🐛 Fix AssertionError when using TestClient directly with APIRouter (#14128)

fastapi/routing.py

@@ -115,6 +115,11 @@ def request_response(
             response_awaited = False
             async with AsyncExitStack() as request_stack:
                 scope["fastapi_inner_astack"] = request_stack
+                # Ensure fastapi_middleware_astack exists in scope even when
+                # the router is used directly without a FastAPI app (which
+                # normally sets it up via AsyncExitStackMiddleware).
+                if "fastapi_middleware_astack" not in scope:
+                    scope["fastapi_middleware_astack"] = request_stack
                 async with AsyncExitStack() as function_stack:
                     scope["fastapi_function_astack"] = function_stack
                     response = await f(request)

tests/test_router_direct_testclient.py

@@ -0,0 +1,62 @@
+"""Test for issue #14128: Router used directly with TestClient and yield dependencies."""
+
+from fastapi import APIRouter, Depends
+from fastapi.testclient import TestClient
+
+
+def test_router_direct_testclient_with_yield_dependency():
+    """TestClient(router) should work even without wrapping in FastAPI()."""
+    router = APIRouter()
+
+    async def yield_dep():
+        yield "value"
+
+    @router.get("/test")
+    async def endpoint(dep: str = Depends(yield_dep)):
+        return {"dep": dep}
+
+    client = TestClient(router)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert response.json() == {"dep": "value"}
+
+
+def test_router_direct_testclient_with_multiple_yield_dependencies():
+    """Multiple yield dependencies should work with direct router TestClient."""
+    router = APIRouter()
+
+    async def dep_a():
+        yield "a"
+
+    async def dep_b():
+        yield "b"
+
+    @router.get("/test")
+    async def endpoint(a: str = Depends(dep_a), b: str = Depends(dep_b)):
+        return {"a": a, "b": b}
+
+    client = TestClient(router)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert response.json() == {"a": "a", "b": "b"}
+
+
+def test_router_direct_testclient_yield_dependency_cleanup():
+    """Yield dependency cleanup should run even with direct router TestClient."""
+    cleaned_up = False
+
+    router = APIRouter()
+
+    async def yield_dep():
+        nonlocal cleaned_up
+        yield "value"
+        cleaned_up = True
+
+    @router.get("/test")
+    async def endpoint(dep: str = Depends(yield_dep)):
+        return {"dep": dep}
+
+    client = TestClient(router)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert cleaned_up