✨ Support dependencies in `app.frontend()`, e.g. for automatic cookie authentication for the frontend (#15908)

docs/en/docs/tutorial/frontend.md

@@ -126,6 +126,12 @@ In this example, frontend paths are served under `/app`.
 
 Any regular *path operations* in the app will still take precedence, including in other routers.
 
+## Dependencies and Middleware { #dependencies-and-middleware }
+
+Frontend responses run inside the normal **FastAPI** application, so HTTP middleware applies to them.
+
+Dependencies from the app, from an `APIRouter`, and from `include_router()` also apply to frontend responses. This can be useful for protecting a frontend with cookie authentication or similar.
+
 ## Static Build Output Only { #static-build-output-only }
 
 `app.frontend()` serves files already generated by your frontend build.

fastapi/routing.py

@@ -797,17 +797,14 @@ class APIWebSocketRoute(routing.WebSocketRoute):
         self.name = get_name(endpoint) if name is None else name
         self.dependencies = list(dependencies or [])
         self.path_regex, self.path_format, self.param_convertors = compile_path(path)
-        self.dependant = get_dependant(
-            path=self.path_format, call=self.endpoint, scope="function"
-        )
-        for depends in self.dependencies[::-1]:
-            self.dependant.dependencies.insert(
-                0,
-                get_parameterless_sub_dependant(depends=depends, path=self.path_format),
-            )
-        self._flat_dependant = get_flat_dependant(self.dependant)
-        self._embed_body_fields = _should_embed_body_fields(
-            self._flat_dependant.body_params
+        (
+            self.dependant,
+            self._flat_dependant,
+            self._embed_body_fields,
+        ) = _build_dependant_with_parameterless_dependencies(
+            path=self.path_format,
+            call=self.endpoint,
+            dependencies=self.dependencies,
         )
         self.app = websocket_session(
             get_websocket_app(
@@ -827,6 +824,7 @@ class APIWebSocketRoute(routing.WebSocketRoute):
 _FASTAPI_SCOPE_KEY = "fastapi"
 _FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY = "effective_route_context"
 _FASTAPI_FRONTEND_PATH_KEY = "frontend_path"
+_FASTAPI_FRONTEND_SPECIFICITY_KEY = "frontend_specificity"
 _FASTAPI_INCLUDED_ROUTER_KEY = "included_router"
 _effective_route_context_var: ContextVar[Any | None] = ContextVar(
     "fastapi_effective_route_context", default=None
@@ -834,6 +832,27 @@ _effective_route_context_var: ContextVar[Any | None] = ContextVar(
 _SCOPE_MISSING = object()
 
 
+def _frontend_dependency_endpoint() -> None:
+    pass  # pragma: no cover
+
+
+def _build_dependant_with_parameterless_dependencies(
+    *,
+    path: str,
+    call: Callable[..., Any],
+    dependencies: Sequence[params.Depends],
+) -> tuple[Dependant, Dependant, bool]:
+    dependant = get_dependant(path=path, call=call, scope="function")
+    for depends in dependencies[::-1]:
+        dependant.dependencies.insert(
+            0,
+            get_parameterless_sub_dependant(depends=depends, path=path),
+        )
+    flat_dependant = get_flat_dependant(dependant)
+    embed_body_fields = _should_embed_body_fields(flat_dependant.body_params)
+    return dependant, flat_dependant, embed_body_fields
+
+
 class _RouteWithPath(Protocol):
     path: str
 
@@ -861,6 +880,15 @@ def _get_scope_included_router(scope: Scope) -> Any | None:
     return scope.get(_FASTAPI_SCOPE_KEY, {}).get(_FASTAPI_INCLUDED_ROUTER_KEY)
 
 
+def _frontend_scope_specificity(scope: Scope) -> int | None:
+    specificity = scope.get(_FASTAPI_SCOPE_KEY, {}).get(
+        _FASTAPI_FRONTEND_SPECIFICITY_KEY
+    )
+    if isinstance(specificity, int):
+        return specificity
+    return None
+
+
 def _restore_fastapi_scope_key(scope: Scope, key: str, previous: Any) -> None:
     fastapi_scope = scope.get(_FASTAPI_SCOPE_KEY)
     if not isinstance(fastapi_scope, dict):
@@ -1053,17 +1081,14 @@ def _populate_api_route_state(
         route.response_fields = {}
 
     assert callable(endpoint), "An endpoint must be a callable"
-    route.dependant = get_dependant(
-        path=route.path_format, call=route.endpoint, scope="function"
-    )
-    for depends in route.dependencies[::-1]:
-        route.dependant.dependencies.insert(
-            0,
-            get_parameterless_sub_dependant(depends=depends, path=route.path_format),
-        )
-    route._flat_dependant = get_flat_dependant(route.dependant)
-    route._embed_body_fields = _should_embed_body_fields(
-        route._flat_dependant.body_params
+    (
+        route.dependant,
+        route._flat_dependant,
+        route._embed_body_fields,
+    ) = _build_dependant_with_parameterless_dependencies(
+        path=route.path_format,
+        call=route.endpoint,
+        dependencies=route.dependencies,
     )
     route.body_field = get_body_field(
         flat_dependant=route._flat_dependant,
@@ -1334,6 +1359,7 @@ class _RouterIncludeContext:
 class _EffectiveRouteContext:
     original_route: BaseRoute
     starlette_route: BaseRoute | None = None
+    frontend_prefix: str = ""
     path: str = ""
     endpoint: Callable[..., Any] | None = None
     stream_item_type: Any | None = None
@@ -1436,7 +1462,34 @@ class _EffectiveRouteContext:
         )
         return context
 
+    @classmethod
+    def from_frontend_route_group(
+        cls,
+        *,
+        original_route: "_FrontendRouteGroup",
+        include_context: _RouterIncludeContext,
+    ) -> "_EffectiveRouteContext":
+        dependencies = [*include_context.dependencies, *original_route.dependencies]
+        context = cls(
+            original_route=original_route,
+            frontend_prefix=include_context.prefix,
+            dependencies=dependencies,
+            dependency_overrides_provider=include_context.dependency_overrides_provider,
+        )
+        (
+            context.dependant,
+            context._flat_dependant,
+            context._embed_body_fields,
+        ) = _build_dependant_with_parameterless_dependencies(
+            path="",
+            call=_frontend_dependency_endpoint,
+            dependencies=dependencies,
+        )
+        return context
+
     def matches(self, scope: Scope) -> tuple[Match, Scope]:
+        if isinstance(self.original_route, _FrontendRouteGroup):
+            return self.original_route.matches_with_prefix(scope, self.frontend_prefix)
         if not isinstance(self.original_route, APIRoute):
             assert self.starlette_route is not None
             return self.starlette_route.matches(scope)
@@ -1579,9 +1632,9 @@ class _IncludedRouter(BaseRoute):
                 include_context=self.include_context,
             )
         if isinstance(route, _FrontendRouteGroup):
-            return _EffectiveRouteContext(
+            return _EffectiveRouteContext.from_frontend_route_group(
                 original_route=route,
-                starlette_route=route.with_prefix(self.include_context.prefix),
+                include_context=self.include_context,
             )
         if isinstance(route, routing.Route):
             starlette_route: BaseRoute = routing.Route(
@@ -1970,28 +2023,31 @@ class _FrontendRoute(BaseRoute):
             directory=directory, fallback=fallback, check_dir=check_dir
         )
 
-    def with_path(self, path: str) -> "_FrontendRoute":
-        route = copy.copy(self)
-        route.path = _normalize_frontend_path(path)
-        return route
-
     def matches(self, scope: Scope) -> tuple[Match, Scope]:
+        return self.matches_with_path(scope, self.path)
+
+    def matches_with_path(self, scope: Scope, path: str) -> tuple[Match, Scope]:
         if scope["type"] != "http":
             return Match.NONE, {}
-        frontend_path = self._get_frontend_path(get_route_path(scope))
+        frontend_path = self._get_frontend_path(path, get_route_path(scope))
         if frontend_path is None:
             return Match.NONE, {}
-        child_scope = {_FASTAPI_SCOPE_KEY: {_FASTAPI_FRONTEND_PATH_KEY: frontend_path}}
+        child_scope = {
+            _FASTAPI_SCOPE_KEY: {
+                _FASTAPI_FRONTEND_PATH_KEY: frontend_path,
+                _FASTAPI_FRONTEND_SPECIFICITY_KEY: _frontend_path_specificity(path),
+            }
+        }
         if scope["method"] not in self.methods:
             return Match.PARTIAL, child_scope
         return Match.FULL, child_scope
 
-    def _get_frontend_path(self, route_path: str) -> str | None:
-        if self.path == "/":
+    def _get_frontend_path(self, path: str, route_path: str) -> str | None:
+        if path == "/":
             return route_path.lstrip("/")
-        if route_path == self.path:
+        if route_path == path:
             return ""
-        prefix = self.path + "/"
+        prefix = path + "/"
         if route_path.startswith(prefix):
             return route_path[len(prefix) :]
         return None
@@ -2004,8 +2060,24 @@ class _FrontendRoute(BaseRoute):
 
 
 class _FrontendRouteGroup(BaseRoute):
-    def __init__(self) -> None:
+    def __init__(
+        self,
+        *,
+        dependencies: Sequence[params.Depends] | None = None,
+        dependency_overrides_provider: Any | None = None,
+    ) -> None:
         self.routes: list[_FrontendRoute] = []
+        self.dependencies = list(dependencies or [])
+        self.dependency_overrides_provider = dependency_overrides_provider
+        (
+            self.dependant,
+            self._flat_dependant,
+            self._embed_body_fields,
+        ) = _build_dependant_with_parameterless_dependencies(
+            path="",
+            call=_frontend_dependency_endpoint,
+            dependencies=self.dependencies,
+        )
 
     def add_frontend_route(
         self,
@@ -2024,51 +2096,116 @@ class _FrontendRouteGroup(BaseRoute):
             )
         )
 
-    def with_prefix(self, prefix: str) -> "_FrontendRouteGroup":
-        route_group = copy.copy(self)
-        route_group.routes = [
-            route.with_path(_join_frontend_paths(prefix, route.path))
-            for route in self.routes
-        ]
-        return route_group
-
     def matches(self, scope: Scope) -> tuple[Match, Scope]:
-        match, child_scope, _ = self._match(scope)
+        match, child_scope, _ = self._match(scope, prefix="")
         return match, child_scope
 
-    def _match(self, scope: Scope) -> tuple[Match, Scope, _FrontendRoute | None]:
-        full: tuple[Scope, _FrontendRoute] | None = None
-        partial: tuple[Scope, _FrontendRoute] | None = None
+    def matches_with_prefix(self, scope: Scope, prefix: str) -> tuple[Match, Scope]:
+        match, child_scope, _ = self._match(scope, prefix=prefix)
+        return match, child_scope
+
+    def _match(
+        self, scope: Scope, *, prefix: str
+    ) -> tuple[Match, Scope, _FrontendRoute | None]:
+        full: tuple[Scope, _FrontendRoute, int] | None = None
+        partial: tuple[Scope, _FrontendRoute, int] | None = None
         for route in self.routes:
-            match, child_scope = route.matches(scope)
+            path = _join_frontend_paths(prefix, route.path)
+            match, child_scope = route.matches_with_path(scope, path)
+            specificity = _frontend_path_specificity(path)
             if match == Match.FULL:
-                if full is None or _frontend_path_specificity(
-                    route.path
-                ) > _frontend_path_specificity(full[1].path):
-                    full = (child_scope, route)
+                if full is None or specificity > full[2]:
+                    full = (child_scope, route, specificity)
             elif match == Match.PARTIAL:
-                if partial is None or _frontend_path_specificity(
-                    route.path
-                ) > _frontend_path_specificity(partial[1].path):
-                    partial = (child_scope, route)
+                if partial is None or specificity > partial[2]:
+                    partial = (child_scope, route, specificity)
         if full is not None:
-            child_scope, route = full
+            child_scope, route, _ = full
             return Match.FULL, child_scope, route
         if partial is not None:
-            child_scope, route = partial
+            child_scope, route, _ = partial
             return Match.PARTIAL, child_scope, route
         return Match.NONE, {}, None
 
     async def handle(self, scope: Scope, receive: Receive, send: Send) -> None:
-        match, child_scope, route = self._match(scope)
+        effective_context = _get_scope_effective_route_context(scope)
+        if (
+            isinstance(effective_context, _EffectiveRouteContext)
+            and effective_context.original_route is self
+        ):
+            prefix = effective_context.frontend_prefix
+            dependant = effective_context.dependant
+            dependency_overrides_provider = (
+                effective_context.dependency_overrides_provider
+            )
+            embed_body_fields = effective_context._embed_body_fields
+        else:
+            prefix = ""
+            dependant = self.dependant
+            dependency_overrides_provider = self.dependency_overrides_provider
+            embed_body_fields = self._embed_body_fields
+        match, child_scope, route = self._match(scope, prefix=prefix)
         if match == Match.NONE or route is None:
             raise HTTPException(status_code=404)
         _update_scope(scope, child_scope)
+        if match == Match.FULL and dependant and dependant.dependencies:
+            async with self._solve_dependencies(
+                scope,
+                receive,
+                send,
+                dependant=dependant,
+                dependency_overrides_provider=dependency_overrides_provider,
+                embed_body_fields=embed_body_fields,
+            ):
+                await route.handle(scope, receive, send)
+            return
         await route.handle(scope, receive, send)
 
     def url_path_for(self, name: str, /, **path_params: Any) -> URLPath:
         raise NoMatchFound(name, path_params)
 
+    # TODO: probably move this out of the Route / Route Group, same in APIRoute
+    # this should probably be top level FastAPI logic, not part of APIRoute and
+    # duplicated here
+    @asynccontextmanager
+    async def _solve_dependencies(
+        self,
+        scope: Scope,
+        receive: Receive,
+        send: Send,
+        *,
+        dependant: Dependant,
+        dependency_overrides_provider: Any | None,
+        embed_body_fields: bool,
+    ) -> AsyncIterator[None]:
+        request = Request(scope, receive, send)
+        previous_inner_astack = scope.get("fastapi_inner_astack", _SCOPE_MISSING)
+        previous_function_astack = scope.get("fastapi_function_astack", _SCOPE_MISSING)
+        try:
+            async with AsyncExitStack() as request_stack:
+                scope["fastapi_inner_astack"] = request_stack
+                async with AsyncExitStack() as function_stack:
+                    scope["fastapi_function_astack"] = function_stack
+                    solved_result = await solve_dependencies(
+                        request=request,
+                        dependant=dependant,
+                        dependency_overrides_provider=dependency_overrides_provider,
+                        async_exit_stack=request_stack,
+                        embed_body_fields=embed_body_fields,
+                    )
+                    if solved_result.errors:
+                        raise RequestValidationError(solved_result.errors)
+                    yield
+        finally:
+            if previous_inner_astack is _SCOPE_MISSING:
+                scope.pop("fastapi_inner_astack", None)
+            else:
+                scope["fastapi_inner_astack"] = previous_inner_astack
+            if previous_function_astack is _SCOPE_MISSING:
+                scope.pop("fastapi_function_astack", None)
+            else:
+                scope["fastapi_function_astack"] = previous_function_astack
+
 
 class APIRouter(routing.Router):
     """
@@ -2515,7 +2652,10 @@ class APIRouter(routing.Router):
         """
         normalized_path = _normalize_frontend_path(path)
         if self._frontend_routes is None:
-            self._frontend_routes = _FrontendRouteGroup()
+            self._frontend_routes = _FrontendRouteGroup(
+                dependencies=self.dependencies,
+                dependency_overrides_provider=self.dependency_overrides_provider,
+            )
             self._low_priority_routes.append(self._frontend_routes)
         self._frontend_routes.add_frontend_route(
             _join_frontend_paths(self.prefix, normalized_path),
@@ -2650,10 +2790,14 @@ class APIRouter(routing.Router):
                 match, child_scope = candidate.matches(scope)
                 route = candidate
             if match == Match.FULL:
-                if full is None:
+                if full is None or self._frontend_match_is_more_specific(
+                    child_scope, full[0]
+                ):
                     full = (child_scope, route, route_context)
             elif match == Match.PARTIAL:
-                if partial is None:
+                if partial is None or self._frontend_match_is_more_specific(
+                    child_scope, partial[0]
+                ):
                     partial = (child_scope, route, route_context)
         if full is not None:
             child_scope, route, route_context = full
@@ -2663,6 +2807,15 @@ class APIRouter(routing.Router):
             return Match.PARTIAL, child_scope, route, route_context
         return Match.NONE, {}, None, None
 
+    def _frontend_match_is_more_specific(
+        self, child_scope: Scope, previous_child_scope: Scope
+    ) -> bool:
+        specificity = _frontend_scope_specificity(child_scope)
+        previous_specificity = _frontend_scope_specificity(previous_child_scope)
+        if specificity is None or previous_specificity is None:
+            return False
+        return specificity > previous_specificity
+
     def route(
         self,
         path: str,

tests/test_frontend.py

@@ -1,12 +1,13 @@
 import errno
 import os
 import runpy
+from contextlib import AsyncExitStack
 from pathlib import Path
 from typing import Literal
 
 import anyio
 import pytest
-from fastapi import APIRouter, FastAPI, HTTPException, Request, WebSocket
+from fastapi import APIRouter, Depends, FastAPI, HTTPException, Request, WebSocket
 from fastapi.testclient import TestClient
 from starlette.exceptions import HTTPException as StarletteHTTPException
 from starlette.responses import PlainTextResponse, Response
@@ -18,11 +19,18 @@ def write_file(path: Path, content: str) -> None:
     path.write_text(content)
 
 
+def record_dependency(calls: list[str], name: str):
+    def dependency() -> None:
+        calls.append(name)
+
+    return dependency
+
+
 def test_frontend_exact_prefix_path_serves_index(tmp_path: Path):
     dist = tmp_path / "dist"
     write_file(dist / "index.html", "app")
     app = FastAPI()
-    app.frontend("/app", directory=dist)
+    app.frontend("/", directory=dist)
 
     response = TestClient(app).get("/app")
 
@@ -109,7 +117,7 @@ def test_frontend_route_group_helpers(tmp_path: Path):
     dist = tmp_path / "dist"
     write_file(dist / "index.html", "app")
     app = FastAPI()
-    app.frontend("/", directory=dist)
+    app.frontend("/app", directory=dist)
     route_group = app.router._frontend_routes
     assert route_group is not None
 
@@ -117,9 +125,22 @@ def test_frontend_route_group_helpers(tmp_path: Path):
     assert match == Match.NONE
     assert child_scope == {}
 
+    match, child_scope = route_group.matches_with_prefix(
+        {"type": "http", "path": "/prefix/app", "method": "GET"},
+        "/prefix",
+    )
+    assert match == Match.FULL
+    assert child_scope["fastapi"]["frontend_path"] == ""
+
+    match, child_scope = route_group.routes[0].matches(
+        {"type": "http", "path": "/app", "method": "GET"}
+    )
+    assert match == Match.FULL
+    assert child_scope["fastapi"]["frontend_path"] == ""
+
     with pytest.raises(StarletteHTTPException) as exc_info:
         anyio.run(
-            route_group.with_prefix("/app").handle,
+            route_group.handle,
             {"type": "http", "path": "/missing", "method": "GET"},
             None,
             None,
@@ -246,6 +267,322 @@ def test_basic_file_serving(tmp_path: Path):
     assert "last-modified" in response.headers
 
 
+def test_app_frontend_dependencies_protect_root_asset_and_fallback(tmp_path: Path):
+    calls: list[str] = []
+
+    def require_cookie(request: Request) -> None:
+        calls.append(request.url.path)
+        if request.cookies.get("session") != "ok":
+            raise HTTPException(status_code=401)
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    write_file(dist / "assets" / "app.js", "console.log('ok')")
+    app = FastAPI(dependencies=[Depends(require_cookie)])
+    app.frontend("/", directory=dist, fallback="index.html")
+    client = TestClient(app)
+
+    response = client.get("/")
+    assert response.status_code == 401
+
+    response = client.get("/", headers={"cookie": "session=ok"})
+    assert response.status_code == 200
+    assert response.text == "app"
+
+    response = client.get("/assets/app.js", headers={"cookie": "session=ok"})
+    assert response.status_code == 200
+    assert response.text == "console.log('ok')"
+
+    response = client.get(
+        "/dashboard",
+        headers={"accept": "text/html", "cookie": "session=ok"},
+    )
+    assert response.status_code == 200
+    assert response.text == "app"
+    assert calls == ["/", "/", "/assets/app.js", "/dashboard"]
+
+
+def test_apirouter_frontend_dependencies_protect_prefixed_frontend(tmp_path: Path):
+    def require_cookie(request: Request) -> None:
+        if request.cookies.get("session") != "ok":
+            raise HTTPException(status_code=401)
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    write_file(dist / "assets" / "app.js", "console.log('ok')")
+    router = APIRouter(dependencies=[Depends(require_cookie)])
+    router.frontend("/", directory=dist, fallback="index.html")
+    app = FastAPI()
+    app.include_router(router, prefix="/app")
+    client = TestClient(app)
+
+    response = client.get("/app/")
+    assert response.status_code == 401
+
+    response = client.get("/app/", headers={"cookie": "session=ok"})
+    assert response.status_code == 200
+    assert response.text == "app"
+
+    response = client.get("/app/assets/app.js", headers={"cookie": "session=ok"})
+    assert response.status_code == 200
+    assert response.text == "console.log('ok')"
+
+    response = client.get(
+        "/app/dashboard",
+        headers={"accept": "text/html", "cookie": "session=ok"},
+    )
+    assert response.status_code == 200
+    assert response.text == "app"
+
+
+def test_included_frontend_does_not_block_url_path_for(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    frontend_router = APIRouter()
+    frontend_router.frontend("/", directory=dist)
+    api_router = APIRouter()
+
+    @api_router.get("/api", name="read_api")
+    def read_api():
+        return {"ok": True}
+
+    app = FastAPI()
+    app.include_router(frontend_router, prefix="/app")
+    app.include_router(api_router)
+    included_frontend = next(
+        route
+        for route in app.router.routes
+        if hasattr(route, "effective_low_priority_routes")
+    )
+
+    with pytest.raises(NoMatchFound):
+        included_frontend.url_path_for("missing")
+    assert app.url_path_for("read_api") == "/api"
+    response = TestClient(app).get("/api")
+    assert response.status_code == 200
+    assert response.json() == {"ok": True}
+
+
+def test_include_router_frontend_dependencies_apply_in_nested_order(tmp_path: Path):
+    calls: list[str] = []
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    child = APIRouter(dependencies=[Depends(record_dependency(calls, "child"))])
+    child.frontend("/ui", directory=dist)
+    parent = APIRouter(dependencies=[Depends(record_dependency(calls, "parent"))])
+    parent.include_router(
+        child,
+        prefix="/child",
+        dependencies=[Depends(record_dependency(calls, "parent-include"))],
+    )
+    app = FastAPI(dependencies=[Depends(record_dependency(calls, "app"))])
+    app.include_router(
+        parent,
+        prefix="/parent",
+        dependencies=[Depends(record_dependency(calls, "app-include"))],
+    )
+
+    response = TestClient(app).get("/parent/child/ui/")
+
+    assert response.status_code == 200
+    assert response.text == "app"
+    assert calls == ["app", "app-include", "parent", "parent-include", "child"]
+
+
+def test_frontend_dependency_overrides_apply(tmp_path: Path):
+    calls: list[str] = []
+
+    def require_cookie() -> None:
+        raise HTTPException(status_code=401)  # pragma: no cover
+
+    def allow_cookie() -> None:
+        calls.append("override")
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI(dependencies=[Depends(require_cookie)])
+    app.dependency_overrides[require_cookie] = allow_cookie
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/")
+
+    assert response.status_code == 200
+    assert response.text == "app"
+    assert calls == ["override"]
+
+
+def test_frontend_dependencies_do_not_run_when_api_route_wins(tmp_path: Path):
+    calls: list[str] = []
+
+    def frontend_dependency() -> None:
+        calls.append("frontend")  # pragma: no cover
+
+    dist = tmp_path / "dist"
+    write_file(dist / "api", "frontend")
+    router = APIRouter(dependencies=[Depends(frontend_dependency)])
+    router.frontend("/", directory=dist)
+    app = FastAPI()
+
+    @app.get("/api")
+    def read_api():
+        return {"source": "api"}
+
+    app.include_router(router)
+
+    response = TestClient(app).get("/api")
+
+    assert response.status_code == 200
+    assert response.json() == {"source": "api"}
+    assert calls == []
+
+
+def test_only_selected_frontend_mount_dependencies_run(tmp_path: Path):
+    calls: list[str] = []
+
+    site = tmp_path / "site"
+    admin = tmp_path / "admin"
+    write_file(site / "index.html", "site")
+    write_file(admin / "index.html", "admin")
+    site_router = APIRouter()
+    site_router.frontend("/", directory=site)
+    admin_router = APIRouter()
+    admin_router.frontend("/", directory=admin)
+    app = FastAPI()
+    app.include_router(
+        site_router, dependencies=[Depends(record_dependency(calls, "site"))]
+    )
+    app.include_router(
+        admin_router,
+        prefix="/admin",
+        dependencies=[Depends(record_dependency(calls, "admin"))],
+    )
+
+    response = TestClient(app).get("/admin/")
+
+    assert response.status_code == 200
+    assert response.text == "admin"
+    assert calls == ["admin"]
+
+
+def test_app_middleware_still_runs_for_frontend_dependencies(tmp_path: Path):
+    calls: list[str] = []
+
+    def frontend_dependency() -> None:
+        calls.append("dependency")
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI(dependencies=[Depends(frontend_dependency)])
+
+    @app.middleware("http")
+    async def record_middleware(request: Request, call_next):
+        calls.append("middleware-before")
+        response = await call_next(request)
+        calls.append("middleware-after")
+        return response
+
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/")
+
+    assert response.status_code == 200
+    assert response.text == "app"
+    assert calls == ["middleware-before", "dependency", "middleware-after"]
+
+
+def test_frontend_dependency_validation_errors_return_422(tmp_path: Path):
+    def require_token(token: str) -> None:
+        pass  # pragma: no cover
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI(dependencies=[Depends(require_token)])
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/")
+
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["query", "token"],
+                "msg": "Field required",
+                "input": None,
+            }
+        ]
+    }
+
+
+@pytest.mark.anyio
+async def test_frontend_dependency_restores_existing_dependency_stacks(
+    tmp_path: Path,
+):
+    def frontend_dependency() -> None:
+        pass
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI(dependencies=[Depends(frontend_dependency)])
+    app.frontend("/", directory=dist)
+    assert app.router._frontend_routes is not None
+    inner_stack = AsyncExitStack()
+    function_stack = AsyncExitStack()
+    scope = {
+        "type": "http",
+        "http_version": "1.1",
+        "method": "GET",
+        "scheme": "http",
+        "path": "/",
+        "root_path": "",
+        "query_string": b"",
+        "headers": [],
+        "client": ("testclient", 50000),
+        "server": ("testserver", 80),
+        "fastapi_inner_astack": inner_stack,
+        "fastapi_function_astack": function_stack,
+    }
+    messages = []
+
+    async def receive():
+        return {  # pragma: no cover
+            "type": "http.request",
+            "body": b"",
+            "more_body": False,
+        }
+
+    async def send(message):
+        messages.append(message)
+
+    async with inner_stack, function_stack:
+        await app.router._frontend_routes.handle(scope, receive, send)
+
+    assert scope["fastapi_inner_astack"] is inner_stack
+    assert scope["fastapi_function_astack"] is function_stack
+    assert messages[0]["type"] == "http.response.start"
+    assert messages[0]["status"] == 200
+
+
+def test_non_frontend_low_priority_route_keeps_order_before_frontend(
+    tmp_path: Path,
+):
+    async def low_priority_endpoint(request: Request):
+        return PlainTextResponse("low")
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "frontend")
+    app = FastAPI()
+    app.router._low_priority_routes.append(Route("/admin", low_priority_endpoint))
+    app.router._mark_routes_changed()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/admin")
+
+    assert response.status_code == 200
+    assert response.text == "low"
+
+
 def test_existing_api_route_wins_over_frontend(tmp_path: Path):
     dist = tmp_path / "dist"
     write_file(dist / "api" / "users", "frontend")