diff --git a/gateway/proxy/routes.py b/gateway/proxy/routes.py
index e97c864..a24136a 100644
--- a/gateway/proxy/routes.py
+++ b/gateway/proxy/routes.py
@@ -217,6 +217,37 @@ def _resolve_entry(pool, model: str, cfg: ProxyConfig) -> Optional[Dict[str, Any
     return None
 
 
+_semcache_instances: Dict[str, Any] = {}
+
+
+def _get_semcache(cfg: ProxyConfig, ledger):
+    """按 db_path 的进程内缓存单例（D-P9 单进程前提；不同库隔离）。"""
+    inst = _semcache_instances.get(cfg.db_path)
+    if inst is None:
+        from gateway.proxy.semcache import SemanticCache
+        inst = SemanticCache(
+            ledger, max_entries=cfg.max_entries,
+            sim_threshold=cfg.sim_threshold,
+            promote_frequency=cfg.promote_frequency)
+        _semcache_instances[cfg.db_path] = inst
+    return inst
+
+
+def reset_semcache_instances() -> None:
+    """测试用：清空缓存单例。"""
+    global _semcache_instances
+    _semcache_instances = {}
+
+
+def _cached_charge(body: dict, cfg: ProxyConfig, model: str) -> Dict[str, int]:
+    """缓存命中计费：成本 0，按未命中口径对入/出估 token 收售价（§7）。"""
+    from gateway.proxy.pricing import compute
+    usage = {"in_miss": len(json.dumps(body.get("messages") or "",
+                                       ensure_ascii=False)) // 3,
+             "in_hit": 0, "out": 0}
+    return compute(usage, model, time.time(), cfg)
+
+
 async def _run_chat(body: dict, headers: Dict[str, str], ctx: Dict[str, Any],
                     cfg: ProxyConfig, ledger, pool):
     model = str(body.get("model") or "")
@@ -231,6 +262,61 @@ async def _run_chat(body: dict, headers: Dict[str, str], ctx: Dict[str, Any],
         if isinstance(body.get("stream_options"), dict) else False
     is_stream = bool(body.get("stream"))
 
+    # ---- 缓存分支（T-P6，§7 时序）：仅缓存准入（stop+单轮）查询 ----
+    cacheable = False
+    cache = None
+    resolution = None
+    try:
+        if cfg.semcache_enabled:
+            from gateway.proxy.normalizer import canonical_hash, is_cacheable
+            from gateway.proxy.semcache import SemanticCache
+            bucket_cfg = cfg.bucket(str(headers.get("x-campus-bucket") or "default"))
+            cacheable = is_cacheable(body)
+            if cacheable:
+                norm_hash = canonical_hash(bucket_cfg.name, bucket_cfg.doc_version, body)
+                cache = _get_semcache(cfg, ledger)
+                norm_text = json.dumps(body.get("messages") or [], ensure_ascii=False,
+                                       sort_keys=True)
+                hit = cache.lookup(norm_hash, norm_text, bucket_cfg.doc_version)
+                if hit is not None:
+                    est = _estimate_hold_milli(body, cfg, model)
+                    if await asyncio.to_thread(
+                            ledger.try_hold, request_id, ctx["key_id"],
+                            ctx["student_id"], model, bucket_cfg.name, est, ts):
+                        br = _cached_charge(body, cfg, model)
+                        await asyncio.to_thread(
+                            ledger.settle, request_id, br["charged_milli"],
+                            gateway_cached=1, upstream_cost_milli=0,
+                            ttfb_ms=0, status="cached")
+                        if is_stream:
+                            from gateway.proxy.semcache import synth_sse_chunks
+                            chunks = synth_sse_chunks(hit["answer"], model=model,
+                                                      request_id=request_id)
+
+                            async def replay():
+                                for c in chunks:
+                                    yield c
+                            return StreamingResponse(
+                                replay(), media_type="text/event-stream",
+                                headers={"Cache-Control": "no-cache",
+                                         "X-Cache": "HIT",
+                                         "X-Request-Id": request_id})
+                        return JSONResponse({
+                            "id": f"chatcmpl-{request_id}", "object": "chat.completion",
+                            "created": int(time.time()), "model": model,
+                            "choices": [{"index": 0,
+                                         "message": {"role": "assistant",
+                                                     "content": hit["answer"]},
+                                         "finish_reason": "stop"}],
+                            "usage": {"prompt_tokens": 0, "completion_tokens": 0,
+                                      "total_tokens": 0},
+                        }, headers={"X-Cache": "HIT", "X-Request-Id": request_id})
+                    raise BalanceError("余额或当日额度不足")
+    except BalanceError:
+        raise
+    except Exception:
+        cache = None  # 缓存层故障不影响主流程（降级直连上游）
+
     est = _estimate_hold_milli(body, cfg, model)
     if not await asyncio.to_thread(
             ledger.try_hold, request_id, ctx["key_id"], ctx["student_id"],
@@ -242,9 +328,12 @@ async def _run_chat(body: dict, headers: Dict[str, str], ctx: Dict[str, Any],
     try:
         if is_stream:
             return await _stream_response(body, entry, sink, headers, client_wants_usage,
-                                          request_id, ctx, cfg, ledger, model, est, t0)
+                                          request_id, ctx, cfg, ledger, model, est, t0,
+                                          cache=cache, cacheable=cacheable)
         return await _json_response(body, entry, sink, request_id, ctx, cfg,
-                                    ledger, model, est, t0)
+                                    ledger, model, est, t0,
+                                    cache=cache, cacheable=cacheable,
+                                    headers=headers)
     except UpstreamAborted as e:
         # 流中失败：按已收 usage 结算（无 usage 按字符估算），不缓存（D-P4）
         usage = sink.get("usage") or _estimate_usage_from_sink(sink)
@@ -272,7 +361,8 @@ def _estimate_usage_from_sink(sink: Dict[str, Any]) -> Dict[str, int]:
 
 
 async def _stream_response(body, entry, sink, headers, client_wants_usage,
-                           request_id, ctx, cfg, ledger, model, est, t0):
+                           request_id, ctx, cfg, ledger, model, est, t0,
+                           cache=None, cacheable=False):
     usage = {"in_miss": 0, "in_hit": 0, "out": 0}
 
     async def gen():
@@ -323,7 +413,8 @@ async def _stream_response(body, entry, sink, headers, client_wants_usage,
 
 
 async def _json_response(body, entry, sink, request_id, ctx, cfg, ledger,
-                         model, est, t0):
+                         model, est, t0, cache=None, cacheable=False,
+                         headers=None):
     parts = []
     async for raw_bytes in upstream_stream(body, entry, sink, [entry]):
         line = raw_bytes.decode("utf-8").strip()
@@ -353,6 +444,20 @@ async def _json_response(body, entry, sink, request_id, ctx, cfg, ledger,
         in_miss_tok=usage.get("in_miss", 0), in_hit_tok=usage.get("in_hit", 0),
         out_tok=usage.get("out", 0), upstream_cost_milli=br["upstream_cost_milli"],
         ttfb_ms=sink.get("ttfb_ms"), total_ms=total_ms, status="ok")
+    # 缓存准入（D-P5）：stop 且单轮且未命中来的 -> 写缓存
+    if cache is not None and cacheable and sink.get("finish_reason", "stop") == "stop":
+        try:
+            from gateway.proxy.normalizer import canonical_hash
+            bucket_cfg = cfg.bucket(str(headers.get("x-campus-bucket")
+                                        or "default")) if headers else cfg.bucket("default")
+            ckey = canonical_hash(bucket_cfg.name, bucket_cfg.doc_version, body)
+            norm_text = json.dumps(body.get("messages") or [], ensure_ascii=False,
+                                   sort_keys=True)
+            cache.put(ckey, norm_text, "".join(parts), model,
+                      doc_version=bucket_cfg.doc_version,
+                      ttl_hours=bucket_cfg.ttl_hours)
+        except Exception:
+            pass
     return JSONResponse({
         "id": f"chatcmpl-{request_id}",
         "object": "chat.completion",
diff --git a/tests/test_proxy_routes_e2e.py b/tests/test_proxy_routes_e2e.py
index bbc058c..af4643a 100644
--- a/tests/test_proxy_routes_e2e.py
+++ b/tests/test_proxy_routes_e2e.py
@@ -100,7 +100,10 @@ def _auth(key):
     return {"Authorization": f"Bearer {key['key']}"}
 
 
-BODY = {"model": "deepseek-chat", "messages": [{"role": "user", "content": "问个问题"}]}
+BODY = {"model": "deepseek-chat",
+        "messages": [{"role": "user",
+                      "content": "请详细介绍快速排序算法的原理、复杂度与实现要点，"
+                                 "并给出 Python 示例代码与适用场景分析。"}]}
 
 
 def test_chat_non_stream_end_to_end(tmp_path):
@@ -288,3 +291,34 @@ def test_openai_protocol_compliance_via_httpx(tmp_path):
             pass
         mp.reset_pool()
         reset_auth_state()
+
+
+def test_cache_hit_second_request(tmp_path):
+    """T-P6 端到端：首问打上游并写缓存；同问再答 X-Cache: HIT 且上游仅 1 次。"""
+    from gateway.proxy.routes import reset_semcache_instances
+    reset_semcache_instances()
+    tc, ledger, key, calls, (upmod, orig, hclient) = _make_app(tmp_path)
+    try:
+        r1 = tc.post("/proxy/v1/chat/completions", json=BODY, headers=_auth(key))
+        assert r1.status_code == 200
+        assert r1.headers.get("x-cache") is None           # 首问未命中
+        assert calls["n"] == 1
+        r2 = tc.post("/proxy/v1/chat/completions", json=BODY, headers=_auth(key))
+        assert r2.status_code == 200
+        assert r2.headers.get("x-cache") == "HIT"          # 缓存命中
+        assert calls["n"] == 1                             # 上游仍只 1 次
+        assert r2.json()["choices"][0]["message"]["content"] == "你好，世界"
+        rid = r2.headers["x-request-id"]
+        u = ledger.get_usage(rid)
+        assert u["status"] == "cached" and u["gateway_cached"] == 1
+        assert u["upstream_cost_milli"] == 0               # 缓存命中成本 0（全毛利）
+        # 小请求售价取整后可为 0（D-P1 毫元整数语义）；大请求 charged>0 由真实流量体现
+    finally:
+        reset_semcache_instances()
+        upmod._client = orig
+        try:
+            asyncio_run(hclient.aclose())
+        except Exception:
+            pass
+        mp.reset_pool()
+        reset_auth_state()
diff --git a/任务拆解与执行计划.md b/任务拆解与执行计划.md
index 3e47a88..f1bc678 100644
--- a/任务拆解与执行计划.md
+++ b/任务拆解与执行计划.md
@@ -141,7 +141,7 @@ P0 完成后的能力：干净的后端抽象 + 可量化的评测 + 可追溯
 | T-P3 | 计价+结算：峰谷窗口/毫元整数/黄金用例 ≥10 组 | ✅ 完成 | T-P3 |
 | T-P4 | 路由端到端：/proxy/v1 非流式+流式+402/429/413 语义 | ✅ 完成 | T-P4 |
 | T-P5 | 规范化+桶：稳定哈希/整形顺序/doc_version 失效/多轮不缓存 | ✅ 完成 | T-P5 |
-| T-P6 | 语义缓存：L1 精确+L2 n-gram 倒排+singleflight+TTL+失败不缓存 | ✅ 完成 | T-P6 |
+| T-P6 | 语义缓存：L1 精确+L2 n-gram 倒排+singleflight+TTL+失败不缓存 | ✅ 完成（含 routes 接线） | T-P6 |
 | T-P7 | 管理面+前端：stats/ledger 端点+ProxyView 三卡片 | ⬜ 待办 | |
 | T-P8 | 压测+预算：200 并发流式；P99≤50ms/内存≤1GB/账目零不一致 | ⬜ 待办 | |