diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4109075..74fefd5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,15 @@ Note that 0.3.3 was never released; 0.3.4 follows 0.3.2.
   compare-and-delete, Memcached `ADD` and `GETS` + `CAS`, memory its lock.
   Third-party backends inherit non-atomic fallbacks. ([#62](https://github.com/allen0099/FastAPI-CacheX/issues/62))
 
+### Fixed
+
+- Concurrent first requests to the `AppCache` dependency, in an app that never
+  configured a backend, no longer each build their own `MemoryBackend` and
+  `CacheManager`. `get_app_cache` runs in worker threads, so the last one
+  registered replaced the others and, for a while, requests used caches that
+  could not see each other's entries. The lazy set-up and the `@cache`
+  fallback now happen under one lock. ([#76](https://github.com/allen0099/FastAPI-CacheX/issues/76))
+
 ### Documentation
 
 - The documentation is published at <https://fastapi-cachex.readthedocs.io/>,
diff --git a/fastapi_cachex/cache.py b/fastapi_cachex/cache.py
index cd075da..52ed75a 100644
--- a/fastapi_cachex/cache.py
+++ b/fastapi_cachex/cache.py
@@ -26,12 +26,12 @@
 from starlette.status import HTTP_300_MULTIPLE_CHOICES
 from starlette.status import HTTP_304_NOT_MODIFIED
 
-from .backends import MemoryBackend
 from .directives import DirectiveType
 from .exceptions import BackendNotFoundError
 from .exceptions import CacheXError
 from .exceptions import RequestNotFoundError
 from .proxy import BackendProxy
+from .proxy import get_backend_or_fallback
 from .types import CACHE_KEY_SEPARATOR
 from .types import CacheEntry
 from .types import CacheKeyBuilder
@@ -470,12 +470,7 @@ def build_cache_control() -> str:
         @wraps(func)
         async def wrapper(*args: Any, **kwargs: Any) -> Response:
             # Resolve backend on every request to support lifespan-configured backends
-            try:
-                cache_backend = BackendProxy.get()
-            except BackendNotFoundError:
-                cache_backend = MemoryBackend()
-                BackendProxy.set(cache_backend)
-                logger.debug("No backend configured; using MemoryBackend fallback")
+            cache_backend = get_backend_or_fallback()
 
             if found_request:
                 req: Request | None = kwargs.get(request_name)
diff --git a/fastapi_cachex/dependencies.py b/fastapi_cachex/dependencies.py
index 0864061..d720ea7 100644
--- a/fastapi_cachex/dependencies.py
+++ b/fastapi_cachex/dependencies.py
@@ -1,15 +1,18 @@
 """FastAPI dependency injection utilities for cache control."""
 
+import threading
 from typing import Annotated
 
 from fastapi import Depends
 
-from .backends import MemoryBackend
 from .backends.base import BaseCacheBackend
 from .exceptions import BackendNotFoundError
 from .manager import CacheManager
 from .manager_proxy import CacheManagerProxy
 from .proxy import BackendProxy
+from .proxy import get_backend_or_fallback
+
+_manager_lock = threading.Lock()
 
 
 def get_cache_backend() -> BaseCacheBackend:
@@ -36,14 +39,17 @@ def get_app_cache() -> CacheManager:
     try:
         return CacheManagerProxy.get()
     except BackendNotFoundError:
+        pass
+    # Checked again under the lock: FastAPI runs this sync dependency in a
+    # worker thread, so concurrent first requests would otherwise each build
+    # and register their own manager (and fallback backend).
+    with _manager_lock:
         try:
-            backend = BackendProxy.get()
+            return CacheManagerProxy.get()
         except BackendNotFoundError:
-            backend = MemoryBackend()
-            BackendProxy.set(backend)
-        manager = CacheManager(backend=backend)
-        CacheManagerProxy.set(manager)
-        return manager
+            manager = CacheManager(backend=get_backend_or_fallback())
+            CacheManagerProxy.set(manager)
+            return manager
 
 
 AppCache = Annotated[CacheManager, Depends(get_app_cache)]
diff --git a/fastapi_cachex/proxy.py b/fastapi_cachex/proxy.py
index ee15e60..613cd46 100644
--- a/fastapi_cachex/proxy.py
+++ b/fastapi_cachex/proxy.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import threading
 import warnings
 from logging import getLogger
 from typing import Generic
@@ -9,12 +10,17 @@
 from typing import TypeVar
 
 from .backends import BaseCacheBackend
+from .backends import MemoryBackend
 from .exceptions import BackendNotFoundError
 
 ProxyInstance = TypeVar("ProxyInstance")
 
 logger = getLogger(__name__)
 
+# Serialises the lazy fallback below. `get_app_cache` is a sync dependency that
+# FastAPI runs in a worker thread, so two first requests can reach it at once.
+_fallback_lock = threading.Lock()
+
 
 class ProxyMeta(type):
     """Metaclass for BackendProxy to prevent instantiation."""
@@ -94,3 +100,25 @@ def set_backend(backend: BaseCacheBackend | None) -> None:
             stacklevel=2,
         )
         BackendProxy.set(backend)
+
+
+def get_backend_or_fallback() -> BaseCacheBackend:
+    """Return the configured backend, registering a `MemoryBackend` if none is.
+
+    Used by `@cache` and the `AppCache` dependency. The check and the
+    registration happen under one lock, so concurrent first callers — including
+    ones on worker threads — all end up with the same fallback instead of each
+    installing its own and overwriting the others.
+    """
+    try:
+        return BackendProxy.get()
+    except BackendNotFoundError:
+        pass
+    with _fallback_lock:
+        try:
+            return BackendProxy.get()
+        except BackendNotFoundError:
+            backend = MemoryBackend()
+            BackendProxy.set(backend)
+            logger.debug("No backend configured; using MemoryBackend fallback")
+            return backend
diff --git a/tests/test_dependencies.py b/tests/test_dependencies.py
index 637d7b0..d017e58 100644
--- a/tests/test_dependencies.py
+++ b/tests/test_dependencies.py
@@ -7,6 +7,7 @@
 from fastapi_cachex.backends import MemoryBackend
 from fastapi_cachex.dependencies import get_app_cache
 from fastapi_cachex.exceptions import BackendNotFoundError
+from fastapi_cachex.manager import CacheManager
 from fastapi_cachex.manager_proxy import CacheManagerProxy
 
 # Setup FastAPI application
@@ -67,3 +68,42 @@ async def test_get_app_cache_uses_the_configured_backend():
     manager = get_app_cache()
 
     assert manager.backend is backend
+
+
+def test_get_app_cache_concurrent_first_calls_share_one_backend(monkeypatch):
+    """Concurrent first calls must agree on one backend and one manager.
+
+    FastAPI runs the sync `get_app_cache` in worker threads. Without a lock,
+    two first requests each built a `MemoryBackend` and a `CacheManager`, and
+    the later `set()` replaced the earlier, leaving two caches that could not
+    see each other's entries.
+    """
+    import threading
+    import time
+    from concurrent.futures import ThreadPoolExecutor
+
+    import fastapi_cachex.proxy
+
+    class SlowMemoryBackend(MemoryBackend):
+        def __init__(self) -> None:
+            time.sleep(0.05)  # widen the window between the check and the set
+            super().__init__()
+
+    monkeypatch.setattr(fastapi_cachex.proxy, "MemoryBackend", SlowMemoryBackend)
+    BackendProxy.set(None)
+    CacheManagerProxy.set(None)
+
+    workers = 8
+    barrier = threading.Barrier(workers)
+
+    def first_call() -> CacheManager:
+        barrier.wait()
+        return get_app_cache()
+
+    with ThreadPoolExecutor(max_workers=workers) as pool:
+        managers = list(pool.map(lambda _: first_call(), range(workers)))
+
+    assert len({id(m) for m in managers}) == 1
+    assert len({id(m.backend) for m in managers}) == 1
+    assert BackendProxy.get() is managers[0].backend
+    assert CacheManagerProxy.get() is managers[0]