diff --git a/web/workspace/src/routes/hosting/setup/+page.svelte b/web/workspace/src/routes/hosting/setup/+page.svelte
index 229311376..81b63c8c7 100644
--- a/web/workspace/src/routes/hosting/setup/+page.svelte
+++ b/web/workspace/src/routes/hosting/setup/+page.svelte
@@ -1,7 +1,7 @@
 <script lang="ts">
-  import { onDestroy } from 'svelte';
-  import { browserSession, sessionMutation } from "#lib/workspace/auth/session-state.ts";
-  import { loadWhoami, loginWithPasskey } from '#lib/workspace/auth/api.ts';
+  import { onDestroy, onMount } from 'svelte';
+  import { browserSession, observeBrowserSession, sessionMutation } from "#lib/workspace/auth/session-state.ts";
+  import { loginWithPasskey } from '#lib/workspace/auth/api.ts';
   import { enrollHostingPasskey } from '#lib/workspace/auth/hosting-enrollment.ts';
   import { loadHostingAccess, notifySessionChanged } from '#lib/workspace/auth/hosting.ts';
   import type { AuthUser } from '#lib/workspace/auth/model.ts';
@@ -14,13 +14,26 @@
   const controller = new AbortController();
   let active = true;
 
+  let verifiedGeneration: number | null = null;
+  const isCurrent = (generation: number) => active && generation === browserSession.authenticationGeneration;
   // 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。
-  async function verifySession(expected: AuthUser): Promise<boolean> {
-    const { actor } = await loadWhoami();
-    if (!active || actor?.user_id !== expected.user_id || actor.account_id !== expected.account_id || actor.auth_method !== 'browser_session') return false;
-    const access = await loadHostingAccess();
-    return active && access.is_admin;
+  function matchesSession(expected: AuthUser): boolean {
+    const snapshot = browserSession.snapshot;
+    if (snapshot.kind !== 'ready' || snapshot.value.session.kind !== 'authenticated') return false;
+    const actor = snapshot.value.actor;
+    return actor?.user_id === expected.user_id && actor.account_id === expected.account_id && actor.auth_method === 'browser_session';
   }
+  onMount(() => {
+    observeBrowserSession();
+    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)) {
+        verifiedLogin = false;
+        verifiedGeneration = null;
+      }
+    });
+  });
 
   async function enroll() {
     if (busy || user || !token.trim()) return;
@@ -47,20 +60,35 @@
     busy = true;
     error = '';
     sessionMutation();
+    let generation = browserSession.authenticationGeneration;
     try {
       const signedIn = await loginWithPasskey(controller.signal);
-      if (!active) return;
-      if (signedIn.user_id !== expected.user_id || signedIn.account_id !== expected.account_id || !await verifySession(expected)) {
-        throw new Error('Login or hosting access not verified');
+      if (!isCurrent(generation)) throw new Error('Login superseded');
+      if (signedIn.user_id !== expected.user_id || signedIn.account_id !== expected.account_id) {
+        throw new Error('Unexpected login actor');
       }
-      if (!active) return;
+      // Set-Cookie後の確認を共有状態へ発行し、操作前/操作中のreadを再利用しない。
+      sessionMutation();
+      generation = browserSession.authenticationGeneration;
+      await browserSession.refresh();
+      if (!isCurrent(generation) || !matchesSession(expected)) throw new Error('Session not verified');
+      const access = await loadHostingAccess();
+      if (!isCurrent(generation) || !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)) throw new Error('Session superseded');
+      verifiedGeneration = generation;
       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)) {
+        sessionMutation();
+        await browserSession.refresh();
+      }
     } finally {
-      sessionMutation();
-      await browserSession.refresh();
       if (active) busy = false;
     }
   }
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 467d4fc70..8f85ab28c 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
@@ -8,6 +8,11 @@ import {
 } from "@testing-library/svelte";
 import { afterEach, expect, test, vi } from "vitest";
 import Setup from "./+page.svelte";
+import {
+  browserSession,
+  observeBrowserSession,
+} from "#lib/workspace/auth/session-state.ts";
+import { SESSION_CHANGED_EVENT } from "#lib/workspace/auth/hosting.ts";
 import {
   authUser,
   credentialFixture,
@@ -17,6 +22,7 @@ import {
 
 afterEach(() => {
   cleanup();
+  browserSession.invalidate();
   vi.unstubAllGlobals();
   vi.restoreAllMocks();
 });
@@ -268,3 +274,150 @@ test("API token identity is not mistaken for a verified browser session", async
   expect(screen.queryByRole("link", { name: "Open hosting settings" }))
     .toBeNull();
 });
+
+function gate<T>() {
+  let resolve!: (value: T) => void;
+  const promise = new Promise<T>((done) => {
+    resolve = done;
+  });
+  return { promise, resolve };
+}
+const setupActor = { ...authUser, auth_method: "browser_session" };
+const replacedActor = {
+  ...setupActor,
+  user_id: "other-user",
+  account_id: "other-account",
+  handle: "bob",
+};
+function identityResponse(actor: typeof setupActor | null) {
+  return Response.json({
+    actor,
+    session: actor
+      ? { kind: "authenticated", expires_at: "2099-01-01T00:00:00Z" }
+      : { kind: "anonymous" },
+  });
+}
+async function changedInAnotherTab(actor: typeof setupActor | null) {
+  window.dispatchEvent(
+    new StorageEvent("storage", {
+      key: "yoi.browser-auth-generation",
+      newValue: JSON.stringify({
+        id: crypto.randomUUID(),
+        reason: actor ? "authentication" : "logout",
+      }),
+    }),
+  );
+  await browserSession.refresh();
+}
+for (const phase of ["login", "whoami", "access", "confirmation"] as const) {
+  for (const replacement of [null, replacedActor]) {
+    test(`newer ${replacement ? "actor" : "logout"} while setup ${phase} is pending cannot publish old verified success`, async () => {
+      const { fetcher } = boundary();
+      const original = fetcher.getMockImplementation()!;
+      const started = gate<void>();
+      const pending = gate<Response>();
+      let current: typeof setupActor | null = setupActor;
+      let reads = 0;
+      fetcher.mockImplementation((input, init) => {
+        const path = String(input);
+        if (path === "/api/auth/whoami") {
+          ++reads;
+          if (
+            (phase === "whoami" && reads === 1) ||
+            (phase === "confirmation" && reads === 2)
+          ) {
+            started.resolve();
+            return pending.promise;
+          }
+          return Promise.resolve(identityResponse(current));
+        }
+        if (
+          (phase === "access" && path === "/api/hosting/access") ||
+          (phase === "login" && path === "/api/auth/passkeys/login/complete")
+        ) {
+          started.resolve();
+          return pending.promise;
+        }
+        return original(input, init);
+      });
+      observeBrowserSession();
+      render(Setup);
+      await enroll();
+      await fireEvent.click(
+        await screen.findByRole("button", { name: "Sign in with new passkey" }),
+      );
+      await started.promise;
+      const notify = vi.fn();
+      window.addEventListener(SESSION_CHANGED_EVENT, notify);
+      try {
+        current = replacement;
+        await changedInAnotherTab(current);
+        const acceptedGeneration = browserSession.authenticationGeneration;
+        pending.resolve(
+          phase === "access"
+            ? Response.json({ is_admin: true })
+            : phase === "login"
+            ? Response.json({ user: authUser })
+            : identityResponse(setupActor),
+        );
+        await waitFor(() =>
+          expect(
+            (screen.getByRole("button", {
+              name: "Sign in with new passkey",
+            }) as HTMLButtonElement).disabled,
+          ).toBe(false)
+        );
+        expect(screen.queryByRole("link", { name: "Open hosting settings" }))
+          .toBeNull();
+        expect(
+          screen.queryByText(
+            "Passkey login and hosting administrator access verified.",
+          ),
+        ).toBeNull();
+        expect(notify).not.toHaveBeenCalled();
+        expect(browserSession.authenticationGeneration).toBe(
+          acceptedGeneration,
+        );
+        expect(browserSession.snapshot).toMatchObject({
+          kind: "ready",
+          value: { actor: current },
+        });
+      } finally {
+        window.removeEventListener(SESSION_CHANGED_EVENT, notify);
+      }
+    });
+  }
+}
+for (const replacement of [null, replacedActor]) {
+  test(`setup verified success is removed on subsequent ${replacement ? "actor replacement" : "logout"}`, async () => {
+    const { fetcher } = boundary();
+    observeBrowserSession();
+    render(Setup);
+    await enroll();
+    await fireEvent.click(
+      await screen.findByRole("button", { name: "Sign in with new passkey" }),
+    );
+    await screen.findByRole("link", { name: "Open hosting settings" });
+    const original = fetcher.getMockImplementation()!;
+    fetcher.mockImplementation((input, init) =>
+      String(input) === "/api/auth/whoami"
+        ? Promise.resolve(identityResponse(replacement))
+        : original(input, init)
+    );
+    await changedInAnotherTab(replacement);
+    await waitFor(() =>
+      expect(screen.queryByRole("link", { name: "Open hosting settings" }))
+        .toBeNull()
+    );
+    expect(
+      screen.queryByText(
+        "Passkey login and hosting administrator access verified.",
+      ),
+    ).toBeNull();
+    expect(
+      (screen.getByRole("button", {
+        name: "Sign in with new passkey",
+      }) as HTMLButtonElement).disabled,
+    ).toBe(false);
+  });
+}