diff --git a/app/llm.py b/app/llm.py
index c18a6db..8b7a6e0 100644
--- a/app/llm.py
+++ b/app/llm.py
@@ -8,6 +8,7 @@ Public API:
 - ``is_configured(cfg)`` — returns True when the client can make calls.
 - ``test_connection(cfg)`` — minimal request to verify credentials.
 - ``expand_query(cfg, query)`` — query-term expansion (step 3 consumer).
+  Returns ``ExpansionResult`` with ``terms`` and optional ``error``.
 - ``analyze_image(...)`` — **reserved stub, not implemented**.
 
 All calls go through ``_call_chat_completion()`` so tests can mock a single
@@ -16,6 +17,8 @@ boundary.
 
 from __future__ import annotations
 
+import json
+import re
 from dataclasses import dataclass
 from typing import Any
 
@@ -26,6 +29,18 @@ from app.settings_store import LLMConfig
 # Sensible defaults
 _TIMEOUT_SECONDS = 30
 
+# ── Prompt for query expansion (Step 3) ──────────────────────────────────
+_EXPAND_QUERY_SYSTEM_PROMPT = (
+    "你是搬家物品搜索助手。用户在搜索自己打包的箱子与物品（家居/搬家场景）。"
+    "给定一个搜索词，列出用户可能用来命名同一类物品的相关词："
+    "近义词、常见别称、上位类别、具体品类。"
+    "规则：用与查询相同的语言；"
+    "只给与该物品紧密相关、有助于在清单里找到它的词；"
+    "不要解释、不要造无关词；最多 8 个；"
+    "只输出一个 JSON 字符串数组，例如 "
+    '`["炒锅","平底锅","汤锅","厨具"]`。'
+)
+
 
 @dataclass
 class LLMResult:
@@ -36,6 +51,20 @@ class LLMResult:
     data: Any = None
 
 
+@dataclass
+class ExpansionResult:
+    """Structured result from ``expand_query``.
+
+    ``terms`` is always a list (may be empty).
+    ``error`` is ``None`` on success (including legitimate empty results);
+    on failure (timeout, network error, HTTP error) it contains a
+    human-friendly error message.
+    """
+
+    terms: list[str]
+    error: str | None = None
+
+
 def is_configured(cfg: LLMConfig) -> bool:
     """Return True only when the LLM is enabled AND has required fields."""
     return bool(cfg.enabled and cfg.model and cfg.api_key)
@@ -87,44 +116,109 @@ def test_connection(cfg: LLMConfig) -> LLMResult:
         )
 
 
-def expand_query(cfg: LLMConfig, query: str) -> list[str]:
+def expand_query(
+    cfg: LLMConfig,
+    query: str,
+    extra_hints: str = "",
+) -> ExpansionResult:
     """Expand a search query into multiple synonymous terms via LLM.
 
-    **Step 3 will consume this.**  Returns a list including the original query.
-    If the LLM call fails or is not configured, returns ``[query]`` as a
-    fallback (graceful degradation).
+    Returns an ``ExpansionResult``.  On success ``terms`` contains the expanded
+    terms (possibly empty) and ``error`` is ``None``.  On failure (network
+    error, timeout, HTTP error) ``terms`` is ``[]`` and ``error`` contains a
+    human-friendly message.
     """
     if not is_configured(cfg):
-        return [query]
+        return ExpansionResult(terms=[])
+
+    system_prompt = _EXPAND_QUERY_SYSTEM_PROMPT
+    if extra_hints and extra_hints.strip():
+        system_prompt += "\n" + extra_hints.strip()
 
     try:
         response = _call_chat_completion(
             cfg,
             messages=[
-                {
-                    "role": "system",
-                    "content": (
-                        "你是一个搜索词扩展助手。用户给你一个搜索词，"
-                        "你返回 3-5 个同义词或相关词，每行一个。"
-                        "不要编号、不要解释、不要标点。"
-                    ),
-                },
+                {"role": "system", "content": system_prompt},
                 {"role": "user", "content": query},
             ],
-            max_tokens=100,
+            max_tokens=200,
+            temperature=0,
+        )
+    except httpx.TimeoutException:
+        return ExpansionResult(
+            terms=[],
+            error="AI 搜索请求超时，请稍后再试。",
+        )
+    except httpx.ConnectError:
+        return ExpansionResult(
+            terms=[],
+            error="无法连接到 AI 服务，请检查网络或设置。",
+        )
+    except httpx.HTTPStatusError:
+        return ExpansionResult(
+            terms=[],
+            error="AI 服务返回错误，请检查配置。",
         )
-        choices = response.get("choices", [])
-        if choices:
-            content = choices[0].get("message", {}).get("content", "")
-            expanded = [
-                line.strip() for line in content.strip().splitlines() if line.strip()
-            ]
-            if expanded:
-                # Always include the original query
-                return [query] + [t for t in expanded if t != query]
-        return [query]
     except Exception:  # noqa: BLE001 — graceful degradation
-        return [query]
+        return ExpansionResult(
+            terms=[],
+            error="AI 搜索暂时不可用，请稍后再试。",
+        )
+
+    choices = response.get("choices", [])
+    if not choices:
+        return ExpansionResult(terms=[])
+    content = choices[0].get("message", {}).get("content", "")
+    return ExpansionResult(terms=_parse_json_string_array(content))
+
+
+# ── Constants for output contract enforcement ────────────────────────────
+_MAX_EXPANSION_TERMS = 8
+_MAX_TERM_LENGTH = 30
+
+
+def _parse_json_string_array(content: str) -> list[str]:
+    """Parse LLM output into a list of strings.
+
+    Strict contract enforcement:
+    1. Strip markdown code fences;
+    2. Try ``json.loads`` — only accept a JSON **array of strings**;
+    3. Anything else (prose, JSON objects, bad JSON) → return ``[]``.
+
+    This ensures the output contract is enforced by code: no matter what
+    the model returns or what ``ai_search_extra_hints`` contains, only a
+    valid JSON string array is accepted.
+    """
+    text = content.strip()
+    if not text:
+        return []
+
+    # Strip markdown code fences
+    text = re.sub(r"^```(?:json)?\s*", "", text)
+    text = re.sub(r"\s*```$", "", text)
+    text = text.strip()
+
+    # Attempt JSON parse — strictly require a list
+    try:
+        parsed = json.loads(text)
+    except (json.JSONDecodeError, ValueError):
+        return []
+
+    if not isinstance(parsed, list):
+        return []
+
+    # Validate every element is a string; reject non-string items
+    terms: list[str] = []
+    for item in parsed:
+        if not isinstance(item, str):
+            return []
+        cleaned = item.strip()
+        if cleaned and len(cleaned) <= _MAX_TERM_LENGTH:
+            terms.append(cleaned)
+
+    # Cap total count
+    return terms[:_MAX_EXPANSION_TERMS]
 
 
 def analyze_image(cfg: LLMConfig, image_data: bytes, prompt: str) -> LLMResult:
@@ -151,6 +245,7 @@ def _call_chat_completion(
     *,
     messages: list[dict[str, str]],
     max_tokens: int = 1,
+    temperature: float | None = None,
 ) -> dict:
     """Call the OpenAI-compatible ``/chat/completions`` endpoint.
 
@@ -164,6 +259,8 @@ def _call_chat_completion(
         "messages": messages,
         "max_tokens": max_tokens,
     }
+    if temperature is not None:
+        payload["temperature"] = temperature
     headers = {
         "Authorization": f"Bearer {cfg.api_key}",
         "Content-Type": "application/json",
diff --git a/app/main.py b/app/main.py
index ff40328..569d6ae 100644
--- a/app/main.py
+++ b/app/main.py
@@ -5,12 +5,12 @@ from fastapi import Depends, FastAPI, File, Form, HTTPException, Request, Upload
 from fastapi.responses import FileResponse, RedirectResponse, Response
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
-from sqlalchemy import func, or_
+from sqlalchemy import func, false, or_
 from sqlalchemy.orm import Session
 
 from app.db import get_db, init_db
 from app.images import process_upload
-from app.llm import test_connection
+from app.llm import expand_query, is_configured, test_connection
 from app.llm import LLMResult
 from app.models import Box, Item, SubItem
 from app.settings_store import LLMConfig, get_app_settings, save_app_settings
@@ -160,100 +160,130 @@ def _build_boxes_overview_summary(db: Session) -> dict[str, int | str]:
     }
 
 
-def _build_search_results(db: Session, query: str) -> list[dict]:
-    keyword = f"%{query.lower()}%"
+def _build_search_results(db: Session, query: str | list[str]) -> list[dict]:
+    """Search Box / Item / SubItem by name and note using case-insensitive LIKE.
+
+    Accepts either a single query string or a list of keywords.
+    When multiple keywords are given, they are combined with OR — a match on
+    *any* keyword is sufficient.
+    """
+    keywords = [query] if isinstance(query, str) else query
+    patterns = [f"%{kw.lower()}%" for kw in keywords]
+
+    def _or_like(column, note_column):
+        """Build an OR filter that matches any pattern on either column."""
+        conditions = []
+        for pat in patterns:
+            conditions.append(func.lower(column).like(pat))
+            conditions.append(func.lower(func.coalesce(note_column, "")).like(pat))
+        return or_(false(), *conditions) if conditions else false()
+
     results: list[dict] = []
+    seen_ids: set[tuple[str, int]] = set()
+
+    def _add(result_type: str, obj_id: int, entry: dict) -> None:
+        key = (result_type, obj_id)
+        if key not in seen_ids:
+            seen_ids.add(key)
+            results.append(entry)
 
     box_matches = (
         db.query(Box)
-        .filter(
-            or_(
-                func.lower(Box.name).like(keyword),
-                func.lower(func.coalesce(Box.note, "")).like(keyword),
-            )
-        )
+        .filter(_or_like(Box.name, Box.note))
         .order_by(Box.id.desc())
         .all()
     )
     for box in box_matches:
-        results.append(
-            {
-                "type": "Box",
-                "name": box.name,
-                "note": box.note,
-                "detail_url": f"/boxes/{box.id}",
-                "detail_label": "查看箱子",
-                "secondary_url": None,
-                "secondary_label": None,
-                "path": "顶层箱子",
-                "is_container": None,
-                "image_url": f"/boxes/{box.id}/image" if box.image_blob else None,
-            }
-        )
+        _add("Box", box.id, {
+            "type": "Box",
+            "name": box.name,
+            "note": box.note,
+            "detail_url": f"/boxes/{box.id}",
+            "detail_label": "查看箱子",
+            "secondary_url": None,
+            "secondary_label": None,
+            "path": "顶层箱子",
+            "is_container": None,
+            "image_url": f"/boxes/{box.id}/image" if box.image_blob else None,
+        })
 
     item_matches = (
         db.query(Item)
         .join(Item.box)
-        .filter(
-            or_(
-                func.lower(Item.name).like(keyword),
-                func.lower(func.coalesce(Item.note, "")).like(keyword),
-            )
-        )
+        .filter(_or_like(Item.name, Item.note))
         .order_by(Item.id.desc())
         .all()
     )
     for item in item_matches:
-        results.append(
-            {
-                "type": "Item",
-                "name": item.name,
-                "note": item.note,
-                "detail_url": f"/items/{item.id}",
-                "detail_label": "查看物品",
-                "secondary_url": f"/boxes/{item.box.id}",
-                "secondary_label": "查看所属箱子",
-                "path": f"位于箱子：{item.box.name}",
-                "is_container": item.is_container,
-                "image_url": f"/items/{item.id}/image" if item.image_blob else None,
-            }
-        )
+        _add("Item", item.id, {
+            "type": "Item",
+            "name": item.name,
+            "note": item.note,
+            "detail_url": f"/items/{item.id}",
+            "detail_label": "查看物品",
+            "secondary_url": f"/boxes/{item.box.id}",
+            "secondary_label": "查看所属箱子",
+            "path": f"位于箱子：{item.box.name}",
+            "is_container": item.is_container,
+            "image_url": f"/items/{item.id}/image" if item.image_blob else None,
+        })
 
     subitem_matches = (
         db.query(SubItem)
         .join(SubItem.parent_item)
         .join(Item.box)
-        .filter(
-            or_(
-                func.lower(SubItem.name).like(keyword),
-                func.lower(func.coalesce(SubItem.note, "")).like(keyword),
-            )
-        )
+        .filter(_or_like(SubItem.name, SubItem.note))
         .order_by(SubItem.id.desc())
         .all()
     )
     for subitem in subitem_matches:
-        results.append(
-            {
-                "type": "SubItem",
-                "name": subitem.name,
-                "note": subitem.note,
-                "detail_url": f"/items/{subitem.parent_item.id}",
-                "detail_label": "查看所属物品",
-                "secondary_url": f"/boxes/{subitem.parent_item.box.id}",
-                "secondary_label": "查看所属箱子",
-                "path": (
-                    f"位于物品：{subitem.parent_item.name} / "
-                    f"箱子：{subitem.parent_item.box.name}"
-                ),
-                "is_container": None,
-                "image_url": f"/subitems/{subitem.id}/image" if subitem.image_blob else None,
-            }
-        )
+        _add("SubItem", subitem.id, {
+            "type": "SubItem",
+            "name": subitem.name,
+            "note": subitem.note,
+            "detail_url": f"/items/{subitem.parent_item.id}",
+            "detail_label": "查看所属物品",
+            "secondary_url": f"/boxes/{subitem.parent_item.box.id}",
+            "secondary_label": "查看所属箱子",
+            "path": (
+                f"位于物品：{subitem.parent_item.name} / "
+                f"箱子：{subitem.parent_item.box.name}"
+            ),
+            "is_container": None,
+            "image_url": f"/subitems/{subitem.id}/image" if subitem.image_blob else None,
+        })
 
     return results
 
 
+def _ai_search(db: Session, cfg: "LLMConfig", query: str) -> tuple[list[str], list[dict], str | None]:
+    """Swappable AI search seam.
+
+    Returns ``(expanded_terms, results, error_message)``.
+    - On success: expanded terms + broadened results, ``error_message`` is ``None``.
+    - On failure (timeout, network error, HTTP error): empty terms + normal LIKE
+      results + friendly error message.
+    - On empty expansion (model returned ``[]`` legitimately): empty terms + normal
+      results, ``error_message`` is ``None``.
+    """
+    result = expand_query(cfg, query, extra_hints=cfg.ai_search_extra_hints)
+
+    if result.error:
+        # Real failure (timeout / network / HTTP) → show error + fallback
+        results = _build_search_results(db, query)
+        return [], results, result.error
+
+    if not result.terms:
+        # Legitimate empty expansion → normal results, no error
+        results = _build_search_results(db, query)
+        return [], results, None
+
+    # Deduplicate: original query + expanded terms
+    all_terms = list(dict.fromkeys([query] + result.terms))
+    results = _build_search_results(db, all_terms)
+    return result.terms, results, None
+
+
 def create_app() -> FastAPI:
     @asynccontextmanager
     async def lifespan(app: FastAPI):
@@ -285,10 +315,28 @@ def create_app() -> FastAPI:
     def search_page(
         request: Request,
         q: str | None = None,
+        ai: str | None = None,
         db: Session = Depends(get_db),
     ):
         query = (q or "").strip()
-        results = _build_search_results(db, query) if query else []
+        cfg = get_app_settings(db)
+        ai_requested = ai == "1"
+        ai_available = cfg.ai_search_enabled and is_configured(cfg)
+        expanded_terms: list[str] = []
+        ai_error: str | None = None
+
+        if query:
+            if ai_requested and ai_available:
+                try:
+                    expanded_terms, results, ai_error = _ai_search(db, cfg, query)
+                except Exception:  # noqa: BLE001 — graceful degradation
+                    ai_error = "AI 搜索暂时不可用，已回退到普通搜索。"
+                    results = _build_search_results(db, query)
+            else:
+                results = _build_search_results(db, query)
+        else:
+            results = []
+
         return templates.TemplateResponse(
             request=request,
             name="search/index.html",
@@ -297,6 +345,10 @@ def create_app() -> FastAPI:
                 "query": query,
                 "results": results,
                 "searched": bool(query),
+                "ai_activated": ai_requested and ai_available and bool(query),
+                "expanded_terms": expanded_terms,
+                "ai_error": ai_error,
+                "ai_available": ai_available,
             },
         )
 
@@ -335,6 +387,8 @@ def create_app() -> FastAPI:
         base_url: str | None = Form(default=None),
         model: str | None = Form(default=None),
         api_key: str | None = Form(default=None),
+        ai_search_enabled: str | None = Form(default=None),
+        ai_search_extra_hints: str | None = Form(default=None),
         db: Session = Depends(get_db),
     ) -> Response:
         # Origin/Referer check for browser requests
@@ -370,6 +424,8 @@ def create_app() -> FastAPI:
                         base_url=resolved_base_url,
                         model=resolved_model,
                         api_key=existing_cfg.api_key,
+                        ai_search_enabled=ai_search_enabled == "on",
+                        ai_search_extra_hints=_clean_text(ai_search_extra_hints) or "",
                     ),
                     "api_key_configured": bool(existing_cfg.api_key),
                     "test_result": LLMResult(
@@ -382,12 +438,16 @@ def create_app() -> FastAPI:
         # submitted_key is None → keep old key; str (including "") → use new value
         resolved_api_key = submitted_key
 
+        resolved_extra_hints = _clean_text(ai_search_extra_hints) or ""
+
         save_app_settings(
             db,
             enabled=enabled == "on",
             base_url=resolved_base_url,
             model=resolved_model,
             api_key=resolved_api_key,
+            ai_search_enabled=ai_search_enabled == "on",
+            ai_search_extra_hints=resolved_extra_hints,
         )
         return RedirectResponse(url="/settings", status_code=status.HTTP_303_SEE_OTHER)
 
@@ -398,6 +458,8 @@ def create_app() -> FastAPI:
         base_url: str | None = Form(default=None),
         model: str | None = Form(default=None),
         api_key: str | None = Form(default=None),
+        ai_search_enabled: str | None = Form(default=None),
+        ai_search_extra_hints: str | None = Form(default=None),
         db: Session = Depends(get_db),
     ):
         # Origin/Referer check for browser requests
@@ -436,6 +498,8 @@ def create_app() -> FastAPI:
                         base_url=resolved_base_url,
                         model=resolved_model,
                         api_key="",
+                        ai_search_enabled=ai_search_enabled == "on",
+                        ai_search_extra_hints=_clean_text(ai_search_extra_hints) or "",
                     ),
                     "api_key_configured": bool(existing_cfg.api_key),
                     "test_result": LLMResult(
@@ -450,6 +514,8 @@ def create_app() -> FastAPI:
             base_url=resolved_base_url,
             model=resolved_model,
             api_key=resolved_api_key or "",
+            ai_search_enabled=ai_search_enabled == "on",
+            ai_search_extra_hints=_clean_text(ai_search_extra_hints) or "",
         )
 
         result = test_connection(test_cfg)
diff --git a/app/settings_store.py b/app/settings_store.py
index 8ee84a4..d91d63a 100644
--- a/app/settings_store.py
+++ b/app/settings_store.py
@@ -25,6 +25,7 @@ class LLMConfig:
     model: str = ""
     api_key: str = ""
     ai_search_enabled: bool = False
+    ai_search_extra_hints: str = ""
 
 
 def _get_value(rows: dict[str, str], key: str, default: str) -> str:
@@ -48,6 +49,7 @@ def get_app_settings(db: Session) -> LLMConfig:
         model=_get_value(rows, "llm_model", ""),
         api_key=_get_value(rows, "llm_api_key", ""),
         ai_search_enabled=_get_bool(rows, "ai_search_enabled", False),
+        ai_search_extra_hints=_get_value(rows, "ai_search_extra_hints", ""),
     )
 
 
@@ -59,6 +61,7 @@ def save_app_settings(
     model: str | None = None,
     api_key: str | None = None,
     ai_search_enabled: bool | None = None,
+    ai_search_extra_hints: str | None = None,
 ) -> None:
     """Write settings to ``app_settings``.
 
@@ -77,6 +80,8 @@ def save_app_settings(
         updates["llm_api_key"] = api_key
     if ai_search_enabled is not None:
         updates["ai_search_enabled"] = str(ai_search_enabled).lower()
+    if ai_search_extra_hints is not None:
+        updates["ai_search_extra_hints"] = ai_search_extra_hints
 
     for key, value in updates.items():
         existing = db.get(AppSetting, key)
diff --git a/app/templates/search/index.html b/app/templates/search/index.html
index 873d6b2..42e87e2 100644
--- a/app/templates/search/index.html
+++ b/app/templates/search/index.html
@@ -20,7 +20,31 @@
     </form>
 </section>
 
+{% if query and ai_available %}
+<section class="card" style="margin-top: 8px;">
+    {% if ai_activated %}
+    <span class="muted">AI 搜索已启用</span>
+    {% else %}
+    <a href="/search?q={{ query | urlencode }}&ai=1" class="button button-secondary" style="display:inline-block; text-decoration:none;">
+        AI 智能搜索
+    </a>
+    {% endif %}
+</section>
+{% endif %}
+
 {% if searched %}
+    {% if ai_error %}
+    <section class="card" style="margin-top: 8px; border-color: #b42318;">
+        <p style="margin:0; color: #b42318;"><strong>{{ ai_error }}</strong></p>
+    </section>
+    {% endif %}
+
+    {% if ai_activated and expanded_terms %}
+    <section class="card" style="margin-top: 8px; border-color: #0b57d0;">
+        <p style="margin:0; color: #0b57d0;"><strong>AI 帮你扩展了：</strong>{{ expanded_terms | join('、') }}</p>
+    </section>
+    {% endif %}
+
     {% if results %}
     <section class="stack">
         <p class="muted">共找到 {{ results|length }} 条结果。</p>
diff --git a/app/templates/settings/form.html b/app/templates/settings/form.html
index 31f69da..4eae15c 100644
--- a/app/templates/settings/form.html
+++ b/app/templates/settings/form.html
@@ -47,6 +47,20 @@
         {% endif %}
     </label>
 
+    <hr style="border:none;border-top:1px solid #ddd;margin:16px 0;">
+
+    <label class="form-field checkbox-row">
+        <input type="checkbox" name="ai_search_enabled" {% if config.ai_search_enabled %}checked{% endif %}>
+        启用 AI 智能搜索
+    </label>
+    <p class="checkbox-help">开启后，搜索页将显示「AI 智能搜索」按钮，通过查询词扩展增强搜索结果。</p>
+
+    <label class="form-field">
+        额外领域提示（可选）
+        <textarea name="ai_search_extra_hints" rows="3" placeholder="例如：用户物品主要涉及厨房用品和电子产品">{% if config.ai_search_extra_hints %}{{ config.ai_search_extra_hints }}{% endif %}</textarea>
+    </label>
+    <p class="checkbox-help">追加到 AI 搜索提示词末尾，帮助模型理解你的物品领域。留空则使用默认提示词。</p>
+
     <div class="form-actions">
         <button type="submit" class="button button-primary">保存设置</button>
         <button type="submit" class="button button-secondary" formaction="/settings/test" formmethod="post">测试连接</button>
diff --git a/docs/design/llm-integration-design.md b/docs/design/llm-integration-design.md
index 4137268..f564169 100644
--- a/docs/design/llm-integration-design.md
+++ b/docs/design/llm-integration-design.md
@@ -210,7 +210,7 @@ OpenAI 兼容的薄客户端，基于 `httpx`，**无新依赖** / A thin OpenAI
 
 - `is_configured(cfg) -> bool`：开关开启且 `model`/`api_key` 齐全。
 - `test_connection(cfg) -> Result`：发一个最小请求验证 `base_url`/`model`/`api_key`，供配置页"测试连接"用。
-- `expand_query(cfg, query) -> list[str]`：把查询词扩成一批近义/相关词（提示词与输出契约见 §5.2）。
+- `expand_query(cfg, query, extra_hints="") -> ExpansionResult`：把查询词扩成一批近义/相关词；`terms` 为扩展词列表（不含原词），`error` 用于区分超时/网络/HTTP 等真实调用失败（提示词与输出契约见 §5.2）。
 - `analyze_image(...)`：**本轮不实现**，仅在文档中预留为未来接口（图片分析轮次）。Reserved for a future round, not implemented now.
 
 要点 / Notes：
@@ -247,8 +247,8 @@ When disabled/unconfigured: the settings page still works; the AI-search button
 
 - **常驻动作 / Persistent action：** 搜索页**始终**提供「AI 智能搜索」，**不以"零结果"为前提**——即便普通搜索已出结果，用户不满意时也能点。
   The "AI search" action is **always** present on the search page, **not gated on zero results** — usable even when normal results exist.
-- **流程 / Flow：** 普通 `LIKE` 照常先出结果 → 用户触发 AI → `expand_query` 把查询词扩成近义/相关词 → 用「原词 + 扩展词」对 `name`/`note` 做 OR `LIKE` 重搜 → 展示，并用横幅标注「AI 帮你扩展了：…」。
-  Normal `LIKE` first → user triggers AI → `expand_query` → OR-`LIKE` over name/note with the original + expanded terms → render with a banner listing the expansion.
+- **流程 / Flow：** 普通 `LIKE` 照常先出结果 → 用户触发 AI → `expand_query` 返回 `ExpansionResult`（扩展词 `terms` 不含原词；调用失败写入 `error`）→ `ai_search` 用「原词 + 扩展词」对 `name`/`note` 做 OR `LIKE` 重搜 → 展示，并用横幅标注「AI 帮你扩展了：…」。
+  Normal `LIKE` first → user triggers AI → `expand_query` returns an `ExpansionResult` (`terms` exclude the original query; failures are represented by `error`) → `ai_search` OR-`LIKE`s over name/note with the original + expanded terms → render with a banner listing the expansion.
 - **只把查询词发出去 / Only the query leaves**，不外泄物品清单；token 恒定、不随上千件物品增长。
   Only the query is sent; the inventory is not. Token cost is constant and does not grow with thousands of items.
 
@@ -261,8 +261,8 @@ Quality hinges on the prompt; integration stability hinges on the output contrac
   Frames the moving/household domain, asks for related naming terms, follows the query's language, caps the count, no prose.
 - **可选「额外领域提示」/ Optional extra hints：** KV `ai_search_extra_hints`（设置页一个多行输入，默认空）。非空时**追加**到基础提示词之后，供业主微调倾向（如"厨房用品多，偏向厨具类"）。**它只能补充，不能改写输出格式。**
   An optional free-text setting appended to the base prompt; it can only add guidance, never alter the output format.
-- **输出契约（代码强制，与提示词解耦）/ Output contract (code-enforced)：** 要求模型只返回 **JSON 字符串数组**；解析时去掉 ` ```json ` 围栏 → `json.loads` → 失败按行/逗号兜底 → 再不行返回 `[]`。`expand_query` 只返回扩展词；**原词由 `ai_search` 并入并去重**，数量在代码侧再封顶一次。
-  Require a JSON string array; tolerant parse with fallbacks to `[]`. `ai_search` adds the original term and dedupes; the count is capped in code.
+- **输出契约（代码强制，与提示词解耦）/ Output contract (code-enforced)：** 要求模型只返回 **JSON 字符串数组**；解析时去掉 ` ```json ` 围栏 → `json.loads` → 只接受字符串数组 → 过滤空串/过长词 → 最多 8 个。散文、坏 JSON、JSON object、非字符串数组都视为**合法空扩展**（`terms=[]`, `error=None`）；网络错误、HTTP 错误、超时等真实调用失败写入 `ExpansionResult.error`。`expand_query` 的 `terms` 只包含扩展词；**原词由 `ai_search` 并入并去重**。
+  Require a JSON string array; strip code fences, `json.loads`, accept only string arrays, filter empty/overlong terms, and cap to 8 terms. Prose, bad JSON, JSON objects, and non-string arrays are successful empty expansions (`terms=[]`, `error=None`); network/HTTP/timeout failures are represented by `ExpansionResult.error`. `expand_query.terms` contains only expanded terms; `ai_search` adds the original term and dedupes.
 - **客户端参数 / Client params：** 低 temperature、较小 max_tokens、设超时。Low temperature, small max_tokens, a timeout.
 - **措辞留松 / Wording left loose：** 默认提示词的具体字句可在 step-3 实测中迭代，不在文档里冻死。
   Exact default wording can be iterated during step-3 testing.
@@ -271,8 +271,8 @@ Quality hinges on the prompt; integration stability hinges on the output contrac
 
 - 路由层扩展现有 `GET /search`：增加 `ai=1` 触发位（如 `GET /search?q=锅&ai=1`），保持单页、可收藏、SSR 友好。
   Extend the existing `GET /search` with an `ai=1` trigger (e.g. `/search?q=…&ai=1`), staying single-page and bookmarkable.
-- 内部定义可替换的检索 seam，例如 `ai_search(db, query) -> (expanded_terms, results)`：
-  Define a replaceable retrieval seam, e.g. `ai_search(db, query) -> (expanded_terms, results)`:
+- 内部定义可替换的检索 seam，例如 `ai_search(db, query) -> (expanded_terms, results, error_message)`：
+  Define a replaceable retrieval seam, e.g. `ai_search(db, query) -> (expanded_terms, results, error_message)`:
   - **本轮 / now：** 内部＝查询词扩展 + 本地 `LIKE`。
   - **未来 / later：** 换成向量嵌入 + 相似度检索，**路由与模板不变**。
     Swap to embeddings + similarity later **without changing the route or template**.
@@ -284,6 +284,9 @@ Quality hinges on the prompt; integration stability hinges on the output contrac
 AI 关闭/未配置 → 不显示按钮（或提示去 `/settings`）；调用失败 → 友好提示并回退到普通结果。
 AI off/unconfigured → no button (or a hint to `/settings`); on failure → a friendly message, fall back to normal results.
 
+合法空扩展（模型返回 `[]` 或输出无法通过严格 JSON 字符串数组契约）不视为调用失败：回退普通结果，不显示故障提示。
+A legitimate empty expansion (model returns `[]` or output fails the strict JSON-string-array contract) is not treated as a call failure: fall back to normal results without an error banner.
+
 ---
 
 ## 6. 数据模型与路由变更 / Data Model & Route Changes
diff --git a/docs/design/step-2-llm-integration.md b/docs/design/step-2-llm-integration.md
index cc063f7..589581c 100644
--- a/docs/design/step-2-llm-integration.md
+++ b/docs/design/step-2-llm-integration.md
@@ -55,7 +55,7 @@ A settings page to enter & test the LLM config, persisted to `app_settings`, plu
 - [ ] 新增 `app/llm.py`（基于 `httpx`）：
   - [ ] `is_configured(cfg) -> bool`
   - [ ] `test_connection(cfg) -> Result`（发最小请求验证 `base_url`/`model`/`api_key`）。
-  - [ ] `expand_query(cfg, query) -> list[str]`（查询词扩展；**步骤 3 会用**，本步先落地+单测）。
+  - [ ] `expand_query(cfg, query) -> ExpansionResult`（查询词扩展；**步骤 3 会校准提示词与输出契约**；`terms` 为扩展词列表，`error` 用于区分超时/网络/HTTP 等真实调用失败）。
   - [ ] 统一超时 + 错误处理；失败优雅降级。
   - [ ] **（预留，不实现）** `analyze_image(...)`：仅留 TODO/签名占位 + 注释指向"未来图片分析轮次"。Reserved, not implemented.
   - [ ] 把所有网络调用收敛到**单一函数边界**，便于测试整体 mock。
diff --git a/docs/design/step-3-ai-search.md b/docs/design/step-3-ai-search.md
index f1e6a9f..c9b03be 100644
--- a/docs/design/step-3-ai-search.md
+++ b/docs/design/step-3-ai-search.md
@@ -17,7 +17,7 @@ A **persistent** "AI search" action on the search page that broadens results via
 
 - 现有搜索：`app/main.py::_build_search_results(db, query)` 对 `Box`/`Item`/`SubItem` 的 `name` 与 `note` 做大小写不敏感 `LIKE`，返回结果列表；路由 `GET /search`（函数 `search_page`，参数 `q`）渲染 `app/templates/search/index.html`。
   Existing search: `_build_search_results(db, query)` does case-insensitive `LIKE` over name/note; route `GET /search` renders `search/index.html`.
-- 步骤 2 已提供：`app/llm.py::expand_query(cfg, query) -> list[str]`、配置读取 `get_app_settings(db)`、开关 `ai_search_enabled` 与 `is_configured(cfg)`、设置页 `app/templates/settings/form.html`。
+- 步骤 2 已提供：`app/llm.py::expand_query` 的基础能力、配置读取 `get_app_settings(db)`、开关 `ai_search_enabled` 与 `is_configured(cfg)`、设置页 `app/templates/settings/form.html`；本步将 `expand_query` 校准为返回结构化 `ExpansionResult(terms, error)`。
 - 本步**新增**配置项 `ai_search_extra_hints`（可选「额外领域提示」）并在设置页加一个多行输入——这是本步**唯一**触及设置页之处。
   This step adds the `ai_search_extra_hints` setting + a textarea on the settings page (the only settings-page change here).
 - 本轮检索范围＝`name` + `note`（`image_description` 本轮不存在，属未来图片分析轮次）。
@@ -27,12 +27,12 @@ A **persistent** "AI search" action on the search page that broadens results via
 
 - **常驻、不依赖零结果。** 普通 `LIKE` 照常先出结果；AI 动作始终可用（开启且已配置时）。
   Persistent and not gated on zero results.
-- **流程：** 触发 AI → `expand_query` 得到"原词 + 一批近义/相关词" → 用这组词对 `name`/`note` 做 OR `LIKE` 重搜 → 展示，并用横幅标注「AI 帮你扩展了：…」。**只把查询词发出去**，不外泄物品清单。
-  Trigger → expand → OR-`LIKE` over the original + expanded terms → render with a banner of the expansion. Only the query leaves.
-- **可替换的检索 seam。** 把 AI 检索抽成一个函数（如 `ai_search(db, query) -> (expanded_terms, results)`），本轮内部＝查询词扩展 + 本地 `LIKE`；**未来换成向量嵌入 + 相似度时，路由与模板不变**。
+- **流程：** 触发 AI → `expand_query` 返回 `ExpansionResult`（扩展词 `terms` 不含原词，调用失败写入 `error`）→ `ai_search` 合并「原词 + 扩展词」并对 `name`/`note` 做 OR `LIKE` 重搜 → 展示，并用横幅标注「AI 帮你扩展了：…」。**只把查询词发出去**，不外泄物品清单。
+  Trigger → `expand_query` returns an `ExpansionResult` (`terms` exclude the original query; failures are represented by `error`) → `ai_search` OR-`LIKE`s over the original + expanded terms → render with a banner of the expansion. Only the query leaves.
+- **可替换的检索 seam。** 把 AI 检索抽成一个函数（如 `ai_search(db, query) -> (expanded_terms, results, error_message)`），本轮内部＝查询词扩展 + 本地 `LIKE`；**未来换成向量嵌入 + 相似度时，路由与模板不变**。
   Wrap AI retrieval behind a swappable seam so embeddings can replace it later without touching route/template.
-- **提示词（决策 C，详见设计 §5.2）。** 基础系统提示词**写死在 `app/llm.py`**；设置页可选的 `ai_search_extra_hints` 非空时**追加**到其后；**输出契约由代码强制**（要求 JSON 字符串数组 → 容忍性解析 → 失败返回 `[]`），用户改 hints 也改不坏解析。
-  Base prompt hardcoded; optional extra hints appended; output contract (JSON array → tolerant parse → `[]`) enforced in code.
+- **提示词（决策 C，详见设计 §5.2）。** 基础系统提示词**写死在 `app/llm.py`**；设置页可选的 `ai_search_extra_hints` 非空时**追加**到其后；**输出契约由代码强制**（只接受 JSON 字符串数组；散文/坏 JSON/非字符串数组解析为合法空扩展；网络/超时/HTTP 失败写入 `ExpansionResult.error`），用户改 hints 也改不坏解析。
+  Base prompt hardcoded; optional extra hints appended; output contract enforced in code: only a JSON string array is accepted; prose/bad JSON/non-string arrays become a successful empty expansion; network/timeout/HTTP failures are represented by `ExpansionResult.error`.
 - **优雅降级。** AI 关闭/未配置 → 不显示按钮（或提示去 `/settings`）；调用失败 → 友好提示 + 回退普通结果。
 
 ---
@@ -43,10 +43,13 @@ A **persistent** "AI search" action on the search page that broadens results via
   - 基础系统提示词写死在 `app/llm.py`（搬家/家居场景、列相关命名词、跟随查询语言、≤ ~8 个、不解释、不造无关词）。默认提示词起点（**可迭代** / a starting point, tune during testing）：
     > 你是搬家物品搜索助手。用户在搜索自己打包的箱子与物品（家居/搬家场景）。给定一个搜索词，列出用户可能用来命名同一类物品的相关词：近义词、常见别称、上位类别、具体品类。规则：用与查询相同的语言；只给与该物品紧密相关、有助于在清单里找到它的词；不要解释、不要造无关词；最多 8 个；只输出一个 JSON 字符串数组，例如 `["炒锅","平底锅","汤锅","厨具"]`。
   - 读取 `ai_search_extra_hints`，非空则**追加**到基础提示词之后（只补充，不改格式）。
-  - **输出契约**：要求模型只回 JSON 字符串数组；解析去 ` ```json ` 围栏 → `json.loads` → 失败按行/逗号兜底 → 再不行返回 `[]`；任何异常/超时都返回 `[]`（不抛错）。
+  - **返回契约**：`expand_query(cfg, query, extra_hints="") -> ExpansionResult`，其中 `terms` 是扩展词列表（**不含原词**），`error` 在成功时为 `None`。
+  - **输出契约**：要求模型只回 JSON 字符串数组；解析去 ` ```json ` 围栏 → `json.loads` → 只接受字符串数组 → 过滤空串/过长词 → 最多 8 个；散文、坏 JSON、JSON object、非字符串数组都返回 `terms=[]` 且 `error=None`（合法空扩展）；网络错误、HTTP 错误、超时等调用失败返回 `terms=[]` 且 `error=<友好错误>`；不向上抛 500。
 - [ ] **新增配置项 `ai_search_extra_hints`**：KV 默认空；纳入 `get_app_settings` / `save_app_settings`；设置页 `app/templates/settings/form.html` 加一个多行输入（沿用 step 2 风格）。
-- [ ] 实现检索 seam：在 `app/main.py`（或抽一个小搜索模块 `app/search.py`）加 `ai_search(db, query) -> (expanded_terms, results)`：
-  - 调 `expand_query(cfg, query)` 得到扩展词；
+- [ ] 实现检索 seam：在 `app/main.py`（或抽一个小搜索模块 `app/search.py`）加 `ai_search(db, query) -> (expanded_terms, results, error_message)`：
+  - 调 `expand_query(cfg, query)` 得到 `ExpansionResult`；
+  - 若 `result.error` 非空：回退普通搜索，并把友好错误传给模板；
+  - 若 `result.terms` 为空且无错误：视为合法空扩展，回退普通搜索，不显示故障提示；
   - 用「原词 + 扩展词」对 `name`/`note` 做 OR `LIKE`（**复用现有 `_build_search_results` 的匹配逻辑**，避免重复实现），去重。
   - 注意：现有 `_build_search_results(db, query)` 只接收单个查询词；建议把它泛化为接收一组关键词（对多个词做 OR），让 AI 搜索与普通搜索共用同一套匹配逻辑，避免分叉。
     Note: `_build_search_results` currently takes a single query — generalize it to accept multiple keywords so AI and normal search share one matching path.
@@ -62,9 +65,9 @@ A **persistent** "AI search" action on the search page that broadens results via
   - [ ] 扩展词驱动命中：原词 `LIKE` 搜不到、扩展后能搜到。
   - [ ] 已有结果时点 AI 仍可用，且结果集被扩大（含原结果）。
   - [ ] 按钮可见性随 `ai_search_enabled` + `is_configured()` 门控。
-  - [ ] 调用失败 → 回退普通结果、页面不报错。
-  - [ ] `expand_query` 输出解析：模型回合法 JSON 数组 → 正确解析；回散文/坏 JSON/超时 → 返回 `[]`、不抛错。
-    Output parsing: valid JSON array → parsed; prose/bad JSON/timeout → `[]`, no raise.
+  - [ ] 调用失败（超时/网络/HTTP）→ 回退普通结果、显示友好提示、页面不报错。
+  - [ ] `expand_query` 输出解析：模型回合法 JSON 数组 → 正确解析；回散文/坏 JSON/非字符串数组 → `terms=[]` 且 `error=None`；超时/网络/HTTP 失败 → `terms=[]` 且 `error` 非空；均不抛错。
+    Output parsing: valid JSON array → parsed; prose/bad JSON/non-string arrays → `terms=[]`, `error=None`; timeout/network/HTTP failures → `terms=[]`, non-empty `error`; no raise.
   - [ ] `ai_search_extra_hints` 非空时确被追加进请求（可对构造的请求体断言）。
     Extra hints, when set, are appended to the request.
 
diff --git a/tests/test_ai_search.py b/tests/test_ai_search.py
new file mode 100644
index 0000000..a3af58f
--- /dev/null
+++ b/tests/test_ai_search.py
@@ -0,0 +1,707 @@
+"""Tests for AI search (Step 3).
+
+All LLM calls are mocked — CI never touches the network.
+
+Coverage areas:
+- expand_query JSON output parsing (valid, fenced, prose, bad JSON, timeout)
+- Output contract enforcement (strict JSON array only)
+- Expansion term count cap and length cap
+- ai_search seam function
+- GET /search with ai=1 trigger
+- AI button visibility on search page
+- Graceful degradation on failure
+- ai_search_extra_hints appended to prompt
+- ai_search_enabled toggle
+"""
+
+from unittest.mock import patch
+
+import httpx
+import pytest
+
+from app.llm import (
+    _MAX_EXPANSION_TERMS,
+    _MAX_TERM_LENGTH,
+    ExpansionResult,
+    LLMResult,
+    _parse_json_string_array,
+    expand_query,
+    is_configured,
+)
+from app.main import _ai_search, _build_search_results
+from app.models import AppSetting, Box, Item, SubItem
+from app.settings_store import LLMConfig, get_app_settings, save_app_settings
+
+
+# ---------------------------------------------------------------------------
+# Helper: configure AI search for route tests
+# ---------------------------------------------------------------------------
+
+_AI_CFG = LLMConfig(
+    enabled=True,
+    base_url="https://api.example.com/v1",
+    model="gpt-4o-mini",
+    api_key="sk-test-key",
+    ai_search_enabled=True,
+)
+
+
+def _enable_ai_search(client, db_session):
+    """Persist a fully-configured AI search setup via the settings route."""
+    client.post(
+        "/settings",
+        data={
+            "enabled": "on",
+            "base_url": "https://api.example.com/v1",
+            "model": "gpt-4o-mini",
+            "api_key": "sk-test-key",
+            "ai_search_enabled": "on",
+        },
+        follow_redirects=False,
+    )
+
+
+# ---------------------------------------------------------------------------
+# _parse_json_string_array: strict JSON contract enforcement
+# ---------------------------------------------------------------------------
+
+
+class TestParseJsonStringArray:
+    def test_valid_json_array(self):
+        result = _parse_json_string_array('["炒锅","平底锅","汤锅"]')
+        assert result == ["炒锅", "平底锅", "汤锅"]
+
+    def test_json_array_with_code_fence(self):
+        result = _parse_json_string_array('```json\n["锅","铲子"]\n```')
+        assert result == ["锅", "铲子"]
+
+    def test_json_array_with_code_fence_no_lang(self):
+        result = _parse_json_string_array('```\n["锅","铲子"]\n```')
+        assert result == ["锅", "铲子"]
+
+    def test_empty_string_returns_empty(self):
+        assert _parse_json_string_array("") == []
+        assert _parse_json_string_array("  ") == []
+
+    def test_prose_returns_empty(self):
+        """Prose text does NOT become expansion terms — strict contract."""
+        assert _parse_json_string_array("I cannot help with that.") == []
+
+    def test_prose_newlines_returns_empty(self):
+        """Line-separated prose does NOT become expansion terms."""
+        assert _parse_json_string_array("炒锅\n平底锅\n汤锅") == []
+
+    def test_prose_commas_returns_empty(self):
+        """Comma-separated prose does NOT become expansion terms."""
+        assert _parse_json_string_array("炒锅, 平底锅, 汤锅") == []
+
+    def test_bad_json_returns_empty(self):
+        """Invalid JSON returns empty — no fallback."""
+        assert _parse_json_string_array("{invalid json") == []
+
+    def test_json_object_returns_empty(self):
+        """JSON object (non-array) returns empty."""
+        assert _parse_json_string_array('{"terms":["锅","厨具"]}') == []
+
+    def test_json_array_with_numbers_returns_empty(self):
+        """Non-string items in array cause rejection — strict contract."""
+        assert _parse_json_string_array('[1, 2, 3]') == []
+
+    def test_json_array_with_mixed_types_returns_empty(self):
+        """Mixed string/number array is rejected."""
+        assert _parse_json_string_array('["锅", 1]') == []
+
+    def test_empty_json_array(self):
+        result = _parse_json_string_array('[]')
+        assert result == []
+
+    def test_capped_at_max_terms(self):
+        """More than _MAX_EXPANSION_TERMS items are truncated."""
+        terms = [f"词{i}" for i in range(20)]
+        json_str = "[" + ",".join(f'"{t}"' for t in terms) + "]"
+        result = _parse_json_string_array(json_str)
+        assert len(result) == _MAX_EXPANSION_TERMS
+
+    def test_long_terms_filtered_out(self):
+        """Terms exceeding _MAX_TERM_LENGTH are silently dropped."""
+        short = "锅"
+        long_term = "A" * (_MAX_TERM_LENGTH + 1)
+        json_str = f'["{short}", "{long_term}"]'
+        result = _parse_json_string_array(json_str)
+        assert result == ["锅"]
+
+    def test_whitespace_stripped(self):
+        result = _parse_json_string_array('["  锅  ", " 平底锅 "]')
+        assert result == ["锅", "平底锅"]
+
+    def test_empty_strings_filtered(self):
+        result = _parse_json_string_array('["锅", "", "  ", "平底锅"]')
+        assert result == ["锅", "平底锅"]
+
+
+# ---------------------------------------------------------------------------
+# expand_query: prompt, hints, graceful degradation
+# ---------------------------------------------------------------------------
+
+
+class TestExpandQueryNew:
+    def test_returns_empty_when_not_configured(self):
+        cfg = LLMConfig(enabled=False)
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is None
+
+    @patch("app.llm._call_chat_completion")
+    def test_parses_valid_json_response(self, mock_call):
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["炒锅","平底锅","汤锅","厨具"]'}}]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert "炒锅" in result.terms
+        assert "平底锅" in result.terms
+        assert "厨具" in result.terms
+        assert result.error is None
+
+    @patch("app.llm._call_chat_completion")
+    def test_handles_json_with_code_fence(self, mock_call):
+        mock_call.return_value = {
+            "choices": [
+                {"message": {"content": '```json\n["炒锅","平底锅"]\n```'}}
+            ]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert "炒锅" in result.terms
+        assert result.error is None
+
+    @patch("app.llm._call_chat_completion")
+    def test_prose_response_returns_empty_no_error(self, mock_call):
+        """Prose from model → empty terms, no error (successful call, unparseable output)."""
+        mock_call.return_value = {
+            "choices": [{"message": {"content": "I cannot help with that."}}]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is None
+
+    @patch("app.llm._call_chat_completion")
+    def test_json_object_response_returns_empty_no_error(self, mock_call):
+        """JSON object (non-array) → empty terms, no error."""
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '{"terms":["锅","厨具"]}'}}]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is None
+
+    @patch("app.llm._call_chat_completion")
+    def test_timeout_returns_error(self, mock_call):
+        """Timeout → empty terms + error message."""
+        mock_call.side_effect = httpx.TimeoutException("timeout")
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is not None
+        assert "超时" in result.error
+
+    @patch("app.llm._call_chat_completion")
+    def test_network_error_returns_error(self, mock_call):
+        """Network error → empty terms + error message."""
+        mock_call.side_effect = httpx.ConnectError("refused")
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is not None
+        assert "无法连接" in result.error
+
+    @patch("app.llm._call_chat_completion")
+    def test_http_error_returns_error(self, mock_call):
+        """HTTP error → empty terms + error message."""
+        mock_call.side_effect = httpx.HTTPStatusError(
+            "401",
+            request=httpx.Request("POST", "http://x"),
+            response=httpx.Response(401),
+        )
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is not None
+        assert "错误" in result.error
+
+    @patch("app.llm._call_chat_completion")
+    def test_returns_empty_on_empty_choices(self, mock_call):
+        mock_call.return_value = {"choices": []}
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        result = expand_query(cfg, "锅")
+        assert result.terms == []
+        assert result.error is None
+
+    @patch("app.llm._call_chat_completion")
+    def test_extra_hints_appended_to_system_prompt(self, mock_call):
+        """When extra_hints is non-empty, it should be appended to the system prompt."""
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["扩展词"]'}}]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        expand_query(cfg, "锅", extra_hints="用户物品主要涉及厨房用品")
+
+        # Verify the system prompt includes the extra hints
+        call_args = mock_call.call_args
+        messages = call_args[1]["messages"] if "messages" in call_args[1] else call_args[0][1]
+        system_content = messages[0]["content"]
+        assert "用户物品主要涉及厨房用品" in system_content
+
+    @patch("app.llm._call_chat_completion")
+    def test_extra_hints_ignored_when_empty(self, mock_call):
+        """When extra_hints is empty, system prompt should not change."""
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["扩展词"]'}}]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        expand_query(cfg, "锅", extra_hints="")
+
+        call_args = mock_call.call_args
+        messages = call_args[1]["messages"] if "messages" in call_args[1] else call_args[0][1]
+        system_content = messages[0]["content"]
+        # Should be the base prompt only
+        assert "搬家物品搜索助手" in system_content
+        assert "JSON 字符串数组" in system_content
+
+    @patch("app.llm._call_chat_completion")
+    def test_temperature_zero_passed(self, mock_call):
+        """expand_query should pass temperature=0 for deterministic output."""
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["扩展词"]'}}]
+        }
+        cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
+        expand_query(cfg, "锅")
+
+        call_args = mock_call.call_args
+        assert call_args[1]["temperature"] == 0
+
+
+# ---------------------------------------------------------------------------
+# _ai_search: seam function
+# ---------------------------------------------------------------------------
+
+
+class TestAiSearchSeam:
+    @patch("app.main.expand_query")
+    def test_returns_expanded_terms_and_results(self, mock_expand, client, db_session):
+        """AI search returns expanded terms and broader results."""
+        box = Box(name="厨房箱", note="装了炒锅和铲子")
+        db_session.add(box)
+        db_session.commit()
+
+        mock_expand.return_value = ExpansionResult(terms=["炒锅", "平底锅", "汤锅"])
+
+        cfg = get_app_settings(db_session)
+        expanded, results, error = _ai_search(db_session, cfg, "平底锅")
+
+        assert "炒锅" in expanded
+        assert error is None
+        assert len(results) >= 1
+        assert any("厨房箱" in r["name"] or "炒锅" in (r.get("note") or "") for r in results)
+
+    @patch("app.main.expand_query")
+    def test_includes_original_query_in_search(self, mock_expand, client, db_session):
+        """AI search includes the original query term in the search."""
+        box = Box(name="冬季衣物箱")
+        db_session.add(box)
+        db_session.commit()
+
+        mock_expand.return_value = ExpansionResult(terms=["羽绒服"])
+
+        cfg = get_app_settings(db_session)
+        expanded, results, error = _ai_search(db_session, cfg, "衣物")
+
+        assert error is None
+        assert any("冬季衣物箱" in r["name"] for r in results)
+
+    @patch("app.main.expand_query")
+    def test_empty_expansion_returns_normal_results_no_error(self, mock_expand, client, db_session):
+        """Legitimate empty expansion (no synonyms found) → normal results, no error."""
+        box = Box(name="书房箱")
+        db_session.add(box)
+        db_session.commit()
+
+        mock_expand.return_value = ExpansionResult(terms=[])
+
+        cfg = get_app_settings(db_session)
+        expanded, results, error = _ai_search(db_session, cfg, "书房")
+
+        assert expanded == []
+        assert error is None
+        assert any("书房箱" in r["name"] for r in results)
+
+    @patch("app.main.expand_query")
+    def test_llm_failure_returns_normal_results_with_error(self, mock_expand, client, db_session):
+        """When expand_query signals failure, seam returns normal results + error message."""
+        box = Box(name="厨房箱", note="装了炒锅")
+        db_session.add(box)
+        db_session.commit()
+
+        mock_expand.return_value = ExpansionResult(terms=[], error="AI 搜索请求超时，请稍后再试。")
+
+        cfg = get_app_settings(db_session)
+        expanded, results, error = _ai_search(db_session, cfg, "厨房")
+
+        assert expanded == []
+        assert error is not None
+        assert "超时" in error
+        assert len(results) >= 1
+
+
+# ---------------------------------------------------------------------------
+# _build_search_results: multi-keyword support
+# ---------------------------------------------------------------------------
+
+
+class TestBuildSearchResultsMultiKeyword:
+    def test_single_keyword_works_as_before(self, db_session):
+        box = Box(name="厨房箱")
+        db_session.add(box)
+        db_session.commit()
+
+        results = _build_search_results(db_session, "厨房")
+        assert len(results) == 1
+        assert results[0]["name"] == "厨房箱"
+
+    def test_multiple_keywords_match_any(self, db_session):
+        box1 = Box(name="厨房箱")
+        box2 = Box(name="卧室箱")
+        db_session.add_all([box1, box2])
+        db_session.commit()
+
+        results = _build_search_results(db_session, ["厨房", "卧室"])
+        assert len(results) == 2
+
+    def test_multiple_keywords_dedupes_results(self, db_session):
+        """A box matching multiple keywords appears only once."""
+        box = Box(name="厨房箱", note="装了厨房用品")
+        db_session.add(box)
+        db_session.commit()
+
+        results = _build_search_results(db_session, ["厨房", "用品"])
+        assert len(results) == 1
+
+    def test_empty_keywords_returns_empty(self, db_session):
+        results = _build_search_results(db_session, [])
+        assert results == []
+
+
+# ---------------------------------------------------------------------------
+# Routes: GET /search with ai=1
+# ---------------------------------------------------------------------------
+
+
+class TestSearchRouteAI:
+    @patch("app.llm._call_chat_completion")
+    def test_ai_search_finds_more_results(self, mock_call, client, db_session):
+        """Original query misses, but expanded term finds items."""
+        box = Box(name="杂物箱")
+        item = Item(name="炒锅", box=box, is_container=False)
+        db_session.add_all([box, item])
+        db_session.commit()
+
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["炒锅","平底锅","汤锅"]'}}]
+        }
+        _enable_ai_search(client, db_session)
+
+        # Normal search for "平底锅" — no results
+        response = client.get("/search?q=平底锅")
+        assert "没有找到匹配结果" in response.text
+
+        # AI search for "平底锅" — finds "炒锅" via expansion
+        response = client.get("/search?q=平底锅&ai=1")
+        assert response.status_code == 200
+        assert "炒锅" in response.text
+        assert "AI 帮你扩展了" in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_ai_search_includes_original_results(self, mock_call, client, db_session):
+        """AI search should also include results from original query."""
+        box = Box(name="厨房箱")
+        item1 = Item(name="锅铲", box=box, is_container=False)
+        item2 = Item(name="平底锅", box=box, is_container=False)
+        db_session.add_all([box, item1, item2])
+        db_session.commit()
+
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["炒锅","汤锅"]'}}]
+        }
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=锅&ai=1")
+        assert response.status_code == 200
+        # Original result "平底锅" should still be there
+        assert "平底锅" in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_ai_search_shows_expansion_banner(self, mock_call, client, db_session):
+        """When AI search is activated, a banner shows expanded terms."""
+        box = Box(name="厨房箱")
+        db_session.add(box)
+        db_session.commit()
+
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["炒锅","平底锅"]'}}]
+        }
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=锅&ai=1")
+        assert response.status_code == 200
+        assert "AI 帮你扩展了" in response.text
+        assert "炒锅" in response.text
+
+    def test_ai_search_without_flag_does_normal_search(self, client, db_session):
+        """Without ai=1, search behaves normally even when AI is configured."""
+        box = Box(name="厨房箱")
+        db_session.add(box)
+        db_session.commit()
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=厨房")
+        assert response.status_code == 200
+        assert "厨房箱" in response.text
+        assert "AI 帮你扩展了" not in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_ai_search_without_configuration_ignores_flag(self, mock_call, client, db_session):
+        """ai=1 is ignored when AI is not configured."""
+        box = Box(name="厨房箱")
+        db_session.add(box)
+        db_session.commit()
+
+        response = client.get("/search?q=厨房&ai=1")
+        assert response.status_code == 200
+        assert "厨房箱" in response.text
+        assert "AI 帮你扩展了" not in response.text
+        mock_call.assert_not_called()
+
+    @patch("app.llm._call_chat_completion")
+    def test_ai_search_graceful_degradation_on_llm_failure(self, mock_call, client, db_session):
+        """LLM failure (timeout) → normal results + friendly error banner."""
+        box = Box(name="厨房箱", note="装了炒锅")
+        db_session.add(box)
+        db_session.commit()
+
+        # expand_query catches timeout and returns ExpansionResult with error
+        mock_call.side_effect = httpx.TimeoutException("timeout")
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=厨房&ai=1")
+        assert response.status_code == 200
+        assert "厨房箱" in response.text
+        # Should show error banner — timeout is a real failure
+        assert "超时" in response.text or "不可用" in response.text
+
+    def test_ai_search_empty_query_does_nothing(self, client, db_session):
+        """ai=1 with empty query does not trigger AI."""
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?ai=1")
+        assert response.status_code == 200
+        assert "AI 帮你扩展了" not in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_ai_search_disabled_ignores_flag(self, mock_call, client, db_session):
+        """ai=1 is ignored when ai_search_enabled is False."""
+        box = Box(name="厨房箱")
+        db_session.add(box)
+        db_session.commit()
+        # Enable LLM but NOT ai_search_enabled
+        client.post(
+            "/settings",
+            data={
+                "enabled": "on",
+                "base_url": "https://api.example.com/v1",
+                "model": "gpt-4o-mini",
+                "api_key": "sk-test-key",
+            },
+            follow_redirects=False,
+        )
+
+        response = client.get("/search?q=厨房&ai=1")
+        assert response.status_code == 200
+        assert "厨房箱" in response.text
+        assert "AI 帮你扩展了" not in response.text
+        mock_call.assert_not_called()
+
+
+# ---------------------------------------------------------------------------
+# Button visibility on search page
+# ---------------------------------------------------------------------------
+
+
+class TestAIButtonVisibility:
+    @patch("app.llm._call_chat_completion")
+    def test_button_visible_when_configured_and_enabled(self, mock_call, client, db_session):
+        """AI search button is visible when ai_search_enabled and configured."""
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=测试")
+        assert response.status_code == 200
+        assert "AI 智能搜索" in response.text
+
+    def test_button_hidden_when_not_configured(self, client, db_session):
+        """AI search button is hidden when LLM is not configured."""
+        response = client.get("/search?q=测试")
+        assert response.status_code == 200
+        assert "AI 智能搜索" not in response.text
+
+    def test_button_hidden_when_ai_search_disabled(self, client, db_session):
+        """AI search button is hidden when ai_search_enabled is False."""
+        client.post(
+            "/settings",
+            data={
+                "enabled": "on",
+                "base_url": "https://api.example.com/v1",
+                "model": "gpt-4o-mini",
+                "api_key": "sk-test-key",
+            },
+            follow_redirects=False,
+        )
+
+        response = client.get("/search?q=测试")
+        assert "AI 智能搜索" not in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_button_hidden_on_empty_query(self, mock_call, client, db_session):
+        """AI search button is not shown when there's no query."""
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search")
+        assert "AI 智能搜索" not in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_button_link_includes_current_query(self, mock_call, client, db_session):
+        """AI button link includes the current query parameter."""
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=锅")
+        assert response.status_code == 200
+        assert "ai=1" in response.text
+        from urllib.parse import quote
+        assert f"q={quote('锅')}" in response.text or "q=锅" in response.text
+
+    @patch("app.llm._call_chat_completion")
+    def test_no_button_when_ai_already_activated(self, mock_call, client, db_session):
+        """When AI is already activated, show status text instead of button."""
+        mock_call.return_value = {
+            "choices": [{"message": {"content": '["炒锅"]'}}]
+        }
+        _enable_ai_search(client, db_session)
+
+        response = client.get("/search?q=锅&ai=1")
+        assert response.status_code == 200
+        assert "AI 搜索已启用" in response.text
+
+
+# ---------------------------------------------------------------------------
+# Settings: ai_search_extra_hints
+# ---------------------------------------------------------------------------
+
+
+class TestExtraHintsSettings:
+    def test_extra_hints_defaults_to_empty(self, db_session):
+        cfg = get_app_settings(db_session)
+        assert cfg.ai_search_extra_hints == ""
+
+    def test_save_extra_hints(self, db_session):
+        save_app_settings(db_session, ai_search_extra_hints="用户物品主要涉及厨房")
+        cfg = get_app_settings(db_session)
+        assert cfg.ai_search_extra_hints == "用户物品主要涉及厨房"
+
+    def test_save_extra_hints_empty_string(self, db_session):
+        save_app_settings(db_session, ai_search_extra_hints="厨房用品")
+        save_app_settings(db_session, ai_search_extra_hints="")
+        cfg = get_app_settings(db_session)
+        assert cfg.ai_search_extra_hints == ""
+
+    def test_settings_page_has_extra_hints_textarea(self, client):
+        response = client.get("/settings")
+        assert response.status_code == 200
+        assert 'name="ai_search_extra_hints"' in response.text
+        assert "额外领域提示" in response.text
+
+    def test_settings_page_has_ai_search_checkbox(self, client):
+        response = client.get("/settings")
+        assert response.status_code == 200
+        assert 'name="ai_search_enabled"' in response.text
+        assert "启用 AI 智能搜索" in response.text
+
+    def test_save_ai_search_settings_via_route(self, client, db_session):
+        client.post(
+            "/settings",
+            data={
+                "enabled": "on",
+                "base_url": "https://api.example.com/v1",
+                "model": "gpt-4o-mini",
+                "api_key": "sk-key",
+                "ai_search_enabled": "on",
+                "ai_search_extra_hints": "用户物品主要涉及厨房用品",
+            },
+            follow_redirects=False,
+        )
+        cfg = get_app_settings(db_session)
+        assert cfg.ai_search_enabled is True
+        assert cfg.ai_search_extra_hints == "用户物品主要涉及厨房用品"
+
+    def test_save_preserves_extra_hints_on_other_changes(self, client, db_session):
+        """Changing LLM settings should not clear extra hints."""
+        client.post(
+            "/settings",
+            data={
+                "enabled": "on",
+                "base_url": "https://api.example.com/v1",
+                "model": "gpt-4o-mini",
+                "api_key": "sk-key",
+                "ai_search_enabled": "on",
+                "ai_search_extra_hints": "厨房用品和电子产品",
+            },
+            follow_redirects=False,
+        )
+        client.post(
+            "/settings",
+            data={
+                "enabled": "on",
+                "base_url": "https://api.example.com/v1",
+                "model": "gpt-4o",
+                "api_key": "",
+                "ai_search_enabled": "on",
+                "ai_search_extra_hints": "厨房用品和电子产品",
+            },
+            follow_redirects=False,
+        )
+        cfg = get_app_settings(db_session)
+        assert cfg.ai_search_extra_hints == "厨房用品和电子产品"
+        assert cfg.model == "gpt-4o"
+
+
+# ---------------------------------------------------------------------------
+# Regression: existing features still work without AI
+# ---------------------------------------------------------------------------
+
+
+class TestRegressionWithoutAI:
+    def test_normal_search_still_works(self, client, db_session):
+        box = Box(name="测试箱")
+        db_session.add(box)
+        db_session.commit()
+
+        response = client.get("/search?q=测试")
+        assert response.status_code == 200
+        assert "测试箱" in response.text
+
+    def test_search_page_no_results(self, client):
+        response = client.get("/search?q=不存在")
+        assert "没有找到匹配结果" in response.text
+
+    def test_search_empty_query(self, client):
+        response = client.get("/search")
+        assert "输入关键词后" in response.text
diff --git a/tests/test_settings.py b/tests/test_settings.py
index b67735b..f17aa4e 100644
--- a/tests/test_settings.py
+++ b/tests/test_settings.py
@@ -9,6 +9,7 @@ import pytest
 
 import app.llm as llm_module
 from app.llm import LLMResult, expand_query, is_configured
+from app.llm import ExpansionResult
 from app.models import AppSetting
 from app.settings_store import LLMConfig, get_app_settings, save_app_settings
 
@@ -29,6 +30,7 @@ class TestLLMConfigDefaults:
         assert cfg.model == ""
         assert cfg.api_key == ""
         assert cfg.ai_search_enabled is False
+        assert cfg.ai_search_extra_hints == ""
 
 
 # ---------------------------------------------------------------------------
@@ -208,37 +210,40 @@ class TestTestConnection:
 
 
 class TestExpandQuery:
-    def test_returns_original_when_not_configured(self):
+    def test_returns_empty_when_not_configured(self):
         cfg = LLMConfig(enabled=False)
         result = expand_query(cfg, "锅")
-        assert result == ["锅"]
+        assert result.terms == []
+        assert result.error is None
 
     @patch("app.llm._call_chat_completion")
     def test_expands_query_successfully(self, mock_call):
         mock_call.return_value = {
             "choices": [
-                {"message": {"content": "平底锅\n炒锅\n锅具\n厨房锅"}}
+                {"message": {"content": '["平底锅","炒锅","锅具","厨房锅"]'}}
             ]
         }
         cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
         result = expand_query(cfg, "锅")
-        assert "锅" in result
-        assert "平底锅" in result
-        assert len(result) >= 4
+        assert "平底锅" in result.terms
+        assert "炒锅" in result.terms
+        assert result.error is None
 
     @patch("app.llm._call_chat_completion")
     def test_fallback_on_api_failure(self, mock_call):
         mock_call.side_effect = Exception("network down")
         cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
         result = expand_query(cfg, "锅")
-        assert result == ["锅"]
+        assert result.terms == []
+        assert result.error is not None
 
     @patch("app.llm._call_chat_completion")
     def test_fallback_on_empty_response(self, mock_call):
         mock_call.return_value = {"choices": [{"message": {"content": ""}}]}
         cfg = LLMConfig(enabled=True, model="gpt-4o", api_key="sk-key")
         result = expand_query(cfg, "锅")
-        assert result == ["锅"]
+        assert result.terms == []
+        assert result.error is None
 
 
 # ---------------------------------------------------------------------------
@@ -441,16 +446,33 @@ class TestSaveSettingsRoute:
         assert cfg.model == "gpt-4o-mini"
         assert cfg.api_key == "sk-original"
 
-    def test_save_does_not_touch_ai_search_enabled(self, client, db_session):
-        """P2 fix: saving LLM settings must not reset ai_search_enabled."""
+    def test_save_includes_ai_search_enabled_checkbox(self, client, db_session):
+        """Saving settings now also persists the ai_search_enabled checkbox."""
+        # Set ai_search_enabled to true first
         db_session.add(AppSetting(key="ai_search_enabled", value="true"))
         db_session.commit()
+        # Save without the checkbox → ai_search_enabled is set to False
         client.post(
             "/settings",
             data={"enabled": "on", "model": "gpt-4o", "api_key": "sk-key"},
             follow_redirects=False,
         )
         cfg = get_app_settings(db_session)
+        assert cfg.ai_search_enabled is False
+
+    def test_save_preserves_ai_search_enabled_when_checked(self, client, db_session):
+        """Saving settings with ai_search_enabled checked persists it."""
+        client.post(
+            "/settings",
+            data={
+                "enabled": "on",
+                "model": "gpt-4o",
+                "api_key": "sk-key",
+                "ai_search_enabled": "on",
+            },
+            follow_redirects=False,
+        )
+        cfg = get_app_settings(db_session)
         assert cfg.ai_search_enabled is True