replying. A hidden document, a visibility
+restore, or an obviously late timer callback clears pending evidence and starts
+a fresh five-second baseline instead of declaring failure.
+
+The response is socket-local, contains no room state, secret, candidate, or raw
+statistics, is not logged, and is limited by the server to at most one response
+per socket per second. The watchdog itself does not close or rebuild media,
+emit `route-failed`, or change route authority. Ordinary P2P and a duplicate
+same-revision SFU authority keep their healthy media while the new socket
+reauthenticates. An existing `peer-selected` TURN lease remains deliberately
+bound to both authenticated sessions: replacing either session invalidates that
+exceptional transport and leaves recovery to the existing bounded route path.
+Relaxing or carrying that security authority is a separate routing/protocol
+change, not part of signaling liveness detection.
 
 ## License Boundary
 
diff --git a/docs/方案设计.md b/docs/方案设计.md
index f15e746..b234b66 100644
--- a/docs/方案设计.md
+++ b/docs/方案设计.md
@@ -142,6 +142,8 @@ Zod 在服务边界拒绝未知类型、超长字符串、非法 SDP/candidate 
 
 信令 WebSocket 暂时中断时，不立即关闭仍健康的 P2P 媒体；客户端指数退避恢复控制通道。明确不可重连的会话替换会释放旧页面的 peer、统计定时器和媒体引用，host 也停止旧捕获。首版只保证同一页面生命周期内恢复，不承诺浏览器进程被系统挂起后的连续播放。
 
+Web 客户端只在当前 socket 已鉴权、分享在线且本端拥有 active 权威路由时运行应用层静默断链探测：五秒后发送带安全整数序号的 challenge，首个精确两秒 miss 只进入内部 suspect 并立即发送新序号 confirm，第二个精确两秒 miss 才废弃旧 socket 代次并直接建立新连接，不等待旧 closing handshake。response 必须匹配当前 socket 代次和序号；旧序号、旧 socket、已替换鉴权会话一律忽略。页面隐藏、恢复可见或定时器明显迟到时清除未决证据并重新等待完整五秒。服务端只向主动 opt-in 的请求 socket 回送相同序号，每 socket 每秒最多一次，不附带房间、密钥、候选或统计信息；因此 strict Native 不会收到未知消息。watchdog 本身不发送 `route-failed`、不修改路由权威，也不要求普通 P2P 或同 revision SFU 关闭健康媒体；现有 `peer-selected` TURN 授权仍严格绑定两端鉴权 session，任一 session 替换后按既有有界恢复路径处理。服务端 30 秒 ping/pong 默认值保持不变。
+
 ## 捕获、编码与质量
 
 捕获只能由按钮产生的用户手势调用 `getDisplayMedia()`。三档目标为：
diff --git a/src/client/lib/signaling.ts b/src/client/lib/signaling.ts
index a86c84a..0125306 100644
--- a/src/client/lib/signaling.ts
+++ b/src/client/lib/signaling.ts
@@ -37,6 +37,15 @@ const INVALID_MESSAGE_CLOSE_CODE = 1008;
 const VIEWER_ACCESS_REVOKED_CLOSE_CODE = 4004;
 const PROTOCOL_REFRESH_MESSAGE = "页面版本已更新，请刷新后重试";
 const TERMINAL_SEND_TIMEOUT_MS = 15_000;
+const SIGNALING_CHALLENGE_INTERVAL_MS = 5_000;
+const SIGNALING_CHALLENGE_TIMEOUT_MS = 2_000;
+const SIGNALING_TIMER_LAG_TOLERANCE_MS = 1_000;
+
+interface PendingSignalingChallenge {
+  generation: number;
+  sequence: number;
+  confirm: boolean;
+}
 
 export function shouldReconnectSignaling(code: number): boolean {
   return (
@@ -61,16 +70,26 @@ export class SignalingClient {
   private authenticationTimer: number | null = null;
   private terminalTimer: number | null = null;
   private terminalMessage: ClientMessage | null = null;
+  private socketGeneration = 0;
+  private hostOnline = false;
+  private authoritativeRoute = false;
+  private challengeSequence = 0;
+  private watchdogTimer: number | null = null;
+  private watchdogDeadlineMs = 0;
+  private pendingChallenge: PendingSignalingChallenge | null = null;
+  private visibilityListenerAttached = false;
 
   constructor(
     private readonly identity: SignalingIdentity,
     private readonly events: SignalingEvents,
+    private readonly now: () => number = Date.now,
   ) {}
 
   start(): void {
     if (this.socket || this.stopped) {
       return;
     }
+    this.attachVisibilityListener();
     this.events.onStatus(this.reconnectAttempt === 0 ? "connecting" : "reconnecting");
     this.connect();
   }
@@ -79,6 +98,7 @@ export class SignalingClient {
     this.stopped = true;
     this.authenticated = false;
     this.clearTimers();
+    this.detachVisibilityListener();
     this.terminalMessage = null;
     const socket = this.socket;
     this.socket = null;
@@ -98,6 +118,7 @@ export class SignalingClient {
     ) {
       return false;
     }
+    this.clearSignalingWatchdog();
     socket.close(CLIENT_RECONNECT_CLOSE_CODE, "client reconnect");
     return true;
   }
@@ -167,11 +188,16 @@ export class SignalingClient {
       return;
     }
 
+    const generation = ++this.socketGeneration;
     const socket = new WebSocket(signalUrl());
     this.socket = socket;
 
     socket.addEventListener("open", () => {
-      if (this.socket !== socket || this.stopped) {
+      if (
+        this.socket !== socket ||
+        generation !== this.socketGeneration ||
+        this.stopped
+      ) {
         return;
       }
       const authenticate: ClientMessage = {
@@ -188,7 +214,11 @@ export class SignalingClient {
     });
 
     socket.addEventListener("message", (event) => {
-      if (this.socket !== socket || typeof event.data !== "string") {
+      if (
+        this.socket !== socket ||
+        generation !== this.socketGeneration ||
+        typeof event.data !== "string"
+      ) {
         return;
       }
       let message: ServerMessage;
@@ -199,6 +229,11 @@ export class SignalingClient {
         return;
       }
 
+      if (message.type === "signaling-challenge-response") {
+        this.acceptSignalingChallengeResponse(message.sequence, generation);
+        return;
+      }
+
       if (
         message.type === "error" &&
         message.code === "INVALID_MESSAGE" &&
@@ -231,15 +266,22 @@ export class SignalingClient {
         }
         this.events.onStatus("connected");
       }
+      this.updateAuthoritativeActivity(message);
       this.events.onMessage(message);
     });
 
     socket.addEventListener("close", (event) => {
-      if (this.socket !== socket) {
+      if (
+        this.socket !== socket ||
+        generation !== this.socketGeneration
+      ) {
         return;
       }
       this.socket = null;
       this.authenticated = false;
+      this.hostOnline = false;
+      this.authoritativeRoute = false;
+      this.clearSignalingWatchdog();
       this.clearAuthenticationTimer();
       if (!this.stopped && shouldReconnectSignaling(event.code)) {
         this.scheduleReconnect();
@@ -287,6 +329,7 @@ export class SignalingClient {
 
   private clearTimers(): void {
     this.clearAuthenticationTimer();
+    this.clearSignalingWatchdog();
     if (this.reconnectTimer !== null) {
       window.clearTimeout(this.reconnectTimer);
       this.reconnectTimer = null;
@@ -296,4 +339,171 @@ export class SignalingClient {
       this.terminalTimer = null;
     }
   }
+
+  private updateAuthoritativeActivity(message: ServerMessage): void {
+    if (message.type === "authenticated") {
+      this.hostOnline = message.hostOnline;
+      this.authoritativeRoute =
+        this.identity.role === "host" ||
+        !("mediaMode" in message) ||
+        message.routeAssignment.upstream.kind !== "none";
+    } else if (message.type === "host-status") {
+      this.hostOnline = message.online;
+    } else if (
+      message.type === "route-update" &&
+      message.phase === "active" &&
+      this.identity.role === "viewer"
+    ) {
+      this.authoritativeRoute = message.assignment.upstream.kind !== "none";
+    } else if (
+      message.type === "sharing-stopped" ||
+      message.type === "room-closed"
+    ) {
+      this.hostOnline = false;
+      this.authoritativeRoute = false;
+    } else {
+      return;
+    }
+    this.refreshSignalingWatchdog();
+  }
+
+  private refreshSignalingWatchdog(): void {
+    if (!this.isWatchdogEligible()) {
+      this.clearSignalingWatchdog();
+      return;
+    }
+    if (this.watchdogTimer === null && this.pendingChallenge === null) {
+      this.armSignalingWatchdog(SIGNALING_CHALLENGE_INTERVAL_MS);
+    }
+  }
+
+  private armSignalingWatchdog(delayMs: number): void {
+    const generation = this.socketGeneration;
+    this.watchdogDeadlineMs = this.now() + delayMs;
+    this.watchdogTimer = window.setTimeout(() => {
+      this.watchdogTimer = null;
+      this.handleSignalingWatchdogTick(generation);
+    }, delayMs);
+  }
+
+  private handleSignalingWatchdogTick(generation: number): void {
+    if (generation !== this.socketGeneration) {
+      return;
+    }
+    if (
+      !this.isWatchdogEligible() ||
+      Math.abs(this.now() - this.watchdogDeadlineMs) >
+        SIGNALING_TIMER_LAG_TOLERANCE_MS
+    ) {
+      this.rebaselineSignalingWatchdog();
+      return;
+    }
+    const pending = this.pendingChallenge;
+    if (!pending) {
+      this.sendSignalingChallenge(false, generation);
+    } else if (pending.confirm) {
+      this.replaceUnresponsiveSocket(generation);
+    } else {
+      this.sendSignalingChallenge(true, generation);
+    }
+  }
+
+  private sendSignalingChallenge(confirm: boolean, generation: number): void {
+    const socket = this.socket;
+    if (!socket || generation !== this.socketGeneration) {
+      return;
+    }
+    this.challengeSequence =
+      this.challengeSequence === Number.MAX_SAFE_INTEGER
+        ? 0
+        : this.challengeSequence + 1;
+    const sequence = this.challengeSequence;
+    socket.send(JSON.stringify({ type: "signaling-challenge", sequence }));
+    this.pendingChallenge = { generation, sequence, confirm };
+    this.armSignalingWatchdog(SIGNALING_CHALLENGE_TIMEOUT_MS);
+  }
+
+  private acceptSignalingChallengeResponse(
+    sequence: number,
+    generation: number,
+  ): void {
+    const pending = this.pendingChallenge;
+    if (
+      !pending ||
+      pending.generation !== generation ||
+      pending.sequence !== sequence ||
+      generation !== this.socketGeneration
+    ) {
+      return;
+    }
+    this.clearSignalingWatchdog();
+    this.refreshSignalingWatchdog();
+  }
+
+  private replaceUnresponsiveSocket(generation: number): void {
+    const socket = this.socket;
+    if (!socket || generation !== this.socketGeneration || this.stopped) {
+      return;
+    }
+    this.socket = null;
+    this.authenticated = false;
+    this.hostOnline = false;
+    this.authoritativeRoute = false;
+    this.clearAuthenticationTimer();
+    this.clearSignalingWatchdog();
+    this.events.onStatus("reconnecting");
+    if (socket.readyState < WebSocket.CLOSING) {
+      socket.close(CLIENT_RECONNECT_CLOSE_CODE, "signaling timeout");
+    }
+    this.connect();
+  }
+
+  private rebaselineSignalingWatchdog(): void {
+    this.clearSignalingWatchdog();
+    this.refreshSignalingWatchdog();
+  }
+
+  private clearSignalingWatchdog(): void {
+    this.pendingChallenge = null;
+    if (this.watchdogTimer !== null) {
+      window.clearTimeout(this.watchdogTimer);
+      this.watchdogTimer = null;
+    }
+  }
+
+  private readonly onVisibilityChange = (): void => {
+    this.rebaselineSignalingWatchdog();
+  };
+
+  private attachVisibilityListener(): void {
+    if (typeof document === "undefined" || this.visibilityListenerAttached) {
+      return;
+    }
+    document.addEventListener("visibilitychange", this.onVisibilityChange);
+    this.visibilityListenerAttached = true;
+  }
+
+  private detachVisibilityListener(): void {
+    if (typeof document === "undefined" || !this.visibilityListenerAttached) {
+      return;
+    }
+    document.removeEventListener("visibilitychange", this.onVisibilityChange);
+    this.visibilityListenerAttached = false;
+  }
+
+  private isDocumentVisible(): boolean {
+    return (
+      typeof document === "undefined" || document.visibilityState === "visible"
+    );
+  }
+
+  private isWatchdogEligible(): boolean {
+    return (
+      this.authenticated &&
+      this.hostOnline &&
+      this.authoritativeRoute &&
+      this.isDocumentVisible() &&
+      this.socket?.readyState === WebSocket.OPEN
+    );
+  }
 }
diff --git a/src/server/signaling.ts b/src/server/signaling.ts
index 962ade0..a25e1eb 100644
--- a/src/server/signaling.ts
+++ b/src/server/signaling.ts
@@ -38,6 +38,7 @@ type ErrorCode = Extract<ServerMessage, { type: "error" }>["code"];
 const MAX_BUFFERED_SIGNAL_BYTES = 256 * 1024;
 const DEFAULT_MAX_SIGNAL_CONNECTIONS = 2_048;
 const DEFAULT_MAX_UNAUTHENTICATED_CONNECTIONS = 256;
+const MIN_SIGNALING_CHALLENGE_INTERVAL_MS = 1_000;
 
 interface AuthenticatedSession {
   roomId: string;
@@ -58,6 +59,7 @@ interface SocketState {
   revoked?: boolean;
   authenticating?: boolean;
   viewerPasswordUpdatePending?: boolean;
+  lastSignalingChallengeAtMs?: number;
   authenticated?: AuthenticatedSession;
 }
 
@@ -634,6 +636,29 @@ export class SignalingServer {
     message: Exclude<ClientMessage, { type: "authenticate" }>,
   ): void {
     switch (message.type) {
+      case "signaling-challenge": {
+        const state = this.socketStates.get(socket);
+        if (!state || state.authenticated !== authenticated) {
+          return;
+        }
+        const now = this.now();
+        if (state.lastSignalingChallengeAtMs !== undefined) {
+          const elapsedMs = now - state.lastSignalingChallengeAtMs;
+          if (elapsedMs < 0) {
+            state.lastSignalingChallengeAtMs = now;
+            return;
+          }
+          if (elapsedMs < MIN_SIGNALING_CHALLENGE_INTERVAL_MS) {
+            return;
+          }
+        }
+        state.lastSignalingChallengeAtMs = now;
+        this.send(socket, {
+          type: "signaling-challenge-response",
+          sequence: message.sequence,
+        });
+        return;
+      }
       case "signal":
         this.routeSignal(socket, authenticated, message);
         return;
diff --git a/src/shared/protocol.ts b/src/shared/protocol.ts
index 83fb885..9448343 100644
--- a/src/shared/protocol.ts
+++ b/src/shared/protocol.ts
@@ -536,8 +536,20 @@ const authenticateMessageSchema = z.discriminatedUnion("role", [
     .strict(),
 ]);
 
+const signalingChallengeSequenceSchema = z
+  .number()
+  .int()
+  .min(0)
+  .max(Number.MAX_SAFE_INTEGER);
+
 export const clientMessageSchema = z.union([
   authenticateMessageSchema,
+  z
+    .object({
+      type: z.literal("signaling-challenge"),
+      sequence: signalingChallengeSequenceSchema,
+    })
+    .strict(),
   z
     .object({
       type: z.literal("signal"),
@@ -680,6 +692,12 @@ const authenticatedMessageSchema = z.union([
 
 export const serverMessageSchema = z.union([
   authenticatedMessageSchema,
+  z
+    .object({
+      type: z.literal("signaling-challenge-response"),
+      sequence: signalingChallengeSequenceSchema,
+    })
+    .strict(),
   z
     .object({
       type: z.literal("peer-joined"),
diff --git a/tests/client-state.test.ts b/tests/client-state.test.ts
index 9babde2..7a42f88 100644
--- a/tests/client-state.test.ts
+++ b/tests/client-state.test.ts
@@ -40,6 +40,7 @@ import {
 } from "../src/client/webrtc/stats.ts";
 
 afterEach(() => {
+  vi.useRealTimers();
   vi.unstubAllGlobals();
 });
 
@@ -659,6 +660,251 @@ describe("client signaling recovery policy", () => {
     expect(signal.reconnect()).toBe(false);
   });
 
+  it("confirms an exact silent partition before replacing the socket immediately", () => {
+    vi.useFakeTimers();
+    let now = 0;
+    const sockets: FakeWebSocket[] = [];
+    class FakeWebSocket extends EventTarget {
+      static readonly OPEN = 1;
+      static readonly CLOSING = 2;
+      readyState = FakeWebSocket.OPEN;
+      readonly send = vi.fn();
+      readonly close = vi.fn((code?: number, reason?: string) => {
+        void code;
+        void reason;
+        this.readyState = FakeWebSocket.CLOSING;
+      });
+
+      constructor(readonly url: string) {
+        super();
+        sockets.push(this);
+      }
+    }
+    vi.stubGlobal("WebSocket", FakeWebSocket);
+    vi.stubGlobal("window", {
+      location: new URL("https://share.test/r/123456789012"),
+      setTimeout: globalThis.setTimeout,
+      clearTimeout: globalThis.clearTimeout,
+    });
+    const statuses: string[] = [];
+    const onMessage = vi.fn();
+    const onTerminated = vi.fn();
+    const signal = new SignalingClient(
+      {
+        roomId: "123456789012",
+        role: "host",
+        token: "h".repeat(43),
+        clientId: "host-client",
+      },
+      {
+        onMessage,
+        onStatus: (status) => statuses.push(status),
+        onTerminated,
+        onAccessRequired: () => undefined,
+      },
+      () => now,
+    );
+    const receive = (socket: FakeWebSocket, value: object) => {
+      const event = new Event("message");
+      Object.defineProperty(event, "data", { value: JSON.stringify(value) });
+      socket.dispatchEvent(event);
+    };
+    const authenticate = (socket: FakeWebSocket) => {
+      socket.dispatchEvent(new Event("open"));
+      receive(socket, {
+        type: "authenticated",
+        protocol: "screener-v5",
+        role: "host",
+        peerId: "host_12345678",
+        roomExpiresAt: null,
+        maxViewers: 8,
+        hostOnline: true,
+        connectionId: null,
+        viewerPeerIds: [],
+        iceConfig: { iceServers: [] },
+        viewerPolicy: "private-link",
+        viewerAuthorizationGeneration: "viewer_generation_12345678",
+      });
+    };
+    const advance = (milliseconds: number) => {
+      now += milliseconds;
+      vi.advanceTimersByTime(milliseconds);
+    };
+
+    signal.start();
+    authenticate(sockets[0]!);
+    advance(5_000);
+    const first = JSON.parse(String(sockets[0]!.send.mock.calls.at(-1)![0]));
+    expect(first).toEqual({ type: "signaling-challenge", sequence: 1 });
+
+    advance(2_000);
+    const confirm = JSON.parse(String(sockets[0]!.send.mock.calls.at(-1)![0]));
+    expect(confirm).toEqual({ type: "signaling-challenge", sequence: 2 });
+    receive(sockets[0]!, {
+      type: "signaling-challenge-response",
+      sequence: first.sequence,
+    });
+    advance(2_000);
+
+    expect(sockets).toHaveLength(2);
+    expect(sockets[0]!.close).toHaveBeenCalledWith(4002, "signaling timeout");
+    expect(statuses.at(-1)).toBe("reconnecting");
+    expect(onTerminated).not.toHaveBeenCalled();
+    expect(
+      sockets[0]!.send.mock.calls
+        .map(([value]) => JSON.parse(String(value)).type)
+        .includes("route-failed"),
+    ).toBe(false);
+
+    receive(sockets[0]!, {
+      type: "signaling-challenge-response",
+      sequence: confirm.sequence,
+    });
+    const oldClose = new Event("close");
+    Object.defineProperties(oldClose, {
+      code: { value: 4002 },
+      reason: { value: "signaling timeout" },
+    });
+    sockets[0]!.dispatchEvent(oldClose);
+    authenticate(sockets[1]!);
+    advance(5_000);
+    const healthy = JSON.parse(String(sockets[1]!.send.mock.calls.at(-1)![0]));
+    receive(sockets[1]!, {
+      type: "signaling-challenge-response",
+      sequence: healthy.sequence,
+    });
+    advance(2_000);
+
+    expect(sockets).toHaveLength(2);
+    expect(onMessage).toHaveBeenCalledTimes(2);
+    signal.stop();
+  });
+
+  it("rebases inactive, hidden, and obviously delayed challenge windows", () => {
+    vi.useFakeTimers();
+    let now = 0;
+    let visibilityState: DocumentVisibilityState = "visible";
+    const documentTarget = new EventTarget();
+    Object.defineProperty(documentTarget, "visibilityState", {
+      get: () => visibilityState,
+    });
+    const sockets: FakeWebSocket[] = [];
+    class FakeWebSocket extends EventTarget {
+      static readonly OPEN = 1;
+      static readonly CLOSING = 2;
+      readyState = FakeWebSocket.OPEN;
+      readonly send = vi.fn();
+      readonly close = vi.fn();
+
+      constructor(readonly url: string) {
+        super();
+        sockets.push(this);
+      }
+    }
+    vi.stubGlobal("WebSocket", FakeWebSocket);
+    vi.stubGlobal("document", documentTarget);
+    vi.stubGlobal("window", {
+      location: new URL("https://share.test/r/123456789012"),
+      setTimeout: globalThis.setTimeout,
+      clearTimeout: globalThis.clearTimeout,
+    });
+    const onMessage = vi.fn();
+    const signal = new SignalingClient(
+      {
+        roomId: "123456789012",
+        role: "viewer",
+        clientId: "viewer-client",
+      },
+      {
+        onMessage,
+        onStatus: () => undefined,
+        onTerminated: () => undefined,
+        onAccessRequired: () => undefined,
+      },
+      () => now,
+    );
+    const receive = (value: object) => {
+      const event = new Event("message");
+      Object.defineProperty(event, "data", { value: JSON.stringify(value) });
+      sockets[0]!.dispatchEvent(event);
+    };
+    const advance = (milliseconds: number) => {
+      now += milliseconds;
+      vi.advanceTimersByTime(milliseconds);
+    };
+
+    signal.start();
+    sockets[0]!.dispatchEvent(new Event("open"));
+    receive({
+      type: "authenticated",
+      protocol: "screener-v5",
+      role: "viewer",
+      peerId: "viewer_12345678",
+      roomExpiresAt: null,
+      maxViewers: 8,
+      hostOnline: true,
+      connectionId: "connection_12345678",
+      viewerPeerIds: [],
+      iceConfig: { iceServers: [] },
+      viewerPolicy: "private-link",
+      viewerAuthorizationGeneration: "viewer_generation_12345678",
+      mediaMode: "peer-assisted",
+      mediaAssignment: { parentPeerId: null, childPeerIds: [] },
+      routeRevision: 1,
+      routeAssignment: {
+        upstream: { kind: "none" },
+        childPeerIds: [],
+        sfuPublicationGeneration: null,
+      },
+      qualitySettings: {
+        resolution: "1080p",
+        maxFramerate: 30,
+        maxBitrate: 5_000_000,
+        degradationPreference: "balanced",
+        videoCodec: "automatic",
+        screenAudioQuality: "music",
+      },
+    });
+    advance(5_000);
+    expect(sockets[0]!.send).toHaveBeenCalledTimes(1);
+
+    receive({
+      type: "route-update",
+      revision: 2,
+      phase: "active",
+      assignment: {
+        upstream: { kind: "peer", peerId: "host_12345678" },
+        childPeerIds: [],
+        sfuPublicationGeneration: null,
+      },
+    });
+    advance(5_000);
+    const first = JSON.parse(String(sockets[0]!.send.mock.calls.at(-1)![0]));
+    expect(first.type).toBe("signaling-challenge");
+
+    visibilityState = "hidden";
+    documentTarget.dispatchEvent(new Event("visibilitychange"));
+    advance(20_000);
+    expect(sockets).toHaveLength(1);
+    expect(sockets[0]!.send).toHaveBeenCalledTimes(2);
+
+    visibilityState = "visible";
+    documentTarget.dispatchEvent(new Event("visibilitychange"));
+    advance(5_000);
+    const visible = JSON.parse(String(sockets[0]!.send.mock.calls.at(-1)![0]));
+    receive({
+      type: "signaling-challenge-response",
+      sequence: visible.sequence,
+    });
+    now += 7_000;
+    vi.advanceTimersByTime(5_000);
+    expect(sockets[0]!.send).toHaveBeenCalledTimes(3);
+    advance(5_000);
+    expect(sockets[0]!.send).toHaveBeenCalledTimes(4);
+    expect(onMessage).toHaveBeenCalledTimes(2);
+    signal.stop();
+  });
+
   it.each([
     JSON.stringify({
       type: "error",
diff --git a/tests/protocol.test.ts b/tests/protocol.test.ts
index 24f6dbd..faa9276 100644
--- a/tests/protocol.test.ts
+++ b/tests/protocol.test.ts
@@ -90,6 +90,39 @@ describe("client signaling protocol", () => {
     );
   });
 
+  it("keeps signaling challenges strict and sequence-only", () => {
+    expect(
+      clientMessageSchema.safeParse({
+        type: "signaling-challenge",
+        sequence: Number.MAX_SAFE_INTEGER,
+      }).success,
+    ).toBe(true);
+    expect(
+      serverMessageSchema.safeParse({
+        type: "signaling-challenge-response",
+        sequence: 0,
+      }).success,
+    ).toBe(true);
+    for (const invalid of [
+      { type: "signaling-challenge", sequence: -1 },
+      { type: "signaling-challenge", sequence: 1.5 },
+      {
+        type: "signaling-challenge",
+        sequence: Number.MAX_SAFE_INTEGER + 1,
+      },
+      { type: "signaling-challenge", sequence: 1, roomId },
+      { type: "signaling-challenge-response", sequence: "1" },
+      { type: "signaling-challenge-response", sequence: 1, sessionId: token },
+    ]) {
+      expect(
+        (invalid.type === "signaling-challenge"
+          ? clientMessageSchema
+          : serverMessageSchema
+        ).safeParse(invalid).success,
+      ).toBe(false);
+    }
+  });
+
   it("accepts only the fixed provisional Host lease", () => {
     expect(
       createRoomRequestSchema.parse({ viewerPolicy: "private-link" }),
diff --git a/tests/server-signal.test.ts b/tests/server-signal.test.ts
index 227ff07..2fc3761 100644
--- a/tests/server-signal.test.ts
+++ b/tests/server-signal.test.ts
@@ -1353,6 +1353,35 @@ async function closeClient(client: TestClient): Promise<void> {
 }
 
 describe("WebSocket signaling", () => {
+  it("answers only opted-in current sockets and rate-limits each socket", async () => {
+    let now = Date.now();
+    const harness = await startHarness({ now: () => now });
+    const host = await openClient(harness.webSocketUrl);
+    await authenticate(host, harness.room, "host", "challenge-host");
+
+    host.socket.send(
+      JSON.stringify({ type: "signaling-challenge", sequence: 7 }),
+    );
+    expect(await host.inbox.next("signaling-challenge-response")).toEqual({
+      type: "signaling-challenge-response",
+      sequence: 7,
+    });
+
+    host.socket.send(
+      JSON.stringify({ type: "signaling-challenge", sequence: 8 }),
+    );
+    await host.inbox.expectNone(40);
+
+    now += 1_000;
+    host.socket.send(
+      JSON.stringify({ type: "signaling-challenge", sequence: 9 }),
+    );
+    expect(await host.inbox.next("signaling-challenge-response")).toEqual({
+      type: "signaling-challenge-response",
+      sequence: 9,
+    });
+  });
+
   it("keeps the exact Native Host wire unchanged without a presence opt-in", async () => {
     const harness = await startHarness();
     const host = await openClient(harness.webSocketUrl);