diff --git a/src/bridge/auth-login.html b/src/bridge/auth-login.html
index 50d6170..0832257 100644
--- a/src/bridge/auth-login.html
+++ b/src/bridge/auth-login.html
@@ -199,6 +199,34 @@
       line-height: 1.5;
     }
 
+    .auth-error {
+      background: #fef2f2;
+      border: 1px solid #fecaca;
+      color: #b91c1c;
+      border-radius: 8px;
+      padding: 10px 14px;
+      font-size: 0.8125rem;
+      line-height: 1.5;
+      margin-bottom: 16px;
+    }
+
+    .view-toggle {
+      margin-top: 16px;
+      font-size: 0.8125rem;
+      color: #64748b;
+      text-align: center;
+    }
+
+    .view-toggle a {
+      color: #4f46e5;
+      font-weight: 600;
+      text-decoration: none;
+    }
+
+    .view-toggle a:hover {
+      text-decoration: underline;
+    }
+
     /* RESPONSIVE */
     @media (max-width: 768px) {
       .container {
@@ -261,6 +289,20 @@
       .footer-note {
         color: #475569;
       }
+
+      .auth-error {
+        background: rgba(239, 68, 68, 0.12);
+        border-color: rgba(239, 68, 68, 0.35);
+        color: #fca5a5;
+      }
+
+      .view-toggle {
+        color: #94a3b8;
+      }
+
+      .view-toggle a {
+        color: #818cf8;
+      }
     }
   </style>
 </head>
@@ -291,9 +333,12 @@
     <!-- Right Panel -->
     <div class="right-panel">
       <div class="login-box">
-        <h2>ログイン</h2>
-        <p class="subtitle">アカウントでサインインしてください</p>
+        <h2 id="view-title">ログイン</h2>
+        <p class="subtitle" id="view-subtitle">アカウントでサインインしてください</p>
 
+        <div id="auth-error" class="auth-error" style="display:none;"></div>
+
+        <div id="view-login">
         <!-- GOOGLE_BUTTON_START -->
         <!-- Google OAuth Button -->
         <a href="/auth/google" class="oauth-button oauth-button-google">
@@ -339,16 +384,29 @@
           </button>
         </form>
         <!-- LOCAL_SIGNUP_START -->
-        <div class="divider">アカウントが無い場合</div>
-        <form method="post" action="/auth/local/signup" style="display:flex;flex-direction:column;gap:10px;">
-          <input type="text" name="email" placeholder="ログインID" required autocomplete="username"
-                 style="padding:11px 12px;border:1px solid #d0d5dd;border-radius:8px;font-size:14px;width:100%;box-sizing:border-box;">
-          <input type="password" name="password" placeholder="パスワード（8文字以上）" minlength="8" required autocomplete="new-password"
-                 style="padding:11px 12px;border:1px solid #d0d5dd;border-radius:8px;font-size:14px;width:100%;box-sizing:border-box;">
-          <button type="submit" class="oauth-button" style="justify-content:center;background:#fff;color:#111827;border:1px solid #d0d5dd;cursor:pointer;">
-            新規登録（管理者の承認後に利用可）
-          </button>
-        </form>
+        <p class="view-toggle">
+          アカウントが無い場合は <a href="#signup" data-show-view="signup">新規登録</a>
+        </p>
+        <!-- LOCAL_SIGNUP_END -->
+        <!-- LOCAL_FORM_END -->
+        </div><!-- /view-login -->
+
+        <!-- LOCAL_FORM_START -->
+        <!-- LOCAL_SIGNUP_START -->
+        <div id="view-signup" style="display:none;">
+          <form method="post" action="/auth/local/signup" style="display:flex;flex-direction:column;gap:10px;">
+            <input type="text" name="email" placeholder="ログインID" required autocomplete="username"
+                   style="padding:11px 12px;border:1px solid #d0d5dd;border-radius:8px;font-size:14px;width:100%;box-sizing:border-box;">
+            <input type="password" name="password" placeholder="パスワード（8文字以上）" minlength="8" required autocomplete="new-password"
+                   style="padding:11px 12px;border:1px solid #d0d5dd;border-radius:8px;font-size:14px;width:100%;box-sizing:border-box;">
+            <button type="submit" class="oauth-button" style="justify-content:center;background:#111827;color:#fff;border:none;cursor:pointer;">
+              新規登録する
+            </button>
+          </form>
+          <p class="view-toggle">
+            <a href="#" data-show-view="login">← ログインに戻る</a>
+          </p>
+        </div><!-- /view-signup -->
         <!-- LOCAL_SIGNUP_END -->
         <!-- LOCAL_FORM_END -->
 
@@ -358,5 +416,58 @@
       </div>
     </div>
   </div>
+  <script>
+    (function () {
+      var loginView = document.getElementById('view-login');
+      var signupView = document.getElementById('view-signup');
+      var title = document.getElementById('view-title');
+      var subtitle = document.getElementById('view-subtitle');
+
+      var ERRORS = {
+        invalid: '入力内容が正しくありません。',
+        credentials: 'ログインID またはパスワードが違います。',
+        disabled: 'このアカウントは無効化されています。管理者にお問い合わせください。',
+        signup: '登録に失敗しました。このログインID は既に使われている可能性があります。',
+        weak: 'パスワードは8文字以上で設定してください。'
+      };
+
+      function show(view) {
+        var isSignup = view === 'signup' && !!signupView;
+        if (loginView) loginView.style.display = isSignup ? 'none' : '';
+        if (signupView) signupView.style.display = isSignup ? '' : 'none';
+        if (title) title.textContent = isSignup ? '新規登録' : 'ログイン';
+        if (subtitle) {
+          subtitle.textContent = isSignup
+            ? '登録後、管理者の承認を経て利用できます'
+            : 'アカウントでサインインしてください';
+        }
+        if (history.replaceState) {
+          history.replaceState(null, '',
+            location.pathname + location.search + (isSignup ? '#signup' : ''));
+        }
+      }
+
+      var toggles = document.querySelectorAll('[data-show-view]');
+      for (var i = 0; i < toggles.length; i++) {
+        (function (el) {
+          el.addEventListener('click', function (e) {
+            e.preventDefault();
+            show(el.getAttribute('data-show-view'));
+          });
+        })(toggles[i]);
+      }
+
+      var err = new URLSearchParams(location.search).get('error');
+      if (err && ERRORS[err]) {
+        var box = document.getElementById('auth-error');
+        if (box) {
+          box.textContent = ERRORS[err];
+          box.style.display = '';
+        }
+      }
+      // signup 由来のエラー、または #signup 直リンクは登録ビューを開く
+      if (location.hash === '#signup' || err === 'signup' || err === 'weak') show('signup');
+    })();
+  </script>
 </body>
 </html>
diff --git a/src/gateway/router.test.ts b/src/gateway/router.test.ts
index 9360e23..0188a10 100644
--- a/src/gateway/router.test.ts
+++ b/src/gateway/router.test.ts
@@ -104,6 +104,67 @@ describe('createRouter.pick', () => {
     expect(counts).toEqual({ a: 2, b: 2, c: 2 });
   });
 
+  // ── Sticky routing (x-aao-preferred-backend) ────────────────────────────
+
+  it('honors the preferred backend even when another is idler', () => {
+    const r = createRouter({
+      getBackends: () => [bk('a', 'qwen3:8b'), bk('b', 'qwen3:8b')],
+      registry: makeRegistry([
+        status('a', true, 3, 4), // busier, but preferred (KV cache lives here)
+        status('b', true, 0, 4), // idler
+      ]),
+    });
+    expect(r.pick('qwen3:8b', 'a')?.id).toBe('a');
+  });
+
+  it('falls back to normal ranking when the preferred backend is saturated', () => {
+    const r = createRouter({
+      getBackends: () => [bk('a', 'qwen3:8b'), bk('b', 'qwen3:8b')],
+      registry: makeRegistry([
+        status('a', true, 4, 4), // preferred but full
+        status('b', true, 1, 4),
+      ]),
+    });
+    expect(r.pick('qwen3:8b', 'a')?.id).toBe('b');
+  });
+
+  it('falls back when the preferred backend is offline', () => {
+    const r = createRouter({
+      getBackends: () => [bk('a', 'qwen3:8b'), bk('b', 'qwen3:8b')],
+      registry: makeRegistry([
+        status('a', false, 0, 4),
+        status('b', true, 1, 4),
+      ]),
+    });
+    expect(r.pick('qwen3:8b', 'a')?.id).toBe('b');
+  });
+
+  it('ignores a preferred backend that does not serve the requested model/role', () => {
+    const r = createRouter({
+      getBackends: () => [bk('a', 'qwen3:8b'), bk('x', 'llama:70b')],
+      registry: makeRegistry([
+        status('a', true, 2, 4),
+        status('x', true, 0, 4), // idle but wrong model — must not leak across pools
+      ]),
+    });
+    expect(r.pick('qwen3:8b', 'x')?.id).toBe('a');
+  });
+
+  it('preferred pick still reserves an inflight slot', () => {
+    const inflight = createBackendInflightCounter();
+    const r = createRouter({
+      getBackends: () => [bk('a', 'qwen3:8b', 2)],
+      registry: makeRegistry([status('a', true, 0, 2)]),
+      inflight,
+    });
+    expect(r.pick('qwen3:8b', 'a')?.id).toBe('a');
+    expect(inflight.get('a')).toBe(1);
+    expect(r.pick('qwen3:8b', 'a')?.id).toBe('a');
+    expect(inflight.get('a')).toBe(2);
+    // Saturated by reservations → preferred no longer admitted → null
+    expect(r.pick('qwen3:8b', 'a')).toBeNull();
+  });
+
   it('treats registry-cold backends as idle (avoids startup outage)', () => {
     // Registry has nothing yet — first request still routes.
     const r = createRouter({
diff --git a/src/gateway/router.ts b/src/gateway/router.ts
index f73d1f2..880babc 100644
--- a/src/gateway/router.ts
+++ b/src/gateway/router.ts
@@ -44,7 +44,13 @@ export interface Router {
    * backend matches the model OR all matching backends are
    * offline/saturated.
    */
-  pick(model: string): GatewayBackendConfig | null;
+  /**
+   * Pick a backend for the routing key. `preferredBackendId` is the sticky
+   * routing hint (x-aao-preferred-backend): honored when that backend is an
+   * admitted candidate (right role/model, online, free capacity), otherwise
+   * the normal least-busy ranking applies.
+   */
+  pick(model: string, preferredBackendId?: string | null): GatewayBackendConfig | null;
   /**
    * Inspection helper — returns the registry-augmented view of every
    * configured backend (regardless of model). Useful for /v1/models
@@ -118,7 +124,7 @@ export function createRouter(deps: CreateRouterDeps): Router {
   }
 
   return {
-    pick(model: string): GatewayBackendConfig | null {
+    pick(model: string, preferredBackendId?: string | null): GatewayBackendConfig | null {
       // Note registry activity so the polling cadence tightens while
       // we're actively routing. Without this, a single-page burst of
       // requests would still see the idle 30s cadence.
@@ -190,6 +196,21 @@ export function createRouter(deps: CreateRouterDeps): Router {
       const candidates = warm.length > 0 ? warm : cold;
       if (candidates.length === 0) return null;
 
+      // Sticky routing (x-aao-preferred-backend): if the client asks for a
+      // specific backend and that backend is an ADMITTED candidate (right
+      // role/model, online, has free capacity), serve it without ranking —
+      // the job's KV cache lives there, so a least-busy re-pick would trade
+      // a large prefill cost for a small load-balance win. When the
+      // preferred backend is saturated or offline it simply isn't in the
+      // candidate list and we fall through to the normal ranking.
+      if (preferredBackendId) {
+        const preferred = candidates.find(c => c.backend.id === preferredBackendId);
+        if (preferred) {
+          deps.inflight?.inc(preferred.backend.id);
+          return preferred.backend;
+        }
+      }
+
       // Stable ascending sort by ratio.
       candidates.sort((a, b) => a.ratio - b.ratio);
       const bestRatio = candidates[0]!.ratio;
diff --git a/src/gateway/stream-proxy.ts b/src/gateway/stream-proxy.ts
index f2e4656..ca4d1d6 100644
--- a/src/gateway/stream-proxy.ts
+++ b/src/gateway/stream-proxy.ts
@@ -297,7 +297,13 @@ export function buildChatCompletionsHandler(deps: StreamProxyDeps) {
       return;
     }
 
-    const backend = deps.router.pick(model);
+    // Sticky routing hint from the worker (see router.pick): prefer the
+    // backend that already holds this job's KV cache when it has capacity.
+    const rawPreferred = req.headers['x-aao-preferred-backend'];
+    const preferredBackendId = typeof rawPreferred === 'string' && rawPreferred.trim().length > 0
+      ? rawPreferred.trim()
+      : null;
+    const backend = deps.router.pick(model, preferredBackendId);
     if (!backend) {
       emitRequestMetric(deps.metrics, {
         team: team0, backend: 'none', model, status: 'no_backend', durationMs: 0,
diff --git a/src/llm/openai-compat.ts b/src/llm/openai-compat.ts
index 112935e..6547a3d 100644
--- a/src/llm/openai-compat.ts
+++ b/src/llm/openai-compat.ts
@@ -281,6 +281,17 @@ export class OpenAICompatClient {
     return `Request timed out (${mins} minutes)`;
   }
 
+  /**
+   * Backend the next request should prefer (gateway sticky routing for
+   * KV-cache reuse). Updated by the worker whenever the resolved backend
+   * changes; per-client so concurrent jobs never share affinity.
+   */
+  private preferredBackendId: string | null = null;
+
+  setPreferredBackendId(backendId: string | null): void {
+    this.preferredBackendId = backendId;
+  }
+
   async *chat(messages: Message[], tools?: ToolDef[], externalSignal?: AbortSignal): AsyncGenerator<LLMEvent> {
     const controller = new AbortController();
     // アイドルタイムアウト: チャンク受信のたびにリセットされる
@@ -308,6 +319,13 @@ export class OpenAICompatClient {
       if (this.apiKey) {
         headers['Authorization'] = `Bearer ${this.apiKey}`;
       }
+      // Sticky routing hint: ask the gateway to keep serving this job from
+      // the backend that already holds its KV cache. The gateway only honors
+      // it while that backend is online with free capacity; otherwise it
+      // re-routes normally. Direct (non-proxy) backends ignore the header.
+      if (this.preferredBackendId) {
+        headers['x-aao-preferred-backend'] = this.preferredBackendId;
+      }
 
       const body: Record<string, unknown> = {
         messages,
diff --git a/src/progress/log-format.test.ts b/src/progress/log-format.test.ts
index 68bb9b6..d9251f9 100644
--- a/src/progress/log-format.test.ts
+++ b/src/progress/log-format.test.ts
@@ -12,6 +12,7 @@ describe('activity log format', () => {
     expect(parseActivityLogMetadata('[2026-03-13T00:00:00.000Z] [worker:worker-074] [mode:fast] Read: {"file_path":"input/a.png"}')).toEqual({
       workerId: 'worker-074',
       mode: 'fast',
+      backendId: null,
     });
   });
 
@@ -20,6 +21,7 @@ describe('activity log format', () => {
     expect(parseActivityLogMetadata('[2026-03-13T00:00:00.000Z] final: completed')).toEqual({
       workerId: null,
       mode: null,
+      backendId: null,
     });
   });
 
@@ -27,6 +29,7 @@ describe('activity log format', () => {
     expect(parseActivityLogMetadata('[2026-03-13T00:00:00.000Z] [worker:worker-148] [mode:quality] [execute] preview: checked draft')).toEqual({
       workerId: 'worker-148',
       mode: 'quality',
+      backendId: null,
     });
   });
 
@@ -54,3 +57,24 @@ describe('activity log format', () => {
     expect(formatDuration(Number.POSITIVE_INFINITY)).toBe('?');
   });
 });
+
+describe('backend tag', () => {
+  it('formats and parses [backend:...] alongside worker/mode', () => {
+    const line = formatActivityLogEntry('LLM call done', {
+      workerId: 'aao-gateway',
+      mode: 'quality',
+      backendId: 'gpu-rtx-a',
+    });
+    expect(line).toContain('[worker:aao-gateway]');
+    expect(line).toContain('[backend:gpu-rtx-a]');
+    const parsed = parseActivityLogMetadata(line);
+    expect(parsed.workerId).toBe('aao-gateway');
+    expect(parsed.backendId).toBe('gpu-rtx-a');
+  });
+
+  it('omits the backend tag when backendId is unset', () => {
+    const line = formatActivityLogEntry('entry', { workerId: 'w1', mode: 'auto' });
+    expect(line).not.toContain('[backend:');
+    expect(parseActivityLogMetadata(line).backendId).toBeNull();
+  });
+});
diff --git a/src/progress/log-format.ts b/src/progress/log-format.ts
index 4113901..fad8005 100644
--- a/src/progress/log-format.ts
+++ b/src/progress/log-format.ts
@@ -1,6 +1,8 @@
 export interface ActivityLogMetadata {
   workerId?: string | null;
   mode?: string | null;
+  /** Physical backend behind a proxy worker (gateway-resolved), if known. */
+  backendId?: string | null;
 }
 
 const TOOL_SUMMARY_KEYS = [
@@ -24,6 +26,7 @@ export function formatActivityLogMetadata(metadata?: ActivityLogMetadata): strin
   const segments: string[] = [];
   if (metadata.workerId) segments.push(`[worker:${metadata.workerId}]`);
   if (metadata.mode) segments.push(`[mode:${metadata.mode}]`);
+  if (metadata.backendId) segments.push(`[backend:${metadata.backendId}]`);
   return segments.join(' ');
 }
 
@@ -35,9 +38,11 @@ export function formatActivityLogEntry(entry: string, metadata?: ActivityLogMeta
 export function parseActivityLogMetadata(line: string): ActivityLogMetadata {
   const workerMatch = /\[worker:([^\]]+)\]/.exec(line);
   const modeMatch = /\[mode:([^\]]+)\]/.exec(line);
+  const backendMatch = /\[backend:([^\]]+)\]/.exec(line);
   return {
     workerId: workerMatch?.[1] ?? null,
     mode: modeMatch?.[1] ?? null,
+    backendId: backendMatch?.[1] ?? null,
   };
 }
 
diff --git a/src/worker.ts b/src/worker.ts
index 4a0e3fe..b98a9f3 100644
--- a/src/worker.ts
+++ b/src/worker.ts
@@ -1164,10 +1164,12 @@ export class Worker {
         isLocalTask,
         localTaskId,
         workspacePath,
-        // Seed the sticky-backend guard with whatever was already persisted
+        // Seed the backend tracker with whatever was already persisted
         // for this job (e.g. on retry / resume from ASK). Only matters for
         // proxy workers; direct workers never produce a backend event.
         isProxyWorker ? (job.lastBackendId ?? null) : null,
+        llmClient,
+        logMetadata,
       );
 
       // 開始コメント
@@ -1500,24 +1502,34 @@ export class Worker {
     localTaskId: number | null,
     workspacePath: string,
     /**
-     * Initial value of jobs.last_backend_id from the DB. Used to seed the
-     * sticky guard so callers don't repeatedly write the same value on
-     * every LLM iteration. Falsy/null = no backend resolved yet.
+     * Initial value of jobs.last_backend_id from the DB. Seeds the backend
+     * tracker (and the sticky-routing hint) so a resumed/retried job goes
+     * back to the backend that already holds its KV cache.
+     * Falsy/null = no backend resolved yet.
      */
     initialLastBackendId: string | null = null,
+    /** LLM client of this job — receives the sticky-routing hint per switch. */
+    llmClient?: { setPreferredBackendId(backendId: string | null): void },
+    /**
+     * The reporter's metadata object (shared by reference): mutating
+     * `backendId` here makes every subsequent activity.log line carry
+     * `[backend:...]` so the Progress tab can show the physical backend
+     * behind a proxy worker.
+     */
+    logMetadata?: ActivityLogMetadata,
   ): PieceRunCallbacks {
     let movementStartTime = Date.now();
     const toolUsageCounts = new Map<string, number>();
-    // Sticky-backend per design Open Question #3: take the first proxy
-    // backend the job sees and never overwrite it. Subsequent calls that
-    // happen to land on a different deployment are ignored at this layer
-    // so the UI Pet doesn't flicker between sprites. The resolver also
-    // guarantees that if the DB persist fails, the local sticky stays
-    // unset so the next event can retry (otherwise a transient DB error
-    // would orphan the worker → backend mapping for the lifetime of the
-    // job). See src/worker/sticky-backend.ts.
+    // Backend tracker (follow-current semantics, 2026-06): persists
+    // jobs.last_backend_id whenever the resolved backend CHANGES so the UI
+    // (pet, badges) follows where the job actually runs. Switches are rare
+    // because the gateway honors the x-aao-preferred-backend sticky hint
+    // (KV-cache reuse) — they only happen when the preferred backend goes
+    // offline or saturates. The tracker still guarantees that a failed DB
+    // persist leaves the in-memory value unchanged so the next event
+    // retries. See src/worker/sticky-backend.ts.
     const workerIdLocal = this.workerId;
-    const onBackendResolvedHandler = createStickyBackendResolver({
+    const backendTracker = createStickyBackendResolver({
       initial: initialLastBackendId,
       persist: (backendId) => this.repo.updateJob(jobId, { lastBackendId: backendId }),
       logger: {
@@ -1528,6 +1540,12 @@ export class Worker {
       workerId: workerIdLocal,
       jobId,
     });
+    // Seed the sticky-routing hint + activity-log backend tag from the DB
+    // value (resume/retry goes straight back to the cache-warm backend).
+    if (initialLastBackendId) {
+      llmClient?.setPreferredBackendId(initialLastBackendId);
+      if (logMetadata) logMetadata.backendId = initialLastBackendId;
+    }
     // Phase 3b: local copy of the sticky backend so the LLM-call metric
     // has a stable backend_id label even before the persist returns.
     // Direct workers (non-proxy) never fire onBackendResolved, so we
@@ -1616,20 +1634,23 @@ export class Worker {
         }
       },
       onBackendResolved: (info) => {
-        // Phase 3b: update the sticky backend id used for LLM-call
-        // metrics. We capture every event (not just the first) so a
-        // routing change mid-job is reflected in the next iteration's
-        // counters; the DB-side sticky still preserves the first.
+        // Phase 3b: update the backend id used for LLM-call metrics.
         if (info.backendId) {
           metricBackendId = info.backendId;
+          // Sticky routing: ask the gateway to keep using this backend on
+          // the next request (KV-cache affinity).
+          llmClient?.setPreferredBackendId(info.backendId);
+          // Tag subsequent activity.log lines with the physical backend so
+          // the Progress tab shows more than the proxy worker's name.
+          if (logMetadata) logMetadata.backendId = info.backendId;
         }
         // Fire-and-forget: agent-loop's onBackendResolved signature is
-        // sync (void). The resolver handles persist errors internally;
+        // sync (void). The tracker handles persist errors internally;
         // we just attach a final guard to log any unexpected throw.
         // cacheKey is observed but not persisted at the job level —
         // Phase B's NodeStatusWidget will track cache hits out-of-band.
-        onBackendResolvedHandler(info).catch(err => {
-          logger.warn(`[worker:${this.workerId}] sticky backend resolver threw for job ${jobId}: ${err}`);
+        backendTracker.onEvent(info).catch(err => {
+          logger.warn(`[worker:${this.workerId}] backend tracker threw for job ${jobId}: ${err}`);
         });
       },
       onMovementComplete: (movementName, result) => {
diff --git a/src/worker/sticky-backend.test.ts b/src/worker/sticky-backend.test.ts
index 572cc8a..e02af98 100644
--- a/src/worker/sticky-backend.test.ts
+++ b/src/worker/sticky-backend.test.ts
@@ -13,11 +13,11 @@ function makeLogger(): StickyBackendLogger & {
   };
 }
 
-describe('createStickyBackendResolver', () => {
-  it('persists the first backend, sets sticky, and logs at info', async () => {
+describe('createStickyBackendResolver (follow-current semantics)', () => {
+  it('persists the first backend, advances current, and logs at info', async () => {
     const logger = makeLogger();
     const persist = vi.fn().mockResolvedValue(undefined);
-    const resolve = createStickyBackendResolver({
+    const tracker = createStickyBackendResolver({
       initial: null,
       persist,
       logger,
@@ -25,18 +25,19 @@ describe('createStickyBackendResolver', () => {
       jobId: 'j1',
     });
 
-    await resolve({ backendId: 'gpu-a', cacheKey: null });
+    await tracker.onEvent({ backendId: 'gpu-a', cacheKey: null });
 
     expect(persist).toHaveBeenCalledTimes(1);
     expect(persist).toHaveBeenCalledWith('gpu-a');
+    expect(tracker.current()).toBe('gpu-a');
     expect(logger.calls.info).toHaveLength(1);
     expect(logger.calls.info[0]).toContain('gpu-a');
   });
 
-  it('short-circuits subsequent events once sticky is set', async () => {
+  it('follows backend switches: each CHANGE persists; repeats do not', async () => {
     const logger = makeLogger();
     const persist = vi.fn().mockResolvedValue(undefined);
-    const resolve = createStickyBackendResolver({
+    const tracker = createStickyBackendResolver({
       initial: null,
       persist,
       logger,
@@ -44,20 +45,24 @@ describe('createStickyBackendResolver', () => {
       jobId: 'j1',
     });
 
-    await resolve({ backendId: 'gpu-a', cacheKey: null });
-    await resolve({ backendId: 'gpu-b', cacheKey: 'sha:xyz' });
-    await resolve({ backendId: 'gpu-a', cacheKey: null });
+    await tracker.onEvent({ backendId: 'gpu-a', cacheKey: null });
+    await tracker.onEvent({ backendId: 'gpu-a', cacheKey: 'sha:1' }); // same — no persist
+    await tracker.onEvent({ backendId: 'gpu-b', cacheKey: null });    // switch — persists
+    await tracker.onEvent({ backendId: 'gpu-b', cacheKey: null });    // same — no persist
 
-    expect(persist).toHaveBeenCalledTimes(1);
-    // gpu-b ≠ sticky → debug; gpu-a == sticky → no log
-    expect(logger.calls.debug).toHaveLength(1);
-    expect(logger.calls.debug[0]).toContain('gpu-b');
+    expect(persist).toHaveBeenCalledTimes(2);
+    expect(persist).toHaveBeenNthCalledWith(1, 'gpu-a');
+    expect(persist).toHaveBeenNthCalledWith(2, 'gpu-b');
+    expect(tracker.current()).toBe('gpu-b');
+    // The switch log mentions both ends
+    expect(logger.calls.info[1]).toContain('gpu-a');
+    expect(logger.calls.info[1]).toContain('gpu-b');
   });
 
-  it('honors initial sticky from DB without re-persisting', async () => {
+  it('honors initial value from DB: same backend does not re-persist, a switch does', async () => {
     const logger = makeLogger();
     const persist = vi.fn().mockResolvedValue(undefined);
-    const resolve = createStickyBackendResolver({
+    const tracker = createStickyBackendResolver({
       initial: 'gpu-seed',
       persist,
       logger,
@@ -65,21 +70,22 @@ describe('createStickyBackendResolver', () => {
       jobId: 'j1',
     });
 
-    await resolve({ backendId: 'gpu-other', cacheKey: null });
-    await resolve({ backendId: 'gpu-seed', cacheKey: null });
-
+    expect(tracker.current()).toBe('gpu-seed');
+    await tracker.onEvent({ backendId: 'gpu-seed', cacheKey: null });
     expect(persist).not.toHaveBeenCalled();
-    expect(logger.calls.debug).toHaveLength(1);
-    expect(logger.calls.debug[0]).toContain('gpu-other');
+
+    await tracker.onEvent({ backendId: 'gpu-other', cacheKey: null });
+    expect(persist).toHaveBeenCalledTimes(1);
+    expect(tracker.current()).toBe('gpu-other');
   });
 
-  it('does NOT set sticky when persist fails — next event retries', async () => {
+  it('does NOT advance when persist fails — next event retries', async () => {
     const logger = makeLogger();
     const persist = vi
       .fn()
       .mockRejectedValueOnce(new Error('SQLITE_BUSY'))
       .mockResolvedValueOnce(undefined);
-    const resolve = createStickyBackendResolver({
+    const tracker = createStickyBackendResolver({
       initial: null,
       persist,
       logger,
@@ -87,23 +93,19 @@ describe('createStickyBackendResolver', () => {
       jobId: 'j1',
     });
 
-    // First call: DB write fails → sticky unset → warn logged
-    await resolve({ backendId: 'gpu-a', cacheKey: null });
+    // First call: DB write fails → current stays null → warn logged
+    await tracker.onEvent({ backendId: 'gpu-a', cacheKey: null });
     expect(persist).toHaveBeenCalledTimes(1);
+    expect(tracker.current()).toBeNull();
     expect(logger.calls.warn).toHaveLength(1);
     expect(logger.calls.warn[0]).toContain('SQLITE_BUSY');
     expect(logger.calls.info).toHaveLength(0);
 
-    // Second call: DB write succeeds → sticky set
-    await resolve({ backendId: 'gpu-b', cacheKey: null });
+    // Second call (same backend again): retries because current ≠ backendId
+    await tracker.onEvent({ backendId: 'gpu-a', cacheKey: null });
     expect(persist).toHaveBeenCalledTimes(2);
-    expect(persist).toHaveBeenLastCalledWith('gpu-b');
+    expect(tracker.current()).toBe('gpu-a');
     expect(logger.calls.info).toHaveLength(1);
-    expect(logger.calls.info[0]).toContain('gpu-b');
-
-    // Third call: sticky is now set → no further persist
-    await resolve({ backendId: 'gpu-c', cacheKey: null });
-    expect(persist).toHaveBeenCalledTimes(2);
   });
 
   it('retries on every event until persist succeeds (multiple failures)', async () => {
@@ -113,7 +115,7 @@ describe('createStickyBackendResolver', () => {
       .mockRejectedValueOnce(new Error('fail 1'))
       .mockRejectedValueOnce(new Error('fail 2'))
       .mockResolvedValueOnce(undefined);
-    const resolve = createStickyBackendResolver({
+    const tracker = createStickyBackendResolver({
       initial: null,
       persist,
       logger,
@@ -121,13 +123,13 @@ describe('createStickyBackendResolver', () => {
       jobId: 'j1',
     });
 
-    await resolve({ backendId: 'gpu-a', cacheKey: null });
-    await resolve({ backendId: 'gpu-b', cacheKey: null });
-    await resolve({ backendId: 'gpu-c', cacheKey: null });
+    await tracker.onEvent({ backendId: 'gpu-a', cacheKey: null });
+    await tracker.onEvent({ backendId: 'gpu-b', cacheKey: null });
+    await tracker.onEvent({ backendId: 'gpu-c', cacheKey: null });
 
     expect(persist).toHaveBeenCalledTimes(3);
     expect(logger.calls.warn).toHaveLength(2);
     expect(logger.calls.info).toHaveLength(1);
-    expect(logger.calls.info[0]).toContain('gpu-c');
+    expect(tracker.current()).toBe('gpu-c');
   });
 });
diff --git a/src/worker/sticky-backend.ts b/src/worker/sticky-backend.ts
index 54de4a5..517c979 100644
--- a/src/worker/sticky-backend.ts
+++ b/src/worker/sticky-backend.ts
@@ -1,24 +1,23 @@
 /**
- * Sticky-backend resolver for proxy worker jobs.
+ * Backend tracker for proxy worker jobs.
  *
- * Design (per
- *   docs/superpowers/specs/2026-05-18-multi-team-gpu-pool-and-node-status-design.md
- * Open Question #3 case 1, "最初に確定したら以後 update しない"):
+ * History: originally "first backend wins" (2026-05-18 design, Open Question
+ * #3 case 1) to keep the UI pet from flickering while the gateway rebalanced
+ * every request. As of 2026-06 the gateway honors `x-aao-preferred-backend`
+ * (client-side sticky routing for KV-cache reuse), so backend switches are
+ * RARE — they only happen when the preferred backend goes offline or
+ * saturates. The tracker therefore now follows the CURRENT backend:
  *
- * - For a proxy worker, every LLM call may resolve to a different
- *   physical backend (LiteLLM rebalances per request). The UI pet should
- *   not flicker, so we record only the FIRST backend a job sees.
- * - Persistence happens via `updateJob({ lastBackendId })`. If that DB
- *   write FAILS, we must remain in the unset state so the next
- *   `onBackendResolved` event has a chance to retry. If we set the local
- *   sticky variable BEFORE persisting, a transient DB error would lose
- *   the binding permanently for the lifetime of the job (sticky check
- *   short-circuits all subsequent events) and the UI would never see
- *   the worker → backend mapping.
+ * - `jobs.last_backend_id` is updated whenever the resolved backend CHANGES,
+ *   so the UI (pet, badges) tracks where the job actually runs.
+ * - Persistence happens via `updateJob({ lastBackendId })`. If that DB write
+ *   FAILS, the in-memory value is left unchanged so the next
+ *   `onBackendResolved` event retries the persist (a transient DB error must
+ *   not permanently lose the worker → backend mapping).
  *
- * This module isolates the "set sticky only after persist succeeds"
- * invariant from `Worker.buildPieceRunCallbacks`, which already has a
- * dozen other concerns and is hard to unit-test in isolation.
+ * This module isolates the "advance only after persist succeeds" invariant
+ * from `Worker.buildPieceCallbacks`, which already has a dozen other
+ * concerns and is hard to unit-test in isolation.
  */
 
 export interface StickyBackendLogger {
@@ -32,16 +31,20 @@ export interface StickyBackendEvent {
   cacheKey: string | null;
 }
 
+export interface BackendTracker {
+  /** The onBackendResolved callback for the agent loop (fire-and-forget safe). */
+  onEvent: (event: StickyBackendEvent) => Promise<void>;
+  /**
+   * The most recently persisted backend id (or the initial DB value).
+   * Used as the `x-aao-preferred-backend` hint on the next LLM request.
+   */
+  current: () => string | null;
+}
+
 /**
- * Build the `onBackendResolved` callback. `persist(backendId)` is the
- * DB write (typically `repo.updateJob(jobId, { lastBackendId })`); it
- * must reject on failure so we can keep sticky unset for retry.
- *
- * Returns an async function the agent-loop can call without awaiting —
- * errors are caught internally and logged. The function resolves once
- * either: (a) sticky was already set and we short-circuited, (b) the
- * persist succeeded and sticky is now set, or (c) the persist failed
- * and sticky remains unset for the next event to retry.
+ * Build the backend tracker. `persist(backendId)` is the DB write
+ * (typically `repo.updateJob(jobId, { lastBackendId })`); it must reject on
+ * failure so the in-memory value stays put for retry.
  */
 export function createStickyBackendResolver(opts: {
   initial: string | null;
@@ -49,33 +52,31 @@ export function createStickyBackendResolver(opts: {
   logger: StickyBackendLogger;
   workerId: string;
   jobId: string;
-}): (event: StickyBackendEvent) => Promise<void> {
+}): BackendTracker {
   const { initial, persist, logger, workerId, jobId } = opts;
-  let sticky: string | null = initial;
+  let current: string | null = initial;
 
-  return async function onBackendResolved({ backendId, cacheKey }: StickyBackendEvent): Promise<void> {
-    if (sticky) {
-      if (sticky !== backendId) {
-        logger.debug(
-          `[worker:${workerId}] job ${jobId} backend re-resolved to ${backendId} (sticky=${sticky}, cache=${cacheKey ?? 'miss'}); keeping sticky`,
-        );
-      }
-      return;
-    }
+  async function onEvent({ backendId, cacheKey }: StickyBackendEvent): Promise<void> {
+    if (current === backendId) return; // unchanged — nothing to persist
     try {
       await persist(backendId);
-      // Only set sticky AFTER persist succeeds. If we set first and persist
-      // failed, the next event would short-circuit on the sticky check and
-      // we'd never recover — the UI would render "no backend" forever.
-      sticky = backendId;
+      // Only advance AFTER persist succeeds. If we advanced first and the
+      // persist failed, the next identical event would short-circuit on the
+      // equality check and the DB would stay stale forever.
+      const previous = current;
+      current = backendId;
       logger.info(
-        `[worker:${workerId}] job ${jobId} backend resolved: ${backendId} cache=${cacheKey ?? 'miss'}`,
+        previous
+          ? `[worker:${workerId}] job ${jobId} backend switched: ${previous} → ${backendId} cache=${cacheKey ?? 'miss'}`
+          : `[worker:${workerId}] job ${jobId} backend resolved: ${backendId} cache=${cacheKey ?? 'miss'}`,
       );
     } catch (err) {
       logger.warn(
-        `[worker:${workerId}] failed to persist lastBackendId for job ${jobId}: ${err} — sticky left unset for retry`,
+        `[worker:${workerId}] failed to persist lastBackendId for job ${jobId}: ${err} — keeping ${current ?? 'unset'} for retry`,
       );
-      // Intentionally do NOT set sticky. Next event retries.
+      // Intentionally do NOT advance. Next event retries.
     }
-  };
+  }
+
+  return { onEvent, current: () => current };
 }
diff --git a/ui/src/components/activity/ActivityEventCard.tsx b/ui/src/components/activity/ActivityEventCard.tsx
index 302450b..2f52ab2 100644
--- a/ui/src/components/activity/ActivityEventCard.tsx
+++ b/ui/src/components/activity/ActivityEventCard.tsx
@@ -22,7 +22,7 @@ const KIND_COLORS: Record<string, { dot: string; badge: string; badgeText: strin
 
 export const ActivityEventCard = memo(function ActivityEventCard({ event, isLast }: { event: ActivityEvent; isLast?: boolean }) {
   const colors = KIND_COLORS[event.kind] ?? KIND_COLORS.other!;
-  const meta = formatActivityMeta(event.workerId, event.mode);
+  const meta = formatActivityMeta(event.workerId, event.mode, event.backendId);
 
   return (
     <div className="grid gap-3" style={{ gridTemplateColumns: '16px minmax(0,1fr)' }}>
diff --git a/ui/src/lib/utils.ts b/ui/src/lib/utils.ts
index 6084656..4a7b390 100644
--- a/ui/src/lib/utils.ts
+++ b/ui/src/lib/utils.ts
@@ -115,10 +115,17 @@ export interface ActivityEvent {
   timestamp: string | null;
   workerId: string | null;
   mode: string | null;
+  /** Physical backend behind a proxy worker (from [backend:...] tags). */
+  backendId: string | null;
 }
 
-export function formatActivityMeta(workerId: string | null, mode: string | null): string {
-  return [workerId ? `worker: ${workerPill(workerId)}` : '', mode ? `mode: ${mode}` : ''].filter(Boolean).join(' · ');
+export function formatActivityMeta(workerId: string | null, mode: string | null, backendId?: string | null): string {
+  return [
+    workerId ? `worker: ${workerPill(workerId)}` : '',
+    // Show the physical backend when it differs from the (proxy) worker name.
+    backendId && backendId !== workerId ? `backend: ${workerPill(backendId)}` : '',
+    mode ? `mode: ${mode}` : '',
+  ].filter(Boolean).join(' · ');
 }
 
 export function parseActivityLog(logText: string): ActivityEvent[] {
@@ -130,13 +137,15 @@ export function parseActivityLog(logText: string): ActivityEvent[] {
     const timestamp = timestampMatch?.[1] ?? null;
     const workerId = /\[worker:([^\]]+)\]/.exec(rawLine)?.[1] ?? null;
     const mode = /\[mode:([^\]]+)\]/.exec(rawLine)?.[1] ?? null;
+    const backendId = /\[backend:([^\]]+)\]/.exec(rawLine)?.[1] ?? null;
     const line = rawLine
       .replace(/^\[[^\]]+\]\s+/, '')
       .replace(/\[worker:[^\]]+\]\s*/g, '')
       .replace(/\[mode:[^\]]+\]\s*/g, '')
+      .replace(/\[backend:[^\]]+\]\s*/g, '')
       .trim();
 
-    const base = { id: `${timestamp ?? 'line'}-${index}`, timestamp, workerId, mode };
+    const base = { id: `${timestamp ?? 'line'}-${index}`, timestamp, workerId, mode, backendId };
 
     const movementStart = /^\[([^\]]+)\] (?:start|ステップ開始)$/.exec(line);
     if (movementStart) {