fix: bring a cached tier back as a tier, not as a bare string

Adding applied_effort to LLMResponse put it through the cache round
trip, where JSON stores a StrEnum as its plain value. Rehydrated raw, a
hit would hand downstream a str while the annotation says Effort, and
every `is Effort.LOW` in the library would quietly answer False on the
hit path only -- the same trap thinking_observation already has a
coercion for.

A value outside this version's vocabulary degrades to None rather than
failing the entry: projects sharing one Redis would otherwise keep
invalidating each other's writes over an attribution field, and None is
the honest reading of a tier this version cannot name.

src/polygateway/middleware/cache.py

@@ -53,6 +53,29 @@ def _coerce_observation(raw: Any) -> ThinkingObservation:
         return ThinkingObservation.UNKNOWN
 
 
+def _coerce_applied_effort(raw: Any) -> Effort | None:
+    """缓存里的档位字符串 → 枚举;域外取值降级为 `None`,**不作废整条缓存**。
+
+    与 `_coerce_observation` 同源同向,理由逐条相同: 多项目共用一个 Redis 时,
+    先升级的进程可能写入本版没有的档位名,未升级的进程若把这些条目判成未命中,
+    两个版本就会互相打对方的缓存。归因字段不该有能力废掉内容完好的响应。
+
+    降级到 `None` 而不是别的档: 它的语义是"库不知道这次跑在哪档",对一个读不懂
+    的取值这是唯一诚实的说法——随便挑一档等于替上游声称了一件它没说过的事。
+    """
+    if raw is None:
+        return None
+    try:
+        return Effort(raw)
+    except ValueError:
+        logger.warning(
+            "缓存条目的 applied_effort 取值 {!r} 不在本版档位词汇内(多半由更新版本的"
+            "进程写入),已降级为 None;响应内容照常复活——归因字段不作废缓存",
+            raw,
+        )
+        return None
+
+
 def digest_messages(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
     """多模态 content part 先各自 sha256 摘要再参与序列化;文本原文参与。
 
@@ -176,6 +199,8 @@ class CacheMW:
             # 键缺失即升级前写入的旧条目,交给 dataclass 默认值
             if "thinking_observation" in fields:
                 fields["thinking_observation"] = _coerce_observation(fields["thinking_observation"])
+            if "applied_effort" in fields:
+                fields["applied_effort"] = _coerce_applied_effort(fields["applied_effort"])
             fields.update(
                 cache_hit=True,
                 latency_ms=0,

tests/unit/test_cache.py

@@ -154,7 +154,6 @@ class TestKeyFormula:
             reasoning_effort=None,
         ) == ("pgw:cache:c54544e8672f4c91373b4a72716a88497445b440b89445aa5379b356b228f58b")
 
-
     def test_declared_tier_key_is_a_golden(self):
         """配了档位那一侧同样要有 golden: 字面量变了就是所有该档缓存冷启动。
 
@@ -402,6 +401,69 @@ class TestThinkingObservationRehydration:
         assert hit.thinking_observation is ThinkingObservation.UNKNOWN
 
 
+class TestAppliedEffortRehydration:
+    """issue #20: 实际档同样必须复活成枚举,理由与 `thinking_observation` 逐条相同。
+
+    JSON 里存的是 `StrEnum` 的字符串值;不转就复活成裸 str,而库内一路是
+    `is Effort.LOW` 的身份比较——命中路径上会静默判否,且下游拿到的类型与字段
+    注解分叉。缓存是档位的**第三条入口**(另两条是 `.env` 解析与 `chat()` 参数),
+    归一化不变式必须在这里也闭合。
+    """
+
+    async def test_hit_replays_enum_instance_not_bare_str(self):
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        terminal = _Terminal(_resp(applied_effort=Effort.LOW))
+        await mw(ChatRequest(messages=_MSGS), terminal)
+        hit = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert hit.cache_hit is True and terminal.calls == 1
+        assert isinstance(hit.applied_effort, Effort)
+        assert hit.applied_effort is Effort.LOW
+
+    async def test_unknown_tier_degrades_to_none_and_still_hits(self):
+        """域外档位降级为 None(=不知道这次跑在哪档),不作废内容完好的条目。
+
+        降级方向与 `thinking_observation` 同源: 共用一个 Redis 的项目里,先升级
+        的那个可能写入本版没有的档位名,未升级的项目若判成未命中,两个版本就会
+        互相打对方的缓存。归因字段不该有能力废掉一条内容完好的响应。
+        """
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        key = build_cache_key("m", _MSGS, "proj", None)
+        poisoned = dataclasses.asdict(_resp(content="from-a-newer-version"))
+        poisoned["applied_effort"] = "ultra"
+        poisoned.pop("structured_data", None)
+        await backend.set(key, json.dumps(poisoned), 3600)
+        terminal = _Terminal(_resp())
+        messages: list[str] = []
+        sink_id = logger.add(messages.append, level="WARNING")
+        try:
+            resp = await mw(ChatRequest(messages=_MSGS), terminal)
+        finally:
+            logger.remove(sink_id)
+        assert terminal.calls == 0 and resp.cache_hit is True
+        assert resp.content == "from-a-newer-version"
+        assert resp.applied_effort is None
+        hits = [m for m in messages if "ultra" in m]
+        assert len(hits) == 1, f"域外档位必须单独告警一次,实得 {len(hits)} 条: {messages}"
+        assert "applied_effort" in hits[0]
+        assert [m for m in messages if "重建失败" in m] == []
+
+    async def test_legacy_entry_without_key_rehydrates_to_none(self):
+        """升级前写入的条目没有该键,必须照常复活并落到默认 None。"""
+        backend = InMemoryCache()
+        mw = _mw(backend)
+        key = build_cache_key("m", _MSGS, "proj", None)
+        legacy = dataclasses.asdict(_resp(content="legacy"))
+        legacy.pop("applied_effort")
+        legacy.pop("structured_data", None)
+        await backend.set(key, json.dumps(legacy), 3600)
+        terminal = _Terminal(_resp())
+        hit = await mw(ChatRequest(messages=_MSGS), terminal)
+        assert hit.content == "legacy" and terminal.calls == 0
+        assert hit.applied_effort is None
+
+
 class _BrokenBackend:
     async def get(self, key):
         raise ConnectionError("redis down")