diff --git a/architecture/container-lifecycle.md b/architecture/container-lifecycle.md
index 7bc8d8c..999c723 100644
--- a/architecture/container-lifecycle.md
+++ b/architecture/container-lifecycle.md
@@ -14,17 +14,19 @@ entry point an application calls at startup. It does three things:
 2. Registers the two *context providers*
    (`fastapi_request_provider`, `fastapi_websocket_provider`) on the container's
    `providers_registry`, so the live `Request` / `WebSocket` can be resolved.
-3. Chains an internal lifespan manager onto the app's existing
-   `lifespan_context` via `fastapi.routing._merge_lifespan_context`, preserving
-   any lifespan the app already had.
+3. Composes the container's open/close onto the app's existing
+   `lifespan_context` via `_compose_lifespan`, preserving any lifespan the app
+   already had (its startup/shutdown still run and its yielded state passes
+   through). The composition is our own — no dependency on FastAPI internals.
 
 It returns the same container for convenience. The application owns container
 construction (groups, overrides); `setup_di` only wires it in.
 
 ## Lifespan — open/close across cycles
 
-The chained `_lifespan_manager` runs `async with fetch_di_container(app):` — the
-root container's `__aenter__` opens it on startup and `__aexit__` closes it on
+The composed lifespan keeps the original as the outer context and opens the
+container inside it with `async with fetch_di_container(app):` — the root
+container's `__aenter__` opens it on startup and `__aexit__` closes it on
 shutdown. Using `async with` (rather than a one-shot open) means a **second
 lifespan cycle against the same container reopens it** instead of raising
 `ContainerClosedError`. This is what lets an app be started, stopped, and
diff --git a/modern_di_fastapi/main.py b/modern_di_fastapi/main.py
index 0f3d0b4..3c04c5b 100644
--- a/modern_di_fastapi/main.py
+++ b/modern_di_fastapi/main.py
@@ -3,9 +3,9 @@
 import typing
 
 import fastapi
-from fastapi.routing import _merge_lifespan_context
 from modern_di import Container, Scope, providers
 from starlette.requests import HTTPConnection
+from starlette.types import Lifespan
 
 
 T_co = typing.TypeVar("T_co", covariant=True)
@@ -25,23 +25,29 @@ def fetch_di_container(app_: fastapi.FastAPI) -> Container:
     return typing.cast(Container, app_.state.di_container)
 
 
-@contextlib.asynccontextmanager
-async def _lifespan_manager(app_: fastapi.FastAPI) -> typing.AsyncIterator[None]:
-    # ``async with`` reopens the root container on each startup (``__aenter__``)
-    # and closes it on shutdown, so a second lifespan cycle against the same
-    # container works instead of raising ContainerClosedError.
-    async with fetch_di_container(app_):
-        yield
+def _compose_lifespan(original: Lifespan[fastapi.FastAPI]) -> Lifespan[fastapi.FastAPI]:
+    """Wrap ``original`` so the root container opens/closes around it.
+
+    The original lifespan stays the outer context and its yielded state passes
+    straight through; the container is opened inside it. ``async with`` reopens the
+    container on each startup and closes it on shutdown, so a second lifespan cycle
+    against the same container works instead of raising ``ContainerClosedError``.
+    """
+
+    @contextlib.asynccontextmanager
+    async def composed(app_: fastapi.FastAPI) -> typing.AsyncIterator[typing.Mapping[str, typing.Any] | None]:
+        async with original(app_) as state, fetch_di_container(app_):
+            yield state
+
+    # ``Lifespan`` is a union of CM[None] | CM[Mapping]; it can't express our
+    # CM[Mapping | None], though that is exactly what a lifespan may yield.
+    return typing.cast(Lifespan[fastapi.FastAPI], composed)
 
 
 def setup_di(app: fastapi.FastAPI, container: Container) -> Container:
     app.state.di_container = container
     container.providers_registry.add_providers(*_CONNECTION_PROVIDERS)
-    old_lifespan_manager = app.router.lifespan_context
-    app.router.lifespan_context = _merge_lifespan_context(
-        old_lifespan_manager,
-        _lifespan_manager,
-    )
+    app.router.lifespan_context = _compose_lifespan(app.router.lifespan_context)
     return container
 
 
diff --git a/tests/test_lifespan.py b/tests/test_lifespan.py
index 4080470..d24f1a8 100644
--- a/tests/test_lifespan.py
+++ b/tests/test_lifespan.py
@@ -1,9 +1,12 @@
+import contextlib
 import typing
 
 import fastapi
+import modern_di
 from starlette import status
 from starlette.testclient import TestClient
 
+import modern_di_fastapi
 from modern_di_fastapi import FromDI, fetch_di_container
 from tests.dependencies import Dependencies, SimpleCreator
 
@@ -23,3 +26,31 @@ async def read_root(instance: typing.Annotated[SimpleCreator, FromDI(Dependencie
     # Second cycle must reopen the same container instead of raising ContainerClosedError.
     with TestClient(app=app) as client:
         assert client.get("/").status_code == status.HTTP_200_OK
+
+
+def test_setup_di_composes_with_existing_lifespan() -> None:
+    events: list[str] = []
+
+    @contextlib.asynccontextmanager
+    async def user_lifespan(app_: fastapi.FastAPI) -> typing.AsyncIterator[dict[str, str]]:
+        assert isinstance(app_, fastapi.FastAPI)
+        events.append("startup")
+        yield {"marker": "from-user-lifespan"}
+        events.append("shutdown")
+
+    app = fastapi.FastAPI(lifespan=user_lifespan)
+    container = modern_di.Container(groups=[Dependencies])
+    modern_di_fastapi.setup_di(app, container)
+
+    @app.get("/")
+    async def read_marker(request: fastapi.Request) -> str:
+        return typing.cast(str, request.state.marker)
+
+    with TestClient(app=app) as client:
+        # original lifespan started; our container opened; its yielded state passes through
+        assert events == ["startup"]
+        assert not container.closed
+        assert client.get("/").json() == "from-user-lifespan"
+    # shutdown ran the original lifespan and closed our container
+    assert events == ["startup", "shutdown"]
+    assert container.closed