diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 887f41c..707698c 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -84,20 +84,31 @@ export type ApiError = {
   detail: string;
   errorCode?: string;
   errorMessage?: string;
+  /**
+   * PR-DocSrv-Web-Ask-Selector-1: backend_unavailable 503 path 가
+   * body level 에 `error_reason` (예: `macbook_unavailable`) 을 둠. 본 필드는
+   * 그 raw 값. 다른 endpoint 응답에 `error_reason` 이 없으면 undefined.
+   */
+  errorReason?: string;
 };
 
 function parseDetail(body: unknown, fallback: string): {
   detail: string;
   errorCode?: string;
   errorMessage?: string;
+  errorReason?: string;
 } {
-  if (body && typeof body === 'object' && 'detail' in body) {
-    const d = (body as { detail: unknown }).detail;
-    if (typeof d === 'string' && d) return { detail: d };
-    if (d && typeof d === 'object') {
-      const obj = d as { error_code?: string; message?: string };
-      const message = obj.message || fallback;
-      return { detail: message, errorCode: obj.error_code, errorMessage: obj.message };
+  if (body && typeof body === 'object') {
+    const top = body as { detail?: unknown; error_reason?: unknown };
+    const errorReason = typeof top.error_reason === 'string' ? top.error_reason : undefined;
+    if ('detail' in body) {
+      const d = top.detail;
+      if (typeof d === 'string' && d) return { detail: d, errorReason };
+      if (d && typeof d === 'object') {
+        const obj = d as { error_code?: string; message?: string };
+        const message = obj.message || fallback;
+        return { detail: message, errorCode: obj.error_code, errorMessage: obj.message, errorReason };
+      }
     }
   }
   return { detail: fallback };
diff --git a/frontend/src/lib/components/ask/AskAnswer.svelte b/frontend/src/lib/components/ask/AskAnswer.svelte
index e94eed6..d125a63 100644
--- a/frontend/src/lib/components/ask/AskAnswer.svelte
+++ b/frontend/src/lib/components/ask/AskAnswer.svelte
@@ -60,8 +60,20 @@
     no_evidence: '근거 없음',
     parse_failed: '형식 오류',
     llm_error: 'AI 오류',
+    backend_unavailable: 'Backend 비가용',
   };
 
+  /**
+   * backend chip label — `backend_requested` 가 명시 opt-in 인 경우만 표시.
+   * 미지정 (null/undefined) default 호출은 chip 없음 (시각 noise 회피).
+   */
+  function backendChipLabel(backend: string | null | undefined): string | null {
+    if (!backend) return null;
+    if (backend === 'qwen-macbook') return 'Qwen 27B (MacBook)';
+    if (backend === 'gemma-macmini') return 'Gemma 26B (Mac mini)';
+    return backend;
+  }
+
   let tokens = $derived(data?.ai_answer ? splitAnswer(data.ai_answer) : []);
   let showFullAnswer = $derived(
     !!data && !!data.ai_answer && data.completeness === 'full'
@@ -88,6 +100,13 @@
         <Badge tone={confidenceTone(data.confidence)} size="sm">
           신뢰도 {confidenceLabel(data.confidence)}
         </Badge>
+        {#if backendChipLabel(data.backend_requested)}
+          <span title={`backend_requested=${data.backend_requested} / backend_used=${data.backend_used ?? 'null'}`}>
+            <Badge tone="neutral" size="sm">
+              {backendChipLabel(data.backend_requested)}
+            </Badge>
+          </span>
+        {/if}
         <Badge tone="neutral" size="sm">
           {STATUS_LABEL[data.synthesis_status]}
         </Badge>
diff --git a/frontend/src/lib/types/ask.ts b/frontend/src/lib/types/ask.ts
index 4a6cd89..826d976 100644
--- a/frontend/src/lib/types/ask.ts
+++ b/frontend/src/lib/types/ask.ts
@@ -11,7 +11,8 @@ export type SynthesisStatus =
   | 'skipped'
   | 'no_evidence'
   | 'parse_failed'
-  | 'llm_error';
+  | 'llm_error'
+  | 'backend_unavailable';
 
 export type Confidence = 'high' | 'medium' | 'low';
 
@@ -74,4 +75,10 @@ export interface AskResponse {
   covered_aspects: string[] | null;
   missing_aspects: string[] | null;
   confirmed_items: ConfirmedItem[] | null;
+  /**
+   * PR-MacBook-RAG-Backend-1: backend dispatcher metadata.
+   * backend 미지정 호출은 둘 다 null (기존 호출자 호환). 명시 opt-in 시만 채워짐.
+   */
+  backend_requested?: string | null;
+  backend_used?: string | null;
 }
diff --git a/frontend/src/routes/ask/+page.svelte b/frontend/src/routes/ask/+page.svelte
index 730e5c9..17cb790 100644
--- a/frontend/src/routes/ask/+page.svelte
+++ b/frontend/src/routes/ask/+page.svelte
@@ -1,10 +1,18 @@
 <!--
   /ask — Phase 3.4 Ask Pipeline Frontend.
 
-  URL-driven: `/ask?q=<encoded>` 가 진입점. $effect 로 q 변화 감지 →
-  `/api/search/ask` 호출 → 3-panel 렌더 (Answer / Evidence / Results).
+  URL-driven: `/ask?q=<encoded>[&backend=qwen-macbook]` 가 진입점.
+  $effect 로 (q, backend) 변화 감지 → `/api/search/ask` 호출 →
+  3-panel 렌더 (Answer / Evidence / Results).
 
-  중복 호출 방지: lastQuery 가드 (hydration + reactive trigger 시 같은 q 2번 발동 방지).
+  중복 호출 방지: lastKey (q+backend) 가드.
+
+  Backend selector (PR-DocSrv-Web-Ask-Selector-1):
+    - `default` (기본 Gemma Mac mini, URL param 없음)
+    - `qwen-macbook` (사용자 명시 opt-in, MacBook)
+    - localStorage / Settings preference 미사용 (별 PR `PR-DocSrv-Ask-Default-Pref-1`)
+    - 503 + error_reason=macbook_unavailable 시 자동 fallback 금지,
+      사용자가 MacBook 깨우거나 Default 로 재요청.
 -->
 <script lang="ts">
   import { page } from '$app/stores';
@@ -12,19 +20,29 @@
   import { api, type ApiError } from '$lib/api';
   import { addToast } from '$lib/stores/toast';
   import EmptyState from '$lib/components/ui/EmptyState.svelte';
+  import Button from '$lib/components/ui/Button.svelte';
   import AskAnswer from '$lib/components/ask/AskAnswer.svelte';
   import AskEvidence from '$lib/components/ask/AskEvidence.svelte';
   import AskResults from '$lib/components/ask/AskResults.svelte';
-  import { Sparkles, Search } from 'lucide-svelte';
+  import { Sparkles, Search, AlertCircle } from 'lucide-svelte';
   import type { AskResponse } from '$lib/types/ask';
 
+  type BackendChoice = 'default' | 'qwen-macbook';
+
+  function parseBackend(raw: string | null): BackendChoice {
+    return raw === 'qwen-macbook' ? 'qwen-macbook' : 'default';
+  }
+
   // ── state ───────────────────────────────────────────
   let queryInput = $state('');
+  let selectedBackend = $state<BackendChoice>('default');
   let data = $state<AskResponse | null>(null);
   let loading = $state(false);
+  let backendUnavailable = $state(false);
+  let backendUnavailableMessage = $state('');
 
   // 중복 호출 방지 가드 (hydration + reactive trigger 이중 발동 방지)
-  let lastQuery = '';
+  let lastKey = '';
 
   // citation scroll 연동: Answer 가 [n] 클릭 → Evidence 카드로 이동 + highlight
   const citationNodes = new Map<number, HTMLElement>();
@@ -45,11 +63,18 @@
     node?.scrollIntoView({ behavior: 'smooth', block: 'center' });
   }
 
+  // ── URL 빌더: backend !== 'default' 일 때만 param 추가 ─────
+  function buildAskUrl(q: string, backend: BackendChoice): string {
+    const params = new URLSearchParams({ q });
+    if (backend !== 'default') params.set('backend', backend);
+    return `/ask?${params.toString()}`;
+  }
+
   // ── submit (URL-driven, back 자동) ──────────────────
   function submit() {
     const q = queryInput.trim();
     if (!q) return;
-    goto(`/ask?q=${encodeURIComponent(q)}`);
+    goto(buildAskUrl(q, selectedBackend));
   }
 
   function handleKeydown(e: KeyboardEvent) {
@@ -59,18 +84,36 @@
     }
   }
 
+  // 503 후 "Default 로 재요청" — backend param 명시 제거.
+  // localStorage preference 도입 시 본 버튼은 preference 무시 invariant 보존 필요.
+  function retryWithDefault() {
+    const q = $page.url.searchParams.get('q') ?? queryInput.trim();
+    if (!q) return;
+    goto(`/ask?q=${encodeURIComponent(q)}`);
+  }
+
   // ── API 호출 ───────────────────────────────────────
-  async function runAsk(q: string) {
+  async function runAsk(q: string, backend: BackendChoice) {
     loading = true;
     activeCitation = null;
+    backendUnavailable = false;
+    backendUnavailableMessage = '';
+    const path = backend !== 'default'
+      ? `/search/ask?q=${encodeURIComponent(q)}&backend=${encodeURIComponent(backend)}&limit=10`
+      : `/search/ask?q=${encodeURIComponent(q)}&limit=10`;
     try {
-      data = await api<AskResponse>(
-        `/search/ask?q=${encodeURIComponent(q)}&limit=10`,
-      );
+      data = await api<AskResponse>(path);
     } catch (err) {
       const apiErr = err as ApiError;
-      addToast('error', apiErr.detail || '답변 생성 실패');
-      data = null;
+      if (apiErr.status === 503 && apiErr.errorReason === 'macbook_unavailable') {
+        backendUnavailable = true;
+        backendUnavailableMessage = apiErr.detail
+          || 'MacBook 이 응답하지 않습니다. 깨우거나 Default (Gemma) 로 다시 요청하세요.';
+        data = null;
+      } else {
+        addToast('error', apiErr.detail || '답변 생성 실패');
+        data = null;
+      }
     } finally {
       loading = false;
     }
@@ -79,16 +122,20 @@
   // ── URL → runAsk (중복 가드) ────────────────────────
   $effect(() => {
     const q = $page.url.searchParams.get('q') ?? '';
+    const backend = parseBackend($page.url.searchParams.get('backend'));
     queryInput = q;
+    selectedBackend = backend;
     if (!q) {
       data = null;
       loading = false;
-      lastQuery = '';
+      backendUnavailable = false;
+      lastKey = '';
       return;
     }
-    if (q === lastQuery) return;
-    lastQuery = q;
-    runAsk(q);
+    const key = `${q}|${backend}`;
+    if (key === lastKey) return;
+    lastKey = key;
+    runAsk(q, backend);
   });
 </script>
 
@@ -114,12 +161,32 @@
           class="w-full pl-9 pr-3 py-2 bg-surface border border-default rounded-lg text-text text-sm focus:border-accent outline-none"
         />
       </div>
+      <select
+        bind:value={selectedBackend}
+        title="Backend 선택 — MacBook 은 사용자 명시 opt-in 시만 호출됩니다 (꺼져 있으면 503)."
+        class="py-2 px-2 bg-surface border border-default rounded-lg text-text text-xs focus:border-accent outline-none"
+      >
+        <option value="default">Default (Gemma)</option>
+        <option value="qwen-macbook">Qwen 27B (MacBook)</option>
+      </select>
     </div>
   </div>
 
   <!-- 본문 -->
   <div class="max-w-5xl mx-auto p-4">
-    {#if !queryInput && !loading && !data}
+    {#if backendUnavailable}
+      <div class="py-16">
+        <EmptyState
+          icon={AlertCircle}
+          title="MacBook 이 응답하지 않습니다"
+          description={backendUnavailableMessage}
+        >
+          <Button variant="primary" size="sm" onclick={retryWithDefault}>
+            Default (Gemma) 로 재요청
+          </Button>
+        </EmptyState>
+      </div>
+    {:else if !queryInput && !loading && !data}
       <div class="py-16">
         <EmptyState
           icon={Sparkles}