diff --git a/gateway/proxy/ledger.py b/gateway/proxy/ledger.py
index 850e8ea..6459ecf 100644
--- a/gateway/proxy/ledger.py
+++ b/gateway/proxy/ledger.py
@@ -187,6 +187,51 @@ class Ledger(BillingMixin):
                 (used + 1, today, key_id))
             return True
 
+    # ---------- 语义缓存持久化（T-P6，供 semcache.SemanticCache 调用） ----------
+    def semcache_rows(self, limit: int = 300000) -> List[Dict[str, Any]]:
+        """全量缓存行（启动重建倒排索引）。"""
+        with self._lock, self._connect() as conn:
+            rows = conn.execute(
+                "SELECT cache_key, q_norm, answer, model, created_ts, ttl_ts,"
+                " doc_version, hits FROM semcache LIMIT ?", (limit,)).fetchall()
+        return [dict(r) for r in rows]
+
+    def put_semcache(self, cache_key: str, bucket: str, q_norm: str, answer: str,
+                     model: str, created_ts: int, ttl_ts: int,
+                     doc_version: int = 1) -> None:
+        """写/覆盖一条缓存（幂等）。"""
+        with self._lock, self._connect() as conn:
+            conn.execute(
+                "INSERT OR REPLACE INTO semcache"
+                "(cache_key, bucket, q_norm, answer, model, created_ts, ttl_ts,"
+                " doc_version, hits) VALUES (?,?,?,?,?,?,?,?,0)",
+                (cache_key, bucket, q_norm, answer, model, created_ts, ttl_ts,
+                 doc_version))
+
+    def promote_semcache(self, source_key: str, alias_key: str, hits: int) -> None:
+        """L2 -> L1 晋升：以别名键复制一行（原行保留审计）。"""
+        with self._lock, self._connect() as conn:
+            row = conn.execute(
+                "SELECT * FROM semcache WHERE cache_key = ?", (source_key,)).fetchone()
+            if row is None:
+                return
+            conn.execute(
+                "INSERT OR REPLACE INTO semcache"
+                "(cache_key, bucket, q_norm, answer, model, created_ts, ttl_ts,"
+                " doc_version, hits) VALUES (?,?,?,?,?,?,?,?,?)",
+                (alias_key, row["bucket"], row["q_norm"], row["answer"],
+                 row["model"], row["created_ts"], row["ttl_ts"],
+                 row["doc_version"], hits))
+            conn.execute("UPDATE semcache SET hits = ? WHERE cache_key = ?",
+                         (hits, source_key))
+
+    def purge_semcache_expired(self, now: Optional[int] = None) -> int:
+        """过期缓存清理（夜间任务顺带）。"""
+        now = int(now if now is not None else time.time())
+        with self._lock, self._connect() as conn:
+            cur = conn.execute("DELETE FROM semcache WHERE ttl_ts < ?", (now,))
+            return cur.rowcount
+
     # ---------- 自省（测试/验收用） ----------
     def table_names(self) -> List[str]:
         """列出已建表名（测试验收）。"""
diff --git a/gateway/proxy/semcache.py b/gateway/proxy/semcache.py
index 00fd821..795558d 100644
--- a/gateway/proxy/semcache.py
+++ b/gateway/proxy/semcache.py
@@ -1,24 +1,261 @@
-"""语义缓存（T-P6 落地；本文件先立签名）。
+"""两级语义缓存（T-P6，M2）：L1 精确 + L2 n-gram 倒排 + singleflight + SSE 回放。
 
-规格（§6）：L1 精确 + L2 字符 2/3-gram 倒排（启动自 sqlite 重建），
-加权 Jaccard（3-gram 权 2）+ 共享 gram>=3 门限 + 阈值 0.92，
-TTL + LRU(max_entries)，L2 命中 promote_frequency 次晋升 L1。
+规格（§6，测试最重）：
+- L1：cache_key（bucket|doc_version|sha256(norm)）-> 条目，LRU（max_entries，默认 30 万）
+- L2：字符 2-gram + 3-gram **集合**，内存倒排索引 gram -> [cache_key]；
+  启动时由 semcache 表 q_norm 重建；加权 Jaccard（3-gram 权 2、2-gram 权 1）；
+  候选门限：共享 gram >= 3 才计分；>= sim_threshold(0.92) 命中；
+  **L2 命中累计 promote_frequency(5) 次晋升 L1**
+- TTL：ttl_ts 过期不可见；命中即续期（滑动过期）
+- 持久化：semcache 表（put 同步写，索引内存维护；调用方 to_thread，D-P10）
 """
 from __future__ import annotations
 
-from typing import Any, Dict, Optional
+import asyncio
+import json
+import re
+import time
+from collections import OrderedDict
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple
+
+_WHITESPACE = re.compile(r"\s+")
+
+
+def grams(text: str) -> set:
+    """字符 2-gram + 3-gram 集合（中文天然适配，无需分词）。"""
+    t = _WHITESPACE.sub("", (text or "").lower())
+    out: set = set()
+    for n in (2, 3):
+        for i in range(len(t) - n + 1):
+            out.add(t[i:i + n])
+    return out or ({t} if t else set())
+
+
+def weighted_jaccard(ga: set, gb: set) -> float:
+    """加权 Jaccard：交集中每个 3-gram 权 2、2-gram 权 1，除以并集加权。"""
+    if not ga or not gb:
+        return 0.0
+    inter = ga & gb
+    if not inter:
+        return 0.0
+    w_inter = sum(2 if g in (3,) or len(g) == 3 else 1 for g in inter)
+    w_union = sum(2 if len(g) == 3 else 1 for g in (ga | gb))
+    return w_inter / w_union if w_union else 0.0
+
+
+class CacheEntry:
+    __slots__ = ("answer", "model", "q_norm", "g", "created_ts", "ttl_ts",
+                 "doc_version", "hits")
+
+    def __init__(self, answer: str, model: str, q_norm: str,
+                 created_ts: float, ttl_ts: float, doc_version: int):
+        self.answer = answer
+        self.model = model
+        self.q_norm = q_norm
+        self.g = grams(q_norm)
+        self.created_ts = created_ts
+        self.ttl_ts = ttl_ts
+        self.doc_version = doc_version
+        self.hits = 0
 
 
 class SemanticCache:
-    """两级语义缓存（T-P6 实现）。"""
+    """L1 精确 + L2 倒排（内存），sqlite 持久化（store 的 semcache 表）。"""
 
-    def lookup(self, bucket: str, doc_version: int, norm_hash: str,
-               norm_text: str, now: float) -> Optional[Dict[str, Any]]:
-        raise NotImplementedError("T-P6")
+    def __init__(self, store, max_entries: int = 300000,
+                 sim_threshold: float = 0.92, promote_frequency: int = 5,
+                 now: Optional[float] = None):
+        self.store = store
+        self.max_entries = max(1, int(max_entries))
+        self.sim_threshold = float(sim_threshold)
+        self.promote_frequency = max(1, int(promote_frequency))
+        self._l1: "OrderedDict[str, CacheEntry]" = OrderedDict()
+        self._inverted: Dict[str, set] = {}
+        self._clock = now            # 假时钟注入（None = time.time）
+        self.hits_exact = 0
+        self.hits_semantic = 0
+        self._rebuild()
 
-    def put(self, bucket: str, doc_version: int, norm_hash: str,
-            norm_text: str, answer: str, model: str, now: float) -> None:
-        raise NotImplementedError("T-P6")
+    # ---------- 时钟 ----------
+    def _now(self) -> float:
+        return time.time() if self._clock is None else self._clock
+
+    # ---------- 启动重建 ----------
+    def _rebuild(self) -> None:
+        try:
+            rows = self.store.semcache_rows(limit=self.max_entries)
+        except Exception:  # noqa: BLE001
+            return
+        for r in rows:
+            entry = CacheEntry(r["answer"], r["model"], r["q_norm"],
+                               r["created_ts"], r["ttl_ts"], r["doc_version"])
+            entry.hits = r["hits"]
+            self._index(r["cache_key"], entry, promote=False)
+
+    def _index(self, cache_key: str, entry: CacheEntry, promote: bool = True) -> None:
+        self._l1[cache_key] = entry
+        self._l1.move_to_end(cache_key)
+        for g in entry.g:
+            self._inverted.setdefault(g, set()).add(cache_key)
+        if promote:
+            self._evict()
+
+    def _evict(self) -> None:
+        """LRU 驱逐（超 max_entries 淘汰最久未用，含倒排回收）。"""
+        while len(self._l1) > self.max_entries:
+            key, entry = self._l1.popitem(last=False)
+            for g in entry.g:
+                bucket = self._inverted.get(g)
+                if bucket is not None:
+                    bucket.discard(key)
+                    if not bucket:
+                        self._inverted.pop(g, None)
+
+    # ---------- 查询 ----------
+    def lookup(self, cache_key: str, norm_text: str,
+               doc_version: int = 1) -> Optional[Dict[str, Any]]:
+        """L1 精确 -> L2 语义（§7 签名）。返回 {answer, model, level} 或 None。"""
+        now = self._now()
+        # L1
+        entry = self._l1.get(cache_key)
+        if entry is not None:
+            if entry.ttl_ts < now:
+                self._invalidate_key(cache_key)
+            else:
+                self.hits_exact += 1
+                self._l1.move_to_end(cache_key)
+                return {"answer": entry.answer, "model": entry.model,
+                        "level": "exact"}
+        # L2
+        g = grams(norm_text)
+        if len(g) < 3:
+            return None
+        candidates: Dict[str, int] = {}
+        for gram in g:
+            for key in self._inverted.get(gram, ()):
+                candidates[key] = candidates.get(key, 0) + 1
+        best_key = None
+        best_score = 0.0
+        for key, shared in candidates.items():
+            if shared < 3:
+                continue                       # 候选门限：共享 gram >= 3
+            cand = self._l1.get(key)
+            if cand is None or cand.ttl_ts < now or cand.doc_version != doc_version:
+                continue
+            score = weighted_jaccard(g, cand.g)
+            if score > best_score:
+                best_score = score
+                best_key = key
+        if best_key is None or best_score < self.sim_threshold:
+            return None
+        cand = self._l1[best_key]
+        cand.hits += 1
+        self.hits_semantic += 1
+        promoted = False
+        if cand.hits >= self.promote_frequency:
+            # L2 -> L1：生成精确键（由 q_norm 重建 cache_key 由调用方语义保证一致——
+            # 这里以 sha256(q_norm) 前缀别名入 L1，桶/版本由 cand 自带）
+            import hashlib
+            alias = f"{best_key.split('|')[0]}|{cand.doc_version}|promoted:" \
+                    f"{hashlib.sha256(cand.q_norm.encode()).hexdigest()[:16]}"
+            self._index(alias, cand, promote=True)
+            try:
+                self.store.promote_semcache(best_key, alias, cand.hits)
+            except Exception:  # noqa: BLE001
+                pass
+            promoted = True
+        _ = promoted
+        return {"answer": cand.answer, "model": cand.model, "level": "semantic"}
+
+    def _invalidate_key(self, cache_key: str) -> None:
+        entry = self._l1.pop(cache_key, None)
+        if entry:
+            for g in entry.g:
+                bucket = self._inverted.get(g)
+                if bucket is not None:
+                    bucket.discard(cache_key)
+                    if not bucket:
+                        self._inverted.pop(g, None)
+
+    # ---------- 写入 ----------
+    def put(self, cache_key: str, q_norm: str, answer: str, model: str,
+            doc_version: int = 1, ttl_hours: int = 72) -> None:
+        """写 L1 + 倒排 + sqlite 持久化（§7 签名）。"""
+        now = self._now()
+        entry = CacheEntry(answer, model, q_norm, now, now + ttl_hours * 3600,
+                           doc_version)
+        self._index(cache_key, entry)
+        try:
+            self.store.put_semcache(cache_key, "default", q_norm, answer, model,
+                                    int(now), int(now + ttl_hours * 3600),
+                                    doc_version)
+        except Exception:  # noqa: BLE001
+            pass  # 持久化失败不影响内存缓存（可重建）
 
     def stats(self) -> Dict[str, Any]:
-        raise NotImplementedError("T-P6")
+        return {"entries": len(self._l1), "hits_exact": self.hits_exact,
+                "hits_semantic": self.hits_semantic}
+
+
+class SingleFlight:
+    """同请求合并（§7）：dict[norm_hash -> Future]，上限 256，60s 超时降级。"""
+
+    MAX = 256
+    TIMEOUT_S = 60.0
+
+    def __init__(self):
+        self._inflight: Dict[str, asyncio.Future] = {}
+
+    def try_claim(self, norm_hash: str):
+        """返回 (future_or_None, slot)。future 非 None = 等待方；slot = 登记句柄。"""
+        fut = self._inflight.get(norm_hash)
+        if fut is not None:
+            return fut, None
+        if len(self._inflight) >= self.MAX:
+            return None, None                        # 超限旁路（不合并）
+        fut = asyncio.get_event_loop().create_future()
+        self._inflight[norm_hash] = fut
+        return None, (norm_hash, fut)
+
+    def release(self, slot, result: Any = None, error: Any = None) -> None:
+        if slot is None:
+            return
+        norm_hash, fut = slot
+        self._inflight.pop(norm_hash, None)
+        if not fut.done():
+            if error is not None:
+                fut.set_exception(error)
+            else:
+                fut.set_result(result)
+
+    async def wait(self, fut, timeout_s: float = TIMEOUT_S):
+        """等待方：超时 -> 降级直连（返回 None）。"""
+        try:
+            return await asyncio.wait_for(asyncio.shield(fut), timeout=timeout_s)
+        except (asyncio.TimeoutError, Exception):
+            return None
+
+
+# ---------------- SSE 合成回放（§6） ----------------
+
+def synth_sse_chunks(answer: str, chunk_size: int = 20,
+                     model: str = "cached", request_id: str = "") -> List[bytes]:
+    """缓存命中且 stream=true：合成合法 SSE（分块 delta + finish + [DONE]）。"""
+    cid = f"chatcmpl-cached-{request_id or '0'}"
+    created = int(time.time())
+    out: List[bytes] = []
+    for i in range(0, max(1, len(answer)), chunk_size):
+        piece = answer[i:i + chunk_size]
+        out.append(("data: " + json.dumps({
+            "id": cid, "object": "chat.completion.chunk", "created": created,
+            "model": model,
+            "choices": [{"index": 0, "delta": {"content": piece}, "finish_reason": None}],
+        }, ensure_ascii=False) + "\n\n").encode("utf-8"))
+    out.append(("data: " + json.dumps({
+        "id": cid, "object": "chat.completion.chunk", "created": created,
+        "model": model,
+        "choices": [{"index": 0, "delta": {}, "finish_reason": "stop"}],
+    }, ensure_ascii=False) + "\n\n").encode("utf-8"))
+    out.append(b"data: [DONE]\n\n")
+    return out
diff --git a/tests/test_proxy_semcache.py b/tests/test_proxy_semcache.py
new file mode 100644
index 0000000..d8206b8
--- /dev/null
+++ b/tests/test_proxy_semcache.py
@@ -0,0 +1,139 @@
+"""语义缓存测试（T-P6，M2）：精确/n-gram 阈值/TTL/LRU/重建/晋升/singleflight/SSE 回放。"""
+import asyncio
+import json
+
+import pytest
+
+from gateway.proxy.semcache import (
+    SingleFlight,
+    SemanticCache,
+    grams,
+    synth_sse_chunks,
+    weighted_jaccard,
+)
+from gateway.proxy.ledger import Ledger
+
+
+@pytest.fixture()
+def cache(tmp_path):
+    led = Ledger.init_db(tmp_path / "p.sqlite3")
+    return SemanticCache(led, max_entries=100, sim_threshold=0.92,
+                         promote_frequency=5)
+
+
+def _put(cache, key, text, answer="答案A", model="m", doc_version=1, ttl_hours=72):
+    cache.put(key, text, answer, model, doc_version=doc_version, ttl_hours=ttl_hours)
+
+
+def test_grams_and_weighted_jaccard():
+    g1 = grams("什么是递归")
+    assert any(len(g) == 2 for g in g1) and any(len(g) == 3 for g in g1)
+    assert weighted_jaccard(g1, g1) == 1.0
+    assert weighted_jaccard(grams("完全不同话题"), g1) == 0.0
+
+
+def test_exact_hit_and_miss(cache):
+    key = "default|1|" + "a" * 16
+    _put(cache, key, "什么是递归", "递归是自调用")
+    hit = cache.lookup(key, "什么是递归")
+    assert hit and hit["level"] == "exact" and hit["answer"] == "递归是自调用"
+    assert cache.lookup(key + "-nope", "完全无关的问题") is None
+
+
+def test_semantic_hit_above_threshold(cache):
+    """同义变体：L2 命中（阈值上）。"""
+    key = "default|1|b1"
+    _put(cache, key, "请解释一下什么叫做递归函数", "递归解释")
+    hit = cache.lookup("default|1|b2", "请解释一下什么叫做递归函数", doc_version=1)
+    assert hit and hit["level"] == "semantic"
+    assert cache.hits_semantic == 1
+
+
+def test_semantic_miss_below_threshold(cache):
+    """完全不同语义：未命中（阈值下）。"""
+    _put(cache, "default|1|c1", "请解释一下什么叫做递归函数", "递归解释")
+    assert cache.lookup("default|1|c2", "今天股市行情怎么样", doc_version=1) is None
+
+
+def test_ttl_expiry_fake_clock(cache):
+    _put(cache, "k", "某个问题文本", "旧答案", ttl_hours=1)
+    cache._clock = cache._now() + 7200                    # 假时钟 +2h
+    assert cache.lookup("k", "某个问题文本", doc_version=1) is None  # 过期不可见
+
+
+def test_lru_eviction(tmp_path):
+    led = Ledger.init_db(tmp_path / "p.sqlite3")
+    cache = SemanticCache(led, max_entries=3)
+    for i in range(5):
+        _put(cache, f"k{i}", f"完全不同的问题编号{i}", f"答{i}")
+    assert len(cache._l1) == 3                            # LRU 上限
+    assert cache.lookup("k0", "完全不同的问题编号0") is None   # 最旧被驱逐
+    assert cache.lookup("k4", "完全不同的问题编号4") is not None
+
+
+def test_rebuild_from_sqlite(tmp_path):
+    """启动时由 semcache 表重建倒排索引。"""
+    led = Ledger.init_db(tmp_path / "p.sqlite3")
+    c1 = SemanticCache(led, max_entries=100)
+    c1.put("k", "解释递归的概念", "持久化答案", "m")
+    c2 = SemanticCache(led, max_entries=100)              # 新实例：重建
+    hit = c2.lookup("k2", "解释递归的概念", doc_version=1)
+    assert hit and hit["answer"] == "持久化答案"
+
+
+def test_promote_after_five_semantic_hits(cache):
+    """L2 命中 5 次 -> 晋升 L1（promote 别名键可精确命中）。"""
+    key = "default|1|p1"
+    _put(cache, key, "请解释一下什么叫做递归函数呢？", "递归解释（变体）")
+    promoted = False
+    for i in range(5):
+        hit = cache.lookup("default|1|p%d" % (i + 2), "请解释一下什么叫做递归函数呢", doc_version=1)
+        assert hit and hit["level"] == "semantic"
+        if any(k.startswith("default|1|promoted:") for k in cache._l1):
+            promoted = True
+    assert promoted and cache.hits_semantic == 5
+
+
+def test_singleflight_merge_and_bypass():
+    """两并发同请求：一登记一等待；超限旁路。"""
+    async def scenario():
+        sf = SingleFlight()
+        fut, slot = sf.try_claim("h1")
+        assert fut is None and slot is not None           # 首个登记
+        fut2, slot2 = sf.try_claim("h1")
+        assert fut2 is not None and slot2 is None         # 第二个等待
+        sf.release(slot, result="共享答案")
+        got = await sf.wait(fut2)
+        assert got == "共享答案"
+        # 超限旁路
+        sf2 = SingleFlight()
+        sf2.MAX = 2
+        _f, s1 = sf2.try_claim("a")
+        _f2, s2 = sf2.try_claim("b")
+        f3, s3 = sf2.try_claim("c")
+        assert f3 is None and s3 is None                  # 第三个旁路
+
+    asyncio.run(scenario())
+
+
+def test_synth_sse_chunks_valid():
+    """命中回放：合法 SSE 形状（delta 分块 + finish + [DONE]）。"""
+    chunks = synth_sse_chunks("你好世界" * 10, chunk_size=20, model="m", request_id="r1")
+    text = b"".join(chunks).decode("utf-8")
+    assert text.count("chat.completion.chunk") >= 2
+    assert '"finish_reason": "stop"' in text or '"finish_reason":"stop"' in text
+    assert text.rstrip("\n").endswith("data: [DONE]")
+    content = ""
+    done = False
+    for raw in chunks:
+        for line in raw.decode("utf-8").splitlines():
+            if not line.startswith("data:"):
+                continue
+            payload = line[5:].strip()
+            if payload == "[DONE]":
+                done = True
+                continue
+            obj = json.loads(payload)
+            assert obj["object"] == "chat.completion.chunk"
+            content += obj["choices"][0]["delta"].get("content") or ""
+    assert done and "你好世界" in content
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 7ed252a..3e47a88 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 | ⬜ 待办 | |
+| 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-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 | |