diff --git a/changelog.d/269.changed.md b/changelog.d/269.changed.md
new file mode 100644
index 0000000..a73395d
--- /dev/null
+++ b/changelog.d/269.changed.md
@@ -0,0 +1,5 @@
+**A query string over 200 bytes is stored in the HTTP cache key as its
+SHA-256 digest.** The key holds `sha256:` and the 64-digit hex digest instead
+of the query, so a client cannot make the query part of the key arbitrarily
+long. The path stays readable, so `clear_path()` still finds such entries, and
+the monitoring routes show the digest as `query_params`.
diff --git a/docs/CACHE_FLOW.md b/docs/CACHE_FLOW.md
index 40a6efb..e6fd7d8 100644
--- a/docs/CACHE_FLOW.md
+++ b/docs/CACHE_FLOW.md
@@ -114,7 +114,9 @@ Query parameters are joined in the order the request sent them
 (`str(request.query_params)`) and are **not sorted** by default, so
 `?page=1&limit=10` and `?limit=10&page=1` are two separate cache entries. To
 treat them as one, set `@cache(sort_query=True)`, which orders the parameters
-by name first (see [Cache keys](HTTP_CACHING.md#cache-keys)).
+by name first (see [Cache keys](HTTP_CACHING.md#cache-keys)). A query longer
+than 200 bytes is then replaced by `sha256:<hex digest>`, so the query part of
+the key stays bounded.
 
 The key format keeps each dimension cached independently:
 
diff --git a/docs/HTTP_CACHING.md b/docs/HTTP_CACHING.md
index 83d51a7..dd0692b 100644
--- a/docs/HTTP_CACHING.md
+++ b/docs/HTTP_CACHING.md
@@ -331,6 +331,16 @@ query order as sent can keep `sort_query=False`.
 Pass `sort_query=True` to `invalidate()` for such a route as well (see
 [Invalidating a single cached route](#invalidating-a-single-cached-route)).
 
+A query string longer than 200 bytes, as encoded in the key, is stored as
+`sha256:` and its 64-digit hex digest instead, so a client cannot make the
+query part of the key arbitrarily long. (Memcached still hashes a whole key over
+250 bytes, prefix included; a query just under the threshold with a long host
+or path can get there.) The digest is taken after sorting, so `sort_query` still
+merges reordered long queries. The path stays readable, so `clear_path()` still
+finds the entry (with `include_params=True`, since the query is not empty), and
+the monitoring routes show the digest as `query_params`. A query as sent never
+looks like one: the key writes `:` as `%3A`.
+
 The host and path come from the client, so `|` and `%` in them are percent-encoded
 (`%7C` and `%25`). A `Host` header or path containing `|` therefore cannot shift
 the components and make one request's key equal another's. The query string is
@@ -352,8 +362,8 @@ proxy keeps its port. A request without a `Host` header uses `unknown`.
 
 Otherwise the host is still whatever the client sends. Unless a reverse proxy
 or load balancer in front of the app already rejects unknown hosts, add
-Starlette's `TrustedHostMiddleware`, so a forged `Host` gets a `400` instead of filling the
-cache with entries no one else will request:
+Starlette's `TrustedHostMiddleware`, so a forged `Host` gets a `400` instead of
+filling the cache with entries no one else will request:
 
 ```python
 from starlette.middleware.trustedhost import TrustedHostMiddleware
@@ -747,7 +757,8 @@ add_routes(
 ```
 
 - `GET {prefix}/cached-hits` — every cached entry split into method, host, path
-  and query, with its ETag and expiry, plus counts of valid and expired entries
+  and query (`sha256:<hex>` for a query over 200 bytes), with its ETag and
+  expiry, plus counts of valid and expired entries
   and the distinct cached paths. It does not count hits.
 - `GET {prefix}/cached-records` — every cached record with its size, expiry,
   `media_type` (the stored response's media type, `null` if it had none) and a
diff --git a/docs/MIGRATING_0_4.md b/docs/MIGRATING_0_4.md
index a18282c..72433c9 100644
--- a/docs/MIGRATING_0_4.md
+++ b/docs/MIGRATING_0_4.md
@@ -253,7 +253,7 @@ CacheManagerProxy.set(CacheManager(lock=True))
 - The separator becomes a single `|` (`CACHE_KEY_SEPARATOR`).
 - Keys start with the format tag `http:v2|` (`CacheKey.FORMAT_TAG`), so the next format change can remove old keys by pattern: `clear_pattern("http:v2|*")` removes every key of this format.
 - The host is normalised: lower-cased, and an empty port or the scheme's default one (`:80` on http, `:443` on https) dropped.
-- A long query string (over about 200 bytes) is stored as `sha256:` and its hex digest; the path stays readable.
+- A query string over 200 bytes (as encoded in the key) is stored as `sha256:` and its hex digest; the path stays readable, and the monitoring routes show the digest.
 - Query parameters are sorted by name: `sort_query` (opt-in since 0.3.9) defaults to `True` in `@cache`, `build_cache_key()` and `invalidate()`, so `?b=2&a=1` and `?a=1&b=2` share one entry.
 - One public `CacheKey` type builds, encodes and parses keys. `CACHE_KEY_MIN_PARTS`, `CACHE_KEY_MAX_SPLIT` and `CACHE_KEY_MAX_PARTS` are removed from `fastapi_cachex.routes`; read a key's components with `CacheKey.parse(key)` instead.
 
diff --git a/fastapi_cachex/cache_key.py b/fastapi_cachex/cache_key.py
index d30bae5..1d9facd 100644
--- a/fastapi_cachex/cache_key.py
+++ b/fastapi_cachex/cache_key.py
@@ -5,6 +5,7 @@
 and nowhere else.
 """
 
+import hashlib
 from dataclasses import dataclass
 from operator import itemgetter
 from typing import ClassVar
@@ -23,6 +24,13 @@
 # The port a scheme implies when the Host header names none.
 _DEFAULT_PORTS = {"http": "80", "https": "443", "ws": "80", "wss": "443"}
 
+# A query component longer than this many bytes is stored as its digest.
+_QUERY_HASH_THRESHOLD = 200
+
+# Marks a hashed query. A query as sent never starts with it: the encoding
+# writes ``:`` as ``%3A``.
+_QUERY_HASH_PREFIX = "sha256:"
+
 # Characters that are live in a Redis glob pattern.
 _GLOB_SPECIAL = frozenset("*?[]\\")
 
@@ -41,10 +49,20 @@ def _query_component(request: Request, sort_query: bool) -> str:
     pairs by name before encoding them the same way, so only the order of
     differently named parameters changes: repeated values of one name keep
     their relative order, and an already sorted query gives the unsorted key.
+
+    A query longer than 200 bytes (``_QUERY_HASH_THRESHOLD``) is replaced by
+    ``sha256:`` and its full hex digest, so a client cannot make the query
+    part of the key arbitrarily long; the digest is taken after sorting, so ``sort_query``
+    still merges reordered long queries.
     """
-    if not sort_query:
-        return str(request.query_params)
-    return urlencode(sorted(request.query_params.multi_items(), key=itemgetter(0)))
+    if sort_query:
+        query = urlencode(sorted(request.query_params.multi_items(), key=itemgetter(0)))
+    else:
+        query = str(request.query_params)
+    encoded = query.encode()
+    if len(encoded) > _QUERY_HASH_THRESHOLD:
+        return _QUERY_HASH_PREFIX + hashlib.sha256(encoded).hexdigest()
+    return query
 
 
 def _host_component(request: Request) -> str:
@@ -103,7 +121,9 @@ class CacheKey:
     ``extra`` item hold the plain text and are percent-encoded by ``to_str()``
     (see ``escape_key_component``), so a client-controlled value cannot
     contain the separator; a method token may contain ``|``. ``query`` is
-    kept URL-encoded, as it appears in the key.
+    kept URL-encoded, as it appears in the key; ``from_request()`` stores a
+    query longer than 200 bytes as ``sha256:`` and its hex digest instead,
+    which no query as sent can look like.
 
     Build one from a request with ``from_request()`` and turn a stored key
     back into one with ``parse()``::
diff --git a/i18n/zh-TW/docs/CACHE_FLOW.md b/i18n/zh-TW/docs/CACHE_FLOW.md
index 6e16505..a0f1274 100644
--- a/i18n/zh-TW/docs/CACHE_FLOW.md
+++ b/i18n/zh-TW/docs/CACHE_FLOW.md
@@ -87,7 +87,7 @@ cache_key = "|".join(
 
 自訂的 `key_builder` 可以用 `build_cache_key(request, *components)` 在查詢字串之後加入其他段；這些段以同樣方式編碼，`clear_path()` 也仍會比對路徑（見 [HTTP 快取](HTTP_CACHING.md#adding-components-to-the-key)中的「在鍵中加入其他段」）。`@cache(vary=[...])` 會在 key builder 回傳的鍵之後，為每個列出的請求標頭附加一個 `name=value` 段，並把這些名稱加入回應的 `Vary` 標頭（見 [HTTP 快取](HTTP_CACHING.md#varying-on-request-headers)中的「依請求標頭區分」）。對於憑證標頭 `Authorization`、`Proxy-Authorization`、`Cookie` 與 `X-Session-Token`，非空的值會寫成 `sha256:<十六進位摘要>`，因此鍵中不會出現任何權杖。
 
-查詢參數依請求送出的順序串接（`str(request.query_params)`），預設**不會排序**，因此 `?page=1&limit=10` 與 `?limit=10&page=1` 是兩個不同的快取項目。若希望兩者視為同一個，請設定 `@cache(sort_query=True)`，先依名稱排序參數（見 [HTTP 快取](HTTP_CACHING.md#cache-keys)中的「快取鍵」）。
+查詢參數依請求送出的順序串接（`str(request.query_params)`），預設**不會排序**，因此 `?page=1&limit=10` 與 `?limit=10&page=1` 是兩個不同的快取項目。若希望兩者視為同一個，請設定 `@cache(sort_query=True)`，先依名稱排序參數（見 [HTTP 快取](HTTP_CACHING.md#cache-keys)中的「快取鍵」）。超過 200 位元組的查詢接著會改寫為 `sha256:<十六進位摘要>`，讓鍵中查詢的部分維持有限長度。
 
 這個快取鍵格式讓每個維度各自獨立快取：
 
diff --git a/i18n/zh-TW/docs/HTTP_CACHING.md b/i18n/zh-TW/docs/HTTP_CACHING.md
index 5c36b84..c8bd774 100644
--- a/i18n/zh-TW/docs/HTTP_CACHING.md
+++ b/i18n/zh-TW/docs/HTTP_CACHING.md
@@ -186,6 +186,8 @@ async def search(q: str, limit: int = 10):
 
 `sort_query` 只套用於預設的 key builder。與自訂的 `key_builder` 一起使用時，套用裝飾器就會拋出 `CacheXError`；請改在 builder 中呼叫 `build_cache_key(request, ..., sort_query=True)`。對這樣的路由呼叫 `invalidate()` 時也要傳入 `sort_query=True`（見[使單一快取路由失效](#invalidating-a-single-cached-route)）。
 
+在鍵中編碼後超過 200 位元組的查詢字串，會改以 `sha256:` 加上 64 個十六進位字元的摘要儲存，因此用戶端無法讓鍵中查詢的部分無限變長。（超過 250 位元組的整個鍵，含前綴，Memcached 仍會整個雜湊；略低於門檻的查詢配上較長的 host 或路徑仍可能超過。）摘要在排序之後計算，因此 `sort_query` 仍會合併順序不同的長查詢。路徑維持可讀，所以 `clear_path()` 仍找得到該項目（需帶上 `include_params=True`，因為查詢不是空的），監控路由則以 `query_params` 顯示這個摘要。用戶端送出的查詢不可能看起來像摘要：鍵中的 `:` 會寫成 `%3A`。
+
 host 與路徑來自用戶端，因此其中的 `|` 與 `%` 會以百分比編碼寫入（`%7C` 與 `%25`）。含有 `|` 的 `Host` 標頭或路徑因此無法讓各段錯位，使某個請求的快取鍵與另一個請求相同。查詢字串本來就經過 URL 編碼。`clear_path()` 接受應用程式看到的路徑（`request.url.path`），並以同樣方式編碼；`clear_pattern()` 比對的是儲存的快取鍵，所以在模式中要把 `|` 寫成 `%7C`。0.3.8 之前兩者都照原樣儲存，因此升級後，host 或路徑含有 `|` 或 `%` 的項目會重新快取一次。
 
 host 會先經過正規化，讓同一個來源的各種寫法共用同一筆項目：轉為小寫（主機名稱不分大小寫），並去除空的連接埠或該 scheme 的預設連接埠（http 為 `:80`，https 為 `:443`）。在 http 上，`Example.com`、`example.com:80` 與 `example.com` 是同一個鍵 `example.com`；`example.com:8080` 保留連接埠，IPv6 位址則保留方括號（`[::1]:8000`）。scheme 以應用程式看到的為準：在終止 TLS 的代理之後，除非套用了代理的標頭（例如 `uvicorn --proxy-headers`），否則 scheme 是 `http`，因此這類代理送來的 `Host: example.com:443` 會保留連接埠。沒有 `Host` 標頭的請求使用 `unknown`。
@@ -448,7 +450,7 @@ add_routes(
 )
 ```
 
-- `GET {prefix}/cached-hits`：列出每筆快取項目，拆分為方法、主機、路徑與查詢，附上 ETag 與到期時間，另外統計有效與已過期的項目數，以及不重複的快取路徑。它不會計算命中次數。
+- `GET {prefix}/cached-hits`：列出每筆快取項目，拆分為方法、主機、路徑與查詢（超過 200 位元組的查詢為 `sha256:<十六進位>`），附上 ETag 與到期時間，另外統計有效與已過期的項目數，以及不重複的快取路徑。它不會計算命中次數。
 - `GET {prefix}/cached-records`：列出每筆快取紀錄的大小、到期時間、`media_type`（儲存的回應的媒體類型，沒有時為 `null`），以及快取內容前 100 個位元組的預覽。設定 `include_content_preview=False` 時，`content_preview` 為 `null`，不會有任何回應本文離開伺服器；鍵、大小與到期時間仍會回報。`content_type` 一律是 `"bytes"`，只為相容而保留；請改讀 `media_type`。
 
 兩個路由都只列出路由項目（格式為 `http:v2|method|host|path|query` 的鍵）；`CacheManager`、Session、state 與鎖的鍵都會略過，未使用 `build_cache_key()` 的 `key_builder` 產生的鍵也一樣。
diff --git a/i18n/zh-TW/docs/MIGRATING_0_4.md b/i18n/zh-TW/docs/MIGRATING_0_4.md
index f297094..3147228 100644
--- a/i18n/zh-TW/docs/MIGRATING_0_4.md
+++ b/i18n/zh-TW/docs/MIGRATING_0_4.md
@@ -252,7 +252,7 @@ CacheManagerProxy.set(CacheManager(lock=True))
 - 分隔符號改為單一的 `|`（`CACHE_KEY_SEPARATOR`）。
 - 鍵以格式標籤 `http:v2|`（`CacheKey.FORMAT_TAG`）開頭，讓下一次格式變更可以用模式移除舊鍵：`clear_pattern("http:v2|*")` 會移除這個格式的所有鍵。
 - host 會正規化：轉為小寫，並去除空的連接埠或該 scheme 的預設連接埠（http 為 `:80`，https 為 `:443`）。
-- 過長的查詢字串（約超過 200 位元組）會以 `sha256:` 加上十六進位摘要儲存；路徑仍保持可讀。
+- 超過 200 位元組（以鍵中的編碼計算）的查詢字串會以 `sha256:` 加上十六進位摘要儲存；路徑仍保持可讀，監控路由則顯示這個摘要。
 - 查詢參數會依名稱排序：`sort_query`（0.3.9 起可選用）在 `@cache`、`build_cache_key()` 與 `invalidate()` 中預設為 `True`，因此 `?b=2&a=1` 與 `?a=1&b=2` 共用同一筆項目。
 - 由單一、公開的 `CacheKey` 型別負責建立、編碼與解析鍵。`fastapi_cachex.routes` 中的 `CACHE_KEY_MIN_PARTS`、`CACHE_KEY_MAX_SPLIT` 與 `CACHE_KEY_MAX_PARTS` 已移除；請改用 `CacheKey.parse(key)` 讀取鍵的各段。
 
diff --git a/tests/test_query_hash.py b/tests/test_query_hash.py
new file mode 100644
index 0000000..818c347
--- /dev/null
+++ b/tests/test_query_hash.py
@@ -0,0 +1,169 @@
+"""A long query string is stored in the key as its SHA-256 digest (#269)."""
+
+import hashlib
+
+from fastapi import FastAPI
+from fastapi import Request
+from fastapi.testclient import TestClient
+
+from fastapi_cachex import CacheKey
+from fastapi_cachex import add_routes
+from fastapi_cachex import build_cache_key
+from fastapi_cachex import cache
+from fastapi_cachex import invalidate
+from fastapi_cachex.backends import MemoryBackend
+from fastapi_cachex.proxy import BackendProxy
+from fastapi_cachex.types import CacheEntry
+
+ENTRY = CacheEntry("e", b"v")
+
+
+def _request(query: str) -> Request:
+    return Request(
+        {
+            "type": "http",
+            "method": "GET",
+            "path": "/p",
+            "raw_path": b"/p",
+            "query_string": query.encode(),
+            "headers": [(b"host", b"h")],
+        }
+    )
+
+
+def _query(length: int) -> str:
+    """A query that is ``length`` bytes once encoded."""
+    return "q=" + "a" * (length - 2)
+
+
+def _digest(query: str) -> str:
+    return "sha256:" + hashlib.sha256(query.encode()).hexdigest()
+
+
+def test_a_query_of_200_bytes_is_kept() -> None:
+    query = _query(200)
+
+    assert CacheKey.from_request(_request(query)).query == query
+
+
+def test_a_longer_query_is_stored_as_its_digest() -> None:
+    query = _query(201)
+
+    key = build_cache_key(_request(query))
+
+    assert key == f"http:v2|GET|h|/p|{_digest(query)}"
+    assert CacheKey.parse(key) == CacheKey("GET", "h", "/p", _digest(query))
+
+
+def test_the_digest_covers_the_encoded_query() -> None:
+    """Length and digest are of the encoded query, not the decoded values."""
+    encoded = "q=" + "%C3%A9" * 40  # 242 bytes; the decoded value is 40 chars
+
+    assert CacheKey.from_request(_request(encoded)).query == _digest(encoded)
+
+
+def test_the_digest_is_taken_after_sorting() -> None:
+    forward = "a=1&" + _query(250)
+    reverse = _query(250) + "&a=1"
+
+    sorted_keys = {
+        build_cache_key(_request(q), sort_query=True) for q in (forward, reverse)
+    }
+    unsorted_keys = {build_cache_key(_request(q)) for q in (forward, reverse)}
+
+    assert len(sorted_keys) == 1
+    assert len(unsorted_keys) == 2
+
+
+def test_extra_components_follow_the_digest() -> None:
+    query = _query(201)
+
+    key = build_cache_key(_request(query), "tenant", 7)
+
+    assert key == f"http:v2|GET|h|/p|{_digest(query)}|tenant|7"
+    assert CacheKey.parse(key) == CacheKey(
+        "GET", "h", "/p", _digest(query), ("tenant", "7")
+    )
+
+
+def test_a_query_cannot_pose_as_a_digest() -> None:
+    """``:`` is percent-encoded, so a sent query never starts with ``sha256:``."""
+    digest = _digest(_query(201))
+
+    assert CacheKey.from_request(_request(digest)).query != digest
+
+
+def _app(backend: MemoryBackend) -> tuple[FastAPI, list[int]]:
+    app = FastAPI()
+    BackendProxy.set(backend)
+    calls: list[int] = []
+
+    @app.get("/p")
+    @cache(ttl=60)
+    async def endpoint() -> dict[str, int]:
+        calls.append(1)
+        return {"calls": len(calls)}
+
+    add_routes(app, dependencies=[])
+    return app, calls
+
+
+def test_long_queries_are_cached_apart_and_hit_again() -> None:
+    backend = MemoryBackend()
+    app, calls = _app(backend)
+    client = TestClient(app)
+    first, second = _query(300), "q=" + "b" * 298
+
+    client.get(f"/p?{first}")
+    client.get(f"/p?{second}")
+    client.get(f"/p?{first}")
+
+    assert len(calls) == 2
+    assert sorted(backend.cache) == sorted(
+        f"http:v2|GET|testserver|/p|{_digest(q)}" for q in (first, second)
+    )
+
+
+def test_monitoring_shows_the_digest() -> None:
+    backend = MemoryBackend()
+    app, _ = _app(backend)
+    client = TestClient(app)
+    query = _query(300)
+
+    client.get(f"/p?{query}")
+    [record] = client.get("/cached-records").json()["cached_records"]
+
+    assert record["path"] == "/p"
+    assert record["query_params"] == _digest(query)
+
+
+async def test_clear_path_counts_a_hashed_query_as_a_query() -> None:
+    backend = MemoryBackend()
+    BackendProxy.set(backend)
+    await backend.set(build_cache_key(_request(_query(300))), ENTRY)
+
+    assert await backend.clear_path("/p") == 0
+    assert await backend.clear_path("/p", include_params=True) == 1
+
+
+async def test_invalidate_finds_a_hashed_entry() -> None:
+    backend = MemoryBackend()
+    BackendProxy.set(backend)
+    request = _request(_query(300))
+    await backend.set(build_cache_key(request), ENTRY)
+
+    assert await invalidate(request) is True
+    assert backend.cache == {}
+
+
+async def test_sorted_invalidate_finds_a_reordered_long_query() -> None:
+    backend = MemoryBackend()
+    BackendProxy.set(backend)
+    stored = _request("a=1&" + _query(250))
+    await backend.set(build_cache_key(stored, sort_query=True), ENTRY)
+
+    reordered = _request(_query(250) + "&a=1")
+
+    assert await invalidate(reordered) is False
+    assert await invalidate(reordered, sort_query=True) is True
+    assert backend.cache == {}