fix: propagate StaticFiles Mount objects when using include_router

Per maintainer feedback on #14924, limit Mount propagation in
include_router() to StaticFiles only, matching the approach in #13944.

Non-StaticFiles sub-app mounts are silently ignored (not propagated)
to preserve existing behavior, since sub-applications should be mounted
directly on the root FastAPI instance.

Fixes #10180

fastapi/routing.py

@@ -74,6 +74,7 @@ from starlette.routing import (
     get_name,
 )
 from starlette.routing import Mount as Mount  # noqa
+from starlette.staticfiles import StaticFiles
 from starlette.types import AppType, ASGIApp, Lifespan, Receive, Scope, Send
 from starlette.websockets import WebSocket
 from typing_extensions import deprecated
@@ -1490,7 +1491,8 @@ class APIRouter(routing.Router):
                     prefix + route.path, route.endpoint, name=route.name
                 )
             elif isinstance(route, routing.Mount):
-                self.mount(prefix + router.prefix + route.path, route.app, name=route.name)
+                if isinstance(route.app, StaticFiles):
+                    self.mount(prefix + router.prefix + route.path, route.app, name=route.name)
         for handler in router.on_startup:
             self.add_event_handler("startup", handler)
         for handler in router.on_shutdown:

tests/test_router_mount.py

@@ -1,11 +1,17 @@
-"""Tests for mounting sub-applications under APIRouter (issue #10180)."""
+"""Tests for mounting StaticFiles under APIRouter (issue #10180)."""
+from pathlib import Path
+
+import pytest
 from fastapi import APIRouter, FastAPI
-from starlette.staticfiles import StaticFiles
-from starlette.testclient import TestClient
+from fastapi.staticfiles import StaticFiles
+from fastapi.testclient import TestClient
+
 
+def test_mount_staticfiles_under_router_with_prefix(tmp_path: Path) -> None:
+    """StaticFiles mounted on APIRouter with prefix should be accessible."""
+    static_file = tmp_path / "hello.txt"
+    static_file.write_text("hello world")
 
-def test_mount_subapp_under_router_with_prefix() -> None:
-    """APIRouter with prefix should propagate Mount to the main app."""
     app = FastAPI()
     api_router = APIRouter(prefix="/api")
 
@@ -13,13 +19,7 @@ def test_mount_subapp_under_router_with_prefix() -> None:
     def read_main() -> dict:
         return {"message": "Hello World from main app"}
 
-    subapi = FastAPI()
-
-    @subapi.get("/sub")
-    def read_sub() -> dict:
-        return {"message": "Hello World from sub API"}
-
-    api_router.mount("/subapi", subapi)
+    api_router.mount("/static", StaticFiles(directory=tmp_path), name="static")
     app.include_router(api_router)
 
     client = TestClient(app)
@@ -27,80 +27,68 @@ def test_mount_subapp_under_router_with_prefix() -> None:
     # Regular route should still work
     r = client.get("/api/app")
     assert r.status_code == 200
-    assert r.json() == {"message": "Hello World from main app"}
 
-    # Mounted sub-app should be accessible under router prefix
-    r = client.get("/api/subapi/sub")
+    # StaticFiles should be accessible under router prefix
+    r = client.get("/api/static/hello.txt")
     assert r.status_code == 200
-    assert r.json() == {"message": "Hello World from sub API"}
+    assert r.text == "hello world"
+
 
+def test_mount_staticfiles_under_router_without_prefix(tmp_path: Path) -> None:
+    """StaticFiles mounted on APIRouter without prefix should be accessible."""
+    static_file = tmp_path / "test.txt"
+    static_file.write_text("test content")
 
-def test_mount_subapp_under_router_without_prefix() -> None:
-    """APIRouter without prefix should propagate Mount correctly."""
     app = FastAPI()
     router = APIRouter()
-
-    subapi = FastAPI()
-
-    @subapi.get("/hello")
-    def hello() -> dict:
-        return {"msg": "hello"}
-
-    router.mount("/sub", subapi)
+    router.mount("/static", StaticFiles(directory=tmp_path), name="static")
     app.include_router(router)
 
     client = TestClient(app)
 
-    r = client.get("/sub/hello")
+    r = client.get("/static/test.txt")
     assert r.status_code == 200
-    assert r.json() == {"msg": "hello"}
+    assert r.text == "test content"
+
 
+def test_mount_staticfiles_with_include_router_prefix(tmp_path: Path) -> None:
+    """include_router prefix + router prefix + mount path should all combine."""
+    static_file = tmp_path / "file.txt"
+    static_file.write_text("combined prefix")
 
-def test_mount_subapp_with_include_router_prefix() -> None:
-    """include_router prefix should be combined with router prefix and mount path."""
     app = FastAPI()
     router = APIRouter(prefix="/api")
-
-    subapi = FastAPI()
-
-    @subapi.get("/test")
-    def test_endpoint() -> dict:
-        return {"msg": "test"}
-
-    router.mount("/sub", subapi)
+    router.mount("/static", StaticFiles(directory=tmp_path), name="static")
     app.include_router(router, prefix="/v1")
 
     client = TestClient(app)
 
-    r = client.get("/v1/api/sub/test")
+    r = client.get("/v1/api/static/file.txt")
     assert r.status_code == 200
-    assert r.json() == {"msg": "test"}
+    assert r.text == "combined prefix"
 
 
-def test_mount_preserves_regular_routes() -> None:
-    """Adding Mount support should not break existing APIRoute handling."""
+def test_mount_non_staticfiles_app_is_ignored() -> None:
+    """Mounting a non-StaticFiles ASGI app on APIRouter should be silently ignored
+    (not propagated to the main app) to preserve existing behavior."""
     app = FastAPI()
     api_router = APIRouter(prefix="/api")
 
-    @api_router.get("/items")
-    def list_items() -> dict:
-        return {"items": [1, 2, 3]}
-
-    @api_router.post("/items")
-    def create_item() -> dict:
-        return {"created": True}
-
     subapi = FastAPI()
 
-    @subapi.get("/status")
-    def status() -> dict:
-        return {"status": "ok"}
+    @subapi.get("/sub")
+    def read_sub() -> dict:
+        return {"message": "sub"}
 
-    api_router.mount("/health", subapi)
+    api_router.mount("/subapi", subapi)
     app.include_router(api_router)
 
     client = TestClient(app)
 
-    assert client.get("/api/items").status_code == 200
-    assert client.post("/api/items").status_code == 200
-    assert client.get("/api/health/status").status_code == 200
+    # Non-StaticFiles sub-app mount is not propagated — returns 404
+    r = client.get("/api/subapi/sub")
+    assert r.status_code == 404
+
+    # Regular routes are unaffected
+    # (no routes defined here, but include_router itself should not crash)
+    assert app is not None