diff --git a/web/workspace/src/lib/workspace/auth/activation.test.ts b/web/workspace/src/lib/workspace/auth/activation.test.ts
index 9148314e2..de5562a3b 100644
--- a/web/workspace/src/lib/workspace/auth/activation.test.ts
+++ b/web/workspace/src/lib/workspace/auth/activation.test.ts
@@ -319,3 +319,74 @@ for (const stage of ["prompt", "completion", "confirmation"] as const) {
     },
   );
 }
+
+for (
+  const phase of ["options", "prompt", "complete", "confirmation"] as const
+) {
+  for (const reason of ["logout", "authentication"] as const) {
+    Deno.test(`activation fences shared ${reason} stamp during ${phase} before event delivery`, async () => {
+      const previous = Object.getOwnPropertyDescriptor(
+        globalThis,
+        "localStorage",
+      );
+      let stamp = "initial-operation-stamp";
+      Object.defineProperty(globalThis, "localStorage", {
+        configurable: true,
+        value: { getItem: () => stamp },
+      });
+      const generation = browserSession.authenticationGeneration;
+      const paths: string[] = [];
+      try {
+        await boundary(async (input) => {
+          const path = String(input);
+          paths.push(path);
+          if (
+            (phase === "options" && path.endsWith("/options")) ||
+            (phase === "complete" && path.endsWith("/complete")) ||
+            (phase === "confirmation" && path === "/api/auth/whoami")
+          ) {
+            stamp = JSON.stringify({ id: "other-tab", reason });
+          }
+          return Response.json(
+            path.endsWith("/options")
+              ? registrationOptions
+              : path.endsWith("/complete")
+              ? {
+                user: authUser,
+                authentication_reference: "fixture-authentication",
+              }
+              : confirmedSession(),
+          );
+        }, async () => {
+          if (phase === "prompt") {
+            stamp = JSON.stringify({ id: "other-tab", reason });
+          }
+          return credentialFixture();
+        }, async () => {
+          await rejects(
+            activateWithPasskey(
+              "alice",
+              "synthetic-input-only",
+              new AbortController().signal,
+            ),
+          );
+          equal(browserSession.authenticationGeneration, generation);
+          equal(stamp === "initial-operation-stamp", false);
+          equal(
+            paths.filter((path) => path.endsWith("/complete")).length,
+            phase === "complete" || phase === "confirmation" ? 1 : 0,
+          );
+          equal(paths.includes("/api/auth/whoami"), phase === "confirmation");
+        });
+      } finally {
+        if (previous) {
+          Object.defineProperty(
+            globalThis,
+            "localStorage",
+            previous,
+          );
+        } else Reflect.deleteProperty(globalThis, "localStorage");
+      }
+    });
+  }
+}
diff --git a/web/workspace/src/lib/workspace/auth/activation.ts b/web/workspace/src/lib/workspace/auth/activation.ts
index 6cd963fb5..a17550fd9 100644
--- a/web/workspace/src/lib/workspace/auth/activation.ts
+++ b/web/workspace/src/lib/workspace/auth/activation.ts
@@ -2,7 +2,7 @@ import type {
   ActivationOptionsRequest,
   PasskeyRegistrationCompleteRequest,
 } from "#lib/generated/auth-api.ts";
-import { readBoundedAuthResponseJson, loadWhoami } from "./api.ts";
+import { loadWhoami, readBoundedAuthResponseJson } from "./api.ts";
 import { browserSession } from "./session-state.ts";
 import { normalizeActivationHandle } from "./hosting-accounts.ts";
 import {
@@ -21,6 +21,15 @@ export async function activateWithPasskey(
   signal: AbortSignal,
 ): Promise<AuthUser> {
   const generation = browserSession.authenticationGeneration;
+  const stamp = browserSession.authenticationStamp;
+  // 別tabのstorage event配送より先にcredential/応答が返っても送信・適用しない。
+  const assertCurrent = () => {
+    signal.throwIfAborted();
+    if (
+      generation !== browserSession.authenticationGeneration ||
+      stamp !== browserSession.authenticationStamp
+    ) throw new Error("Activation superseded");
+  };
   try {
     handle = normalizeActivationHandle(handle);
     if (window.isSecureContext !== true || !code || code.length > 4096) {
@@ -30,7 +39,7 @@ export async function activateWithPasskey(
       path: "/api/auth/activation/options" | "/api/auth/activation/complete",
       body: ActivationOptionsRequest | PasskeyRegistrationCompleteRequest,
     ): Promise<unknown> => {
-      signal.throwIfAborted();
+      assertCurrent();
       const response = await fetch(path, {
         method: "POST",
         credentials: "same-origin",
@@ -46,7 +55,9 @@ export async function activateWithPasskey(
         await response.body?.cancel();
         throw new Error("Activation rejected");
       }
-      return await readBoundedAuthResponseJson(response);
+      const result = await readBoundedAuthResponseJson(response);
+      assertCurrent();
+      return result;
     };
     const options = await request("/api/auth/activation/options", {
       handle,
@@ -56,25 +67,32 @@ export async function activateWithPasskey(
     code = "";
     const publicKey = prepareRegistrationOptions(options);
     const challengeId = (options as { challenge_id: string }).challenge_id;
-    signal.throwIfAborted();
+    assertCurrent();
     const credential = await navigator.credentials.create({
       publicKey,
       signal,
     });
     if (!credential) throw new Error("Registration cancelled");
-    signal.throwIfAborted();
-    if (generation !== browserSession.authenticationGeneration) throw new Error("Activation superseded");
+    assertCurrent();
     const result = parseAuthUserResponse(
       await request("/api/auth/activation/complete", {
         challenge_id: challengeId,
         credential: registrationCredentialToJson(credential),
       }),
     );
-    signal.throwIfAborted();
-    if (result.user.handle !== handle || generation !== browserSession.authenticationGeneration) throw new Error("Activation target mismatch");
+    assertCurrent();
+    if (result.user.handle !== handle) {
+      throw new Error("Activation target mismatch");
+    }
     const current = await loadWhoami(signal);
-    signal.throwIfAborted();
-    if (generation !== browserSession.authenticationGeneration || current.session.kind !== "authenticated" || current.actor?.auth_method !== "browser_session" || current.actor.user_id !== result.user.user_id || current.actor.account_id !== result.user.account_id || current.authentication_reference !== result.authentication_reference) throw new Error("Activation session unconfirmed");
+    assertCurrent();
+    if (
+      current.session.kind !== "authenticated" ||
+      current.actor?.auth_method !== "browser_session" ||
+      current.actor.user_id !== result.user.user_id ||
+      current.actor.account_id !== result.user.account_id ||
+      current.authentication_reference !== result.authentication_reference
+    ) throw new Error("Activation session unconfirmed");
     return result.user;
   } catch {
     // HTTP/WebAuthn/JSON例外はsecretを含む可能性があるため、本文も例外も転載しない。
diff --git a/web/workspace/src/routes/activate/+page.svelte b/web/workspace/src/routes/activate/+page.svelte
index 84eaaaefb..9c864169d 100644
--- a/web/workspace/src/routes/activate/+page.svelte
+++ b/web/workspace/src/routes/activate/+page.svelte
@@ -3,6 +3,7 @@
   import { activateWithPasskey } from '#lib/workspace/auth/activation.ts';
   import { normalizeActivationHandle } from '#lib/workspace/auth/hosting-accounts.ts';
   import { notifySessionChanged } from '#lib/workspace/auth/hosting.ts';
+  import { browserSession } from '#lib/workspace/auth/session-state.ts';
   import type { AuthUser } from '#lib/workspace/auth/model.ts';
 
   let handle = $state('');
@@ -19,12 +20,15 @@
     try { handle = normalizeActivationHandle(handle); }
     catch { error = 'Check the account ID supplied by your administrator.'; return; }
     const current = ++generation;
+    const authGeneration = browserSession.authenticationGeneration;
+    const authStamp = browserSession.authenticationStamp;
     controller = new AbortController(); busy = true; error = '';
     const pending = activateWithPasskey(handle, code, controller.signal);
     code = '';
     try {
       const confirmed = await pending;
       if (current !== generation) return;
+      if (authGeneration !== browserSession.authenticationGeneration || authStamp !== browserSession.authenticationStamp) throw new Error('Activation superseded');
       user = confirmed;
       notifySessionChanged();
     } catch {
diff --git a/web/workspace/src/routes/activate/activation.browser.spec.ts b/web/workspace/src/routes/activate/activation.browser.spec.ts
index 7073c54e4..6df8f1490 100644
--- a/web/workspace/src/routes/activate/activation.browser.spec.ts
+++ b/web/workspace/src/routes/activate/activation.browser.spec.ts
@@ -236,3 +236,45 @@ test("logout during the activation prompt fences a late credential before comple
   expect(screen.queryByText(/You are signed in/)).toBeNull();
   expect(screen.queryByRole("link", { name: "Open Account" })).toBeNull();
 });
+
+for (const phase of ["prompt", "confirmation"] as const) {
+  for (const reason of ["logout", "authentication"] as const) {
+    test(`activation UI rejects shared ${reason} stamp during ${phase} before event delivery`, async () => {
+      const { create, fetcher } = boundary("success");
+      const started = gate<void>();
+      const pending = gate<void>();
+      const request = fetcher.getMockImplementation()!;
+      if (phase === "prompt") {
+        create.mockImplementation(async () => {
+          started.resolve();
+          await pending.promise;
+          return credentialFixture();
+        });
+      } else {fetcher.mockImplementation(async (input, init) => {
+          if (String(input) === "/api/auth/whoami") {
+            started.resolve();
+            await pending.promise;
+          }
+          return await request(input, init);
+        });}
+      render(Activate);
+      await submit();
+      await started.promise;
+      const generation = browserSession.authenticationGeneration;
+      localStorage.setItem(
+        "yoi.browser-auth-generation",
+        JSON.stringify({ id: crypto.randomUUID(), reason }),
+      );
+      expect(browserSession.authenticationGeneration).toBe(generation);
+      pending.resolve();
+      await screen.findByRole("alert");
+      expect(screen.queryByRole("link", { name: "Open Account" })).toBeNull();
+      expect(screen.queryByText(/You are signed in/)).toBeNull();
+      expect(
+        fetcher.mock.calls.filter(([path]) =>
+          path === "/api/auth/activation/complete"
+        ),
+      ).toHaveLength(phase === "prompt" ? 0 : 1);
+    });
+  }
+}
diff --git a/web/workspace/src/routes/hosting/setup/+page.svelte b/web/workspace/src/routes/hosting/setup/+page.svelte
index db24aa040..03c621a53 100644
--- a/web/workspace/src/routes/hosting/setup/+page.svelte
+++ b/web/workspace/src/routes/hosting/setup/+page.svelte
@@ -17,7 +17,8 @@
   let active = true;
 
   let verifiedGeneration: number | null = null;
-  const isCurrent = (generation: number) => active && generation === browserSession.authenticationGeneration;
+  let verifiedStamp: string | null = null;
+  const isCurrent = (generation: number, stamp: string | null) => active && generation === browserSession.authenticationGeneration && stamp === browserSession.authenticationStamp;
   // 初期Bearerでの鍵追加はcookieを使わない。明示Passkey loginのsessionを検証する。
   function matchesSession(expected: AuthUser): boolean {
     const snapshot = browserSession.snapshot;
@@ -30,9 +31,10 @@
     return browserSession.subscribe(() => {
       // 公開pageにはprivate SessionBoundaryの取消がない。成功表示も確認した世代に限る。
       // 後続の再確認/失効/主体変更では明示loginとaccess確認からやり直す。
-      if (!user || verifiedGeneration !== browserSession.authenticationGeneration || !matchesSession(user)) {
+      if (!user || verifiedGeneration !== browserSession.authenticationGeneration || verifiedStamp !== browserSession.authenticationStamp || !matchesSession(user)) {
         verifiedLogin = false;
         verifiedGeneration = null;
+        verifiedStamp = null;
       }
     });
   });
@@ -63,26 +65,27 @@
     error = '';
     sessionMutation();
     let generation = browserSession.authenticationGeneration;
+    let stamp = browserSession.authenticationStamp;
     const operation = new AbortController();
     loginController = operation;
     let promptGeneration: number | null = generation;
     const unsubscribe = browserSession.subscribe(() => {
-      if (promptGeneration !== null && promptGeneration !== browserSession.authenticationGeneration) operation.abort();
+      if (promptGeneration !== null && !isCurrent(promptGeneration, stamp)) operation.abort();
     });
     try {
       const initial = await loadWhoami(operation.signal);
-      if (!isCurrent(generation)) throw new Error('Login superseded');
+      if (!isCurrent(generation, stamp)) throw new Error('Login superseded');
       observeAuthOperation(operation.signal, {
         expected: initial, phase: () => {}, additionAttempt: () => {},
         verifySession: async () => {
-          if (!isCurrent(generation)) operation.abort();
+          if (!isCurrent(generation, stamp)) operation.abort();
           operation.signal.throwIfAborted();
           const current = await loadWhoami(operation.signal);
-          if (!isCurrent(generation) || initial.actor?.user_id !== current.actor?.user_id || initial.actor?.account_id !== current.actor?.account_id || initial.authentication_reference !== current.authentication_reference) throw new Error('Session changed');
+          if (!isCurrent(generation, stamp) || initial.actor?.user_id !== current.actor?.user_id || initial.actor?.account_id !== current.actor?.account_id || initial.authentication_reference !== current.authentication_reference) throw new Error('Session changed');
         },
       });
       const signedIn = await loginWithPasskey(operation.signal);
-      if (!isCurrent(generation)) throw new Error('Login superseded');
+      if (!isCurrent(generation, stamp)) throw new Error('Login superseded');
       if (signedIn.user.user_id !== expected.user_id || signedIn.user.account_id !== expected.account_id) {
         throw new Error('Unexpected login actor');
       }
@@ -90,21 +93,23 @@
       promptGeneration = null;
       sessionMutation();
       generation = browserSession.authenticationGeneration;
+      stamp = browserSession.authenticationStamp;
       await browserSession.refresh();
-      if (!isCurrent(generation) || !matchesSession(expected) || browserSession.snapshot.kind !== 'ready' || browserSession.snapshot.value.authentication_reference !== signedIn.authentication_reference) throw new Error('Session not verified');
+      if (!isCurrent(generation, stamp) || !matchesSession(expected) || browserSession.snapshot.kind !== 'ready' || browserSession.snapshot.value.authentication_reference !== signedIn.authentication_reference) throw new Error('Session not verified');
       const access = await loadHostingAccess();
-      if (!isCurrent(generation) || !access.is_admin) throw new Error('Access not verified');
+      if (!isCurrent(generation, stamp) || !access.is_admin) throw new Error('Access not verified');
       // accessのawait中にもcookie/actorは変わり得る。success公開前の最終確認を行う。
       await browserSession.refresh();
-      if (!isCurrent(generation) || !matchesSession(expected) || browserSession.snapshot.kind !== 'ready' || browserSession.snapshot.value.authentication_reference !== signedIn.authentication_reference) throw new Error('Session superseded');
+      if (!isCurrent(generation, stamp) || !matchesSession(expected) || browserSession.snapshot.kind !== 'ready' || browserSession.snapshot.value.authentication_reference !== signedIn.authentication_reference) throw new Error('Session superseded');
       verifiedGeneration = generation;
+      verifiedStamp = stamp;
       verifiedLogin = true;
       notifySessionChanged();
     } catch {
       if (active) error = 'Passkey login or hosting administrator access could not be verified. Try signing in again, or ask the host operator to check this account.';
       // 応答不明時の再確認は自身がまだ現世代の時だけ。古いfinallyで新logout/loginを
       // invalidateしたり、その確定結果を再取得のloadingへ戻したりしない。
-      if (isCurrent(generation)) {
+      if (isCurrent(generation, stamp)) {
         sessionMutation();
         await browserSession.refresh();
       }
diff --git a/web/workspace/src/routes/hosting/setup/hosting-setup.browser.spec.ts b/web/workspace/src/routes/hosting/setup/hosting-setup.browser.spec.ts
index d83532a16..41d5f6459 100644
--- a/web/workspace/src/routes/hosting/setup/hosting-setup.browser.spec.ts
+++ b/web/workspace/src/routes/hosting/setup/hosting-setup.browser.spec.ts
@@ -467,33 +467,151 @@ for (const replacement of [null, replacedActor]) {
 
 test("setup logout during native login prompt aborts and never submits the late credential", async () => {
   const { get, fetcher } = boundary();
-  const prompted = gate<void>(); const credential = gate<Credential>();
-  get.mockImplementation(() => { prompted.resolve(); return credential.promise; });
-  render(Setup); await enroll();
-  await fireEvent.click(await screen.findByRole("button", { name: "Sign in with new passkey" }));
+  const prompted = gate<void>();
+  const credential = gate<Credential>();
+  get.mockImplementation(() => {
+    prompted.resolve();
+    return credential.promise;
+  });
+  render(Setup);
+  await enroll();
+  await fireEvent.click(
+    await screen.findByRole("button", { name: "Sign in with new passkey" }),
+  );
   await prompted.promise;
   await changedInAnotherTab(null);
-  const signal = (get.mock.calls[0] as unknown as [CredentialRequestOptions])[0].signal!;
+  const signal = (get.mock.calls[0] as unknown as [CredentialRequestOptions])[0]
+    .signal!;
   expect(signal.aborted).toBe(true);
   credential.resolve(credentialFixture(true));
   await screen.findByRole("alert");
-  expect(fetcher.mock.calls.some(([path]) => path === "/api/auth/passkeys/login/complete")).toBe(false);
-  expect(screen.queryByRole("link", { name: "Open hosting settings" })).toBeNull();
+  expect(
+    fetcher.mock.calls.some(([path]) =>
+      path === "/api/auth/passkeys/login/complete"
+    ),
+  ).toBe(false);
+  expect(screen.queryByRole("link", { name: "Open hosting settings" }))
+    .toBeNull();
 });
 
 test("setup final confirmation rejects replacement session for the same owner", async () => {
-  const { fetcher } = boundary(); const request = fetcher.getMockImplementation()!;
+  const { fetcher } = boundary();
+  const request = fetcher.getMockImplementation()!;
   let accessRead = false;
   fetcher.mockImplementation(async (input, init) => {
     if (String(input) === "/api/hosting/access") accessRead = true;
-    if (String(input) === "/api/auth/whoami" && accessRead) return Response.json({
-      actor: setupActor, authentication_reference: "same-owner-different-session",
-      session: { kind: "authenticated", expires_at: "2099-01-01T00:00:00Z" },
-    });
+    if (String(input) === "/api/auth/whoami" && accessRead) {
+      return Response.json({
+        actor: setupActor,
+        authentication_reference: "same-owner-different-session",
+        session: { kind: "authenticated", expires_at: "2099-01-01T00:00:00Z" },
+      });
+    }
     return await request(input, init);
   });
-  render(Setup); await enroll();
-  await fireEvent.click(await screen.findByRole("button", { name: "Sign in with new passkey" }));
+  render(Setup);
+  await enroll();
+  await fireEvent.click(
+    await screen.findByRole("button", { name: "Sign in with new passkey" }),
+  );
   await screen.findByRole("alert");
-  expect(screen.queryByRole("link", { name: "Open hosting settings" })).toBeNull();
+  expect(screen.queryByRole("link", { name: "Open hosting settings" }))
+    .toBeNull();
 });
+
+for (
+  const phase of [
+    "initial",
+    "options",
+    "prompt",
+    "pre-complete",
+    "complete",
+    "whoami",
+    "access",
+    "confirmation",
+  ] as const
+) {
+  for (const reason of ["logout", "authentication"] as const) {
+    test(`setup fences shared ${reason} stamp during ${phase} before event delivery`, async () => {
+      const { fetcher, get } = boundary();
+      const request = fetcher.getMockImplementation()!;
+      const started = gate<void>();
+      const pending = gate<void>();
+      let optionsRead = false,
+        promptReturned = false,
+        completionSent = false,
+        accessRead = false,
+        held = false;
+      const hold = async () => {
+        held = true;
+        started.resolve();
+        await pending.promise;
+      };
+      get.mockImplementation(async () => {
+        if (phase === "prompt") await hold();
+        promptReturned = true;
+        return credentialFixture(true);
+      });
+      fetcher.mockImplementation(async (input, init) => {
+        if (new Headers(init?.headers).has("authorization")) {
+          return await request(input, init);
+        }
+        const path = String(input);
+        const point = path === "/api/auth/whoami"
+          ? !optionsRead
+            ? "initial"
+            : !completionSent && promptReturned
+            ? "pre-complete"
+            : accessRead
+            ? "confirmation"
+            : "whoami"
+          : path.endsWith("/login/options")
+          ? "options"
+          : path.endsWith("/login/complete")
+          ? "complete"
+          : path === "/api/hosting/access"
+          ? "access"
+          : null;
+        if (point === "options") optionsRead = true;
+        if (point === "complete") completionSent = true;
+        if (point === "access") accessRead = true;
+        const response = await request(input, init);
+        if (!held && point === phase) await hold();
+        return response;
+      });
+      render(Setup);
+      await enroll();
+      await fireEvent.click(
+        await screen.findByRole("button", { name: "Sign in with new passkey" }),
+      );
+      await started.promise;
+      const generation = browserSession.authenticationGeneration;
+      const stamp = JSON.stringify({ id: crypto.randomUUID(), reason });
+      // 同一documentのsetItemはstorage eventを配送しない。generationを直接変えず順序を固定する。
+      localStorage.setItem("yoi.browser-auth-generation", stamp);
+      expect(browserSession.authenticationGeneration).toBe(generation);
+      pending.resolve();
+      await screen.findByRole("alert");
+      await waitFor(() =>
+        expect(
+          (screen.getByRole("button", {
+            name: "Sign in with new passkey",
+          }) as HTMLButtonElement).disabled,
+        ).toBe(false)
+      );
+      expect(
+        fetcher.mock.calls.filter(([path]) =>
+          path === "/api/auth/passkeys/login/complete"
+        ),
+      ).toHaveLength(
+        ["complete", "whoami", "access", "confirmation"].includes(phase)
+          ? 1
+          : 0,
+      );
+      expect(screen.queryByRole("link", { name: "Open hosting settings" }))
+        .toBeNull();
+      // 古いcatchが新tabのstampを自身のsessionMutationで上書きしない。
+      expect(browserSession.authenticationStamp).toBe(stamp);
+    });
+  }
+}