Address review feedback: plain text errors, configurable on_error, perf docs

- Changed default error response from JSON to plain text (browser-friendly)
- Added optional `on_error` callback for custom error responses (redirect
  to login page, HTML error pages, etc.)
- Added performance note about keeping auth checks lightweight
- Added tests for redirect and HTML custom error responses

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

fastapi/staticfiles.py

@@ -2,7 +2,7 @@ from collections.abc import Awaitable, Callable
 from typing import Any
 
 from starlette.requests import Request
-from starlette.responses import JSONResponse
+from starlette.responses import PlainTextResponse, Response
 from starlette.staticfiles import StaticFiles as StaticFiles  # noqa
 from starlette.types import Receive, Scope, Send
 
@@ -45,12 +45,23 @@ class AuthStaticFiles(StaticFiles):
     * `auth`: An async callable that takes a `Request` object and performs
       authentication. It should raise an `HTTPException` if authentication
       fails, or return `None` if authentication succeeds.
+    * `on_error`: An optional callable that takes a `Request` and an
+      `HTTPException` and returns a `Response`. Use this to customize
+      error responses (e.g., redirect to login, return HTML instead of
+      plain text). If not provided, a plain text error response is returned.
     * `directory`: The directory to serve files from.
     * `packages`: A list of Python packages to serve files from.
     * `html`: If `True`, serves `index.html` files for directories.
     * `check_dir`: If `True`, checks that the directory exists on startup.
     * `follow_symlink`: If `True`, follows symbolic links.
 
+    ## Performance Note
+
+    The `auth` callable runs on **every static file request** (CSS, JS,
+    images, etc.). Prefer lightweight checks (header presence, JWT signature
+    verification) over expensive operations (database lookups) to avoid
+    slowing down page loads.
+
     Ref: https://github.com/fastapi/fastapi/issues/858
     """
 
@@ -63,6 +74,7 @@ class AuthStaticFiles(StaticFiles):
         check_dir: bool = True,
         follow_symlink: bool = False,
         auth: Callable[[Request], Awaitable[Any]],
+        on_error: Callable[[Request, Any], Awaitable[Response]] | None = None,
     ) -> None:
         super().__init__(
             directory=directory,
@@ -72,6 +84,7 @@ class AuthStaticFiles(StaticFiles):
             follow_symlink=follow_symlink,
         )
         self.auth = auth
+        self.on_error = on_error
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         if scope["type"] == "http":
@@ -82,11 +95,14 @@ class AuthStaticFiles(StaticFiles):
                 from fastapi.exceptions import HTTPException
 
                 if isinstance(exc, HTTPException):
-                    response = JSONResponse(
-                        {"detail": exc.detail},
-                        status_code=exc.status_code,
-                        headers=getattr(exc, "headers", None),
-                    )
+                    if self.on_error is not None:
+                        response = await self.on_error(request, exc)
+                    else:
+                        response = PlainTextResponse(
+                            str(exc.detail),
+                            status_code=exc.status_code,
+                            headers=getattr(exc, "headers", None),
+                        )
                     await response(scope, receive, send)
                     return
                 raise

tests/test_auth_static_files.py

@@ -1,7 +1,9 @@
 import pytest
 from fastapi import FastAPI, HTTPException, Request
+from fastapi.responses import RedirectResponse
 from fastapi.staticfiles import AuthStaticFiles
 from fastapi.testclient import TestClient
+from starlette.responses import HTMLResponse, Response
 
 
 @pytest.fixture(scope="module")
@@ -19,6 +21,11 @@ async def verify_token(request: Request) -> None:
         raise HTTPException(status_code=401, detail="Not authenticated")
 
 
+async def _allow_all(request: Request) -> None:
+    """Auth function that allows all requests."""
+    pass
+
+
 @pytest.fixture(scope="module")
 def app(static_dir):
     app = FastAPI()
@@ -46,11 +53,6 @@ def app(static_dir):
     return app
 
 
-async def _allow_all(request: Request) -> None:
-    """Auth function that allows all requests."""
-    pass
-
-
 @pytest.fixture(scope="module")
 def client(app):
     with TestClient(app) as c:
@@ -61,7 +63,7 @@ def test_private_file_without_auth(client: TestClient):
     """Requesting a private file without auth should return 401."""
     response = client.get("/private/secret.txt")
     assert response.status_code == 401
-    assert response.json() == {"detail": "Not authenticated"}
+    assert response.text == "Not authenticated"
 
 
 def test_private_file_with_wrong_token(client: TestClient):
@@ -71,7 +73,7 @@ def test_private_file_with_wrong_token(client: TestClient):
         headers={"Authorization": "Bearer wrong-token"},
     )
     assert response.status_code == 401
-    assert response.json() == {"detail": "Not authenticated"}
+    assert response.text == "Not authenticated"
 
 
 def test_private_file_with_valid_token(client: TestClient):
@@ -121,7 +123,7 @@ def test_auth_headers_forwarded(static_dir):
         response = client.get("/protected/public.txt")
         assert response.status_code == 401
         assert response.headers["WWW-Authenticate"] == "Bearer"
-        assert response.json() == {"detail": "Login required"}
+        assert response.text == "Login required"
 
 
 def test_cookie_based_auth(static_dir):
@@ -149,3 +151,58 @@ def test_cookie_based_auth(static_dir):
         response = client.get("/dashboard/public.txt")
         assert response.status_code == 200
         assert response.text == "public content"
+
+
+def test_custom_on_error_redirect(static_dir):
+    """on_error can redirect to a login page."""
+
+    async def deny_all(request: Request) -> None:
+        raise HTTPException(status_code=401, detail="Unauthorized")
+
+    async def redirect_to_login(request: Request, exc: HTTPException) -> Response:
+        return RedirectResponse(url="/login", status_code=302)
+
+    app = FastAPI()
+    app.mount(
+        "/protected",
+        AuthStaticFiles(
+            directory=str(static_dir),
+            auth=deny_all,
+            on_error=redirect_to_login,
+        ),
+        name="protected",
+    )
+
+    with TestClient(app, follow_redirects=False) as client:
+        response = client.get("/protected/public.txt")
+        assert response.status_code == 302
+        assert response.headers["location"] == "/login"
+
+
+def test_custom_on_error_html(static_dir):
+    """on_error can return a custom HTML error page."""
+
+    async def deny_all(request: Request) -> None:
+        raise HTTPException(status_code=403, detail="Forbidden")
+
+    async def html_error(request: Request, exc: HTTPException) -> Response:
+        return HTMLResponse(
+            f"<h1>{exc.status_code} {exc.detail}</h1>",
+            status_code=exc.status_code,
+        )
+
+    app = FastAPI()
+    app.mount(
+        "/protected",
+        AuthStaticFiles(
+            directory=str(static_dir),
+            auth=deny_all,
+            on_error=html_error,
+        ),
+        name="protected",
+    )
+
+    with TestClient(app) as client:
+        response = client.get("/protected/public.txt")
+        assert response.status_code == 403
+        assert "<h1>403 Forbidden</h1>" in response.text