🔖 Release version 0.138.0 (#15808)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

.github/workflows/test.yml

@@ -245,9 +245,10 @@ jobs:
       - run: uv run coverage report --fail-under=100
 
   # https://github.com/marketplace/actions/alls-green#why
-  check:  # This job does nothing and is only used for the branch protection
+  test-alls-green:  # This job does nothing and is only used for the branch protection
     if: always()
     needs:
+      - test
       - coverage-combine
       - benchmark
     runs-on: ubuntu-latest

docs/en/docs/reference/apirouter.md

@@ -13,6 +13,7 @@ from fastapi import APIRouter
         members:
             - websocket
             - include_router
+            - frontend
             - get
             - put
             - post

docs/en/docs/reference/fastapi.md

@@ -18,6 +18,7 @@ from fastapi import FastAPI
             - openapi
             - websocket
             - include_router
+            - frontend
             - get
             - put
             - post

docs/en/docs/release-notes.md

@@ -7,8 +7,19 @@ hide:
 
 ## Latest Changes
 
+## 0.138.0 (2026-06-20)
+
+### Features
+
+* ✨ Add support for `app.frontend("/", directory="dist")` and `router.frontend("/", directory="dist")`. PR [#15800](https://github.com/fastapi/fastapi/pull/15800) by [@tiangolo](https://github.com/tiangolo).
+    * Read the docs: [Frontend](https://fastapi.tiangolo.com/tutorial/frontend/).
+
 ### Docs
 
+* 📝 Fix typo in release notes. PR [#15807](https://github.com/fastapi/fastapi/pull/15807) by [@tiangolo](https://github.com/tiangolo).
+* 📝 Add `app.frontend()` instructions to Agent Library Skill. PR [#15805](https://github.com/fastapi/fastapi/pull/15805) by [@tiangolo](https://github.com/tiangolo).
+* 📝 Update release notes link. PR [#15802](https://github.com/fastapi/fastapi/pull/15802) by [@tiangolo](https://github.com/tiangolo).
+* ✏️ Update white space characters in bigger apps. PR [#15801](https://github.com/fastapi/fastapi/pull/15801) by [@tiangolo](https://github.com/tiangolo).
 * ✏️ Fix grammar, typos, and broken links in docs. PR [#15694](https://github.com/fastapi/fastapi/pull/15694) by [@YuriiMotov](https://github.com/YuriiMotov).
 
 ### Translations
@@ -17,6 +28,8 @@ hide:
 
 ### Internal
 
+* 🐛 Fix failing test, update format for raised errors. PR [#15804](https://github.com/fastapi/fastapi/pull/15804) by [@tiangolo](https://github.com/tiangolo).
+* 👷 Fix test-alls-green. PR [#15803](https://github.com/fastapi/fastapi/pull/15803) by [@tiangolo](https://github.com/tiangolo).
 * 🔧 Enable checking `release-notes.md` for typos. PR [#15796](https://github.com/fastapi/fastapi/pull/15796) by [@YuriiMotov](https://github.com/YuriiMotov).
 * 📝 Tweak wording about deploying to FastAPI Cloud. PR [#15793](https://github.com/fastapi/fastapi/pull/15793) by [@tiangolo](https://github.com/tiangolo).
 * 🔨 Use `gpt-5.5` model in `translate.py`, specify `-chat` to avoid warnings. PR [#15792](https://github.com/fastapi/fastapi/pull/15792) by [@YuriiMotov](https://github.com/YuriiMotov).

docs/en/docs/tutorial/bigger-applications.md

@@ -17,16 +17,16 @@ Let's say you have a file structure like this:
 ```
 .
 ├── app
-│   ├── __init__.py
-│   ├── main.py
-│   ├── dependencies.py
-│   └── routers
-│   │   ├── __init__.py
-│   │   ├── items.py
-│   │   └── users.py
-│   └── internal
-│       ├── __init__.py
-│       └── admin.py
+│   ├── __init__.py
+│   ├── main.py
+│   ├── dependencies.py
+│   └── routers
+│   │   ├── __init__.py
+│   │   ├── items.py
+│   │   └── users.py
+│   └── internal
+│       ├── __init__.py
+│       └── admin.py
 ```
 
 /// tip

docs/en/docs/tutorial/frontend.md

@@ -0,0 +1,131 @@
+# Frontend { #frontend }
+
+You can serve static frontend apps with `app.frontend()` (or `router.frontend()`).
+
+This is useful for frontend tools that generate static files, like React with Vite, TanStack Router, Astro, Vue, Svelte, Angular, Solid, and others.
+
+With these tools, you normally have a step that builds the frontend, with a command like:
+
+```bash
+npm run build
+```
+
+That would generate a directory like `./dist/` with your frontend files.
+
+You can use `app.frontend()` to serve that directory following the conventions needed by these frontend frameworks.
+
+**FastAPI** checks *path operations* first. The frontend files are checked only if no normal route matched, so your API won't be affected.
+
+## Serve a Frontend { #serve-a-frontend }
+
+After building your frontend, for example with `npm run build`, put the generated files in a directory, for example, `dist`.
+
+Your project structure could look like this:
+
+```text
+.
+├── pyproject.toml
+├── app
+│   ├── __init__.py
+│   └── main.py
+└── dist
+    ├── index.html
+    └── assets
+        └── app.js
+```
+
+Then serve it with `app.frontend()`:
+
+{* ../../docs_src/frontend/tutorial001_py310.py hl[5] *}
+
+With this, a request for `/assets/app.js` can serve `dist/assets/app.js`.
+
+If you also have a **FastAPI** *path operation*, the *path operation* wins.
+
+## Client-Side Routing { #client-side-routing }
+
+Many frontend apps, including **single-page apps** (SPAs), use client-side routing. A path like `/dashboard/settings` might not be a real file but the framework would take care of handling it.
+
+So, if accessing that URL directly (instead of navigating through the app), the backend should serve the frontend app from `index.html`, so that the frontend framework can then handle the client-side routing.
+
+For that, use `fallback="index.html"`:
+
+{* ../../docs_src/frontend/tutorial002_py310.py hl[5] *}
+
+**FastAPI** uses this fallback only for requests that look like browser navigation. Missing files like JavaScript, CSS, and images still return `404`.
+
+/// tip
+
+By default, `fallback` has a value of `fallback="auto"`. In most cases you won't need to specify `fallback`. Read below for details.
+
+///
+
+This is what you would want with many frontend apps that use client-side routing, for example, React with TanStack Router, Vue, Angular, SvelteKit, or Solid.
+
+## Custom 404 Page { #custom-404-page }
+
+You can also serve a static `404.html` page for missing frontend paths:
+
+{* ../../docs_src/frontend/tutorial003_py310.py hl[5] *}
+
+That response keeps a status code of `404`.
+
+In this case, **FastAPI** won't serve `index.html` for missing frontend paths. It will return the `404.html` file instead.
+
+/// tip
+
+By default, `fallback` has a value of `fallback="auto"`. With this, if a `404.html` file is found, it will be used as the fallback automatically.
+
+So, you can normally omit the `fallback` argument.
+
+///
+
+This is useful with frontend tools that generate static HTML files for each page, like Astro.
+
+## Fallback Auto { #fallback-auto }
+
+By default, `app.frontend()` uses `fallback="auto"`.
+
+If there is a `404.html` file in the frontend directory, missing frontend paths serve that file with status code `404`.
+
+Otherwise, if there is an `index.html` file, missing browser navigation paths serve `index.html`, which is what many frontend apps with client-side routing expect.
+
+So, in most cases you can use `app.frontend("/", directory="dist")` without specifying the `fallback` argument.
+
+{* ../../docs_src/frontend/tutorial001_py310.py hl[5] *}
+
+## Disable Fallback { #disable-fallback }
+
+If you don't want to serve a fallback file for missing frontend paths, use `fallback=None`:
+
+{* ../../docs_src/frontend/tutorial005_py310.py hl[5] *}
+
+Then missing frontend paths return the normal `404`.
+
+## Check Directory { #check-directory }
+
+By default, `app.frontend()` checks that the directory exists when the app is created.
+
+This helps catch configuration errors early. For example, if the frontend build output directory is missing, **FastAPI** will raise an error on startup.
+
+If your frontend files are created later, for example by a separate build step after the app object is created, set `check_dir=False`:
+
+{* ../../docs_src/frontend/tutorial006_py310.py hl[5] *}
+
+With `check_dir=False`, **FastAPI** will not check the directory when the app is created. If the configured directory is still missing when a request is handled, **FastAPI** will raise an error then.
+
+## Use it with `APIRouter` { #use-it-with-apirouter }
+
+You can also add frontend files to an `APIRouter` and include it with a prefix:
+
+{* ../../docs_src/frontend/tutorial004_py310.py hl[6,7] *}
+
+In this example, frontend paths are served under `/app`.
+
+Any regular *path operations* in the app will still take precedence, including in other routers.
+
+## Static Build Output Only { #static-build-output-only }
+
+`app.frontend()` serves files already generated by your frontend build.
+
+It does not run server-side rendering. It is for frontend frameworks that generate static files, not for frameworks that need dynamic rendering on the server for each request.

docs/en/docs/tutorial/static-files.md

@@ -2,6 +2,14 @@
 
 You can serve static files automatically from a directory using `StaticFiles`.
 
+/// tip
+
+If you need to host a frontend, use `app.frontend()` instead, read about it in [Frontend](frontend.md).
+
+`app.frontend()` uses `StaticFiles` underneath, with several additional advantages for frontends, like handling client-side routing.
+
+///
+
 ## Use `StaticFiles` { #use-staticfiles }
 
 * Import `StaticFiles`.

docs/en/mkdocs.yml

@@ -133,6 +133,7 @@ nav:
     - tutorial/server-sent-events.md
     - tutorial/background-tasks.md
     - tutorial/metadata.md
+    - tutorial/frontend.md
     - tutorial/static-files.md
     - tutorial/testing.md
     - tutorial/debugging.md

docs_src/frontend/tutorial001_py310.py

@@ -0,0 +1,5 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+app.frontend("/", directory="dist")

docs_src/frontend/tutorial002_py310.py

@@ -0,0 +1,5 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+app.frontend("/", directory="dist", fallback="index.html")

docs_src/frontend/tutorial003_py310.py

@@ -0,0 +1,5 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+app.frontend("/", directory="dist", fallback="404.html")

docs_src/frontend/tutorial004_py310.py

@@ -0,0 +1,7 @@
+from fastapi import APIRouter, FastAPI
+
+app = FastAPI()
+router = APIRouter()
+
+router.frontend("/", directory="dist", fallback="index.html")
+app.include_router(router, prefix="/app")

docs_src/frontend/tutorial005_py310.py

@@ -0,0 +1,5 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+app.frontend("/", directory="dist", fallback=None)

docs_src/frontend/tutorial006_py310.py

@@ -0,0 +1,5 @@
+from fastapi import FastAPI
+
+app = FastAPI()
+
+app.frontend("/", directory="dist", check_dir=False)

fastapi/.agents/skills/fastapi/SKILL.md

@@ -288,6 +288,32 @@ There could be exceptions, but try to follow this convention.
 
 Apply shared dependencies at the router level via `dependencies=[Depends(...)]`.
 
+## Serve Frontend Apps
+
+Use `app.frontend()` to serve a built static frontend app, for example a directory generated by Vite, Astro, Angular, Svelte, Vue, or a similar tool.
+
+```python
+from fastapi import FastAPI
+
+app = FastAPI()
+
+app.frontend("/", directory="dist")
+```
+
+Use `router.frontend()` when the frontend belongs to an `APIRouter`; normal router prefix behavior applies when the router is included.
+
+```python
+from fastapi import APIRouter, FastAPI
+
+app = FastAPI()
+router = APIRouter(prefix="/admin")
+
+router.frontend("/", directory="admin-dist")
+app.include_router(router)
+```
+
+`app.frontend()` and `router.frontend()` are low-priority routes: regular API routes are matched first, then frontend files and client-side routing fallbacks. Use this for single-page apps and built frontend assets instead of mounting `StaticFiles` manually.
+
 ## Dependency Injection
 
 See [the dependency injection reference](references/dependencies.md) for detailed patterns including `yield` with `scope`, and class dependencies.

fastapi/__init__.py

@@ -1,6 +1,6 @@
 """FastAPI framework, high performance, easy to learn, fast to code, ready for production"""
 
-__version__ = "0.137.2"
+__version__ = "0.138.0"
 
 from starlette import status as status
 

fastapi/applications.py

@@ -1,6 +1,7 @@
+import os
 from collections.abc import Awaitable, Callable, Coroutine, Sequence
 from enum import Enum
-from typing import Annotated, Any, TypeVar
+from typing import Annotated, Any, Literal, TypeVar
 
 from annotated_doc import Doc
 from fastapi import routing
@@ -1218,6 +1219,79 @@ class FastAPI(Starlette):
             generate_unique_id_function=generate_unique_id_function,
         )
 
+    def frontend(
+        self,
+        path: Annotated[
+            str,
+            Doc(
+                """
+                The URL path prefix where the frontend build should be served.
+                """
+            ),
+        ],
+        *,
+        directory: Annotated[
+            str | os.PathLike[str],
+            Doc(
+                """
+                The directory containing the static frontend build output.
+                """
+            ),
+        ],
+        fallback: Annotated[
+            Literal["auto", "index.html", "404.html"] | None,
+            Doc(
+                """
+                The fallback file behavior for missing frontend paths.
+                """
+            ),
+        ] = "auto",
+        check_dir: Annotated[
+            bool,
+            Doc(
+                """
+                Check that the frontend directory exists when the app is created.
+                """
+            ),
+        ] = True,
+    ) -> None:
+        """
+        Serve a static frontend build as low-priority routes.
+
+        Use this for frontend tools that build static files into a directory,
+        such as `dist`. **FastAPI** path operations are checked first, and
+        the frontend files are checked only if no normal route matched.
+
+        A typical project could look like this:
+
+        ```text
+        .
+        ├── pyproject.toml
+        ├── app
+        │   ├── __init__.py
+        │   └── main.py
+        └── dist
+            ├── index.html
+            └── assets
+                └── app.js
+        ```
+
+        Then in `app/main.py`:
+
+        ```python
+        from fastapi import FastAPI
+
+        app = FastAPI()
+        app.frontend("/", directory="dist")
+        ```
+        """
+        self.router.frontend(
+            path,
+            directory=directory,
+            fallback=fallback,
+            check_dir=check_dir,
+        )
+
     def api_route(
         self,
         path: str,

fastapi/routing.py

@@ -1,9 +1,12 @@
 import contextlib
 import copy
 import email.message
+import errno
 import functools
 import inspect
 import json
+import os
+import stat
 import types
 from collections.abc import (
     AsyncIterator,
@@ -28,6 +31,7 @@ from enum import Enum, IntEnum
 from typing import (
     Annotated,
     Any,
+    Literal,
     Protocol,
     TypeVar,
     cast,
@@ -80,22 +84,25 @@ from starlette import routing
 from starlette._exception_handler import wrap_app_handling_exceptions
 from starlette._utils import get_route_path, is_async_callable
 from starlette.concurrency import iterate_in_threadpool, run_in_threadpool
-from starlette.datastructures import FormData, URLPath
+from starlette.datastructures import URL, FormData, URLPath
 from starlette.exceptions import HTTPException
 from starlette.requests import Request
 from starlette.responses import (
     JSONResponse,
     PlainTextResponse,
+    RedirectResponse,
     Response,
     StreamingResponse,
 )
 from starlette.routing import (
     BaseRoute,
     Match,
+    NoMatchFound,
     compile_path,
     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
@@ -819,6 +826,7 @@ class APIWebSocketRoute(routing.WebSocketRoute):
 
 _FASTAPI_SCOPE_KEY = "fastapi"
 _FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY = "effective_route_context"
+_FASTAPI_FRONTEND_PATH_KEY = "frontend_path"
 _FASTAPI_INCLUDED_ROUTER_KEY = "included_router"
 _effective_route_context_var: ContextVar[Any | None] = ContextVar(
     "fastapi_effective_route_context", default=None
@@ -826,12 +834,25 @@ _effective_route_context_var: ContextVar[Any | None] = ContextVar(
 _SCOPE_MISSING = object()
 
 
+class _RouteWithPath(Protocol):
+    path: str
+
+
 def _get_fastapi_scope(scope: Scope) -> dict[str, Any]:
     fastapi_scope = scope.setdefault(_FASTAPI_SCOPE_KEY, {})
     assert isinstance(fastapi_scope, dict)
     return fastapi_scope
 
 
+def _update_scope(scope: Scope, child_scope: Scope) -> None:
+    fastapi_child_scope = child_scope.get(_FASTAPI_SCOPE_KEY)
+    for key, value in child_scope.items():
+        if key != _FASTAPI_SCOPE_KEY:
+            scope[key] = value
+    if isinstance(fastapi_child_scope, dict):
+        _get_fastapi_scope(scope).update(fastapi_child_scope)
+
+
 def _get_scope_effective_route_context(scope: Scope) -> Any | None:
     return scope.get(_FASTAPI_SCOPE_KEY, {}).get(_FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY)
 
@@ -1305,9 +1326,7 @@ class _RouterIncludeContext:
             dependency_overrides_provider=self.dependency_overrides_provider,
         )
 
-    def path_for(
-        self, route: APIRoute | routing.Route | routing.WebSocketRoute | routing.Mount
-    ) -> str:
+    def path_for(self, route: _RouteWithPath) -> str:
         return self.prefix + route.path
 
 
@@ -1503,6 +1522,10 @@ class _IncludedRouter(BaseRoute):
         default_factory=list
     )
     _effective_candidates_version: int | None = None
+    _effective_low_priority_routes: list["_EffectiveRouteContext"] = field(
+        default_factory=list
+    )
+    _effective_low_priority_routes_version: int | None = None
 
     def effective_candidates(self) -> list["_EffectiveRouteContext | _IncludedRouter"]:
         routes_version = self.original_router._get_routes_version()
@@ -1525,6 +1548,28 @@ class _IncludedRouter(BaseRoute):
         self._effective_candidates_version = routes_version
         return self._effective_candidates
 
+    def effective_low_priority_routes(self) -> list["_EffectiveRouteContext"]:
+        routes_version = self.original_router._get_routes_version()
+        if routes_version == self._effective_low_priority_routes_version:
+            return self._effective_low_priority_routes
+        self._effective_low_priority_routes = []
+        for route in self.original_router._low_priority_routes:
+            route_context = self._build_effective_context(route)
+            if route_context is not None:
+                self._effective_low_priority_routes.append(route_context)
+        for route in self.original_router.routes:
+            if isinstance(route, _IncludedRouter):
+                child_context = self.include_context.combine(route.include_context)
+                child_branch = _IncludedRouter(
+                    original_router=route.original_router,
+                    include_context=child_context,
+                )
+                self._effective_low_priority_routes.extend(
+                    child_branch.effective_low_priority_routes()
+                )
+        self._effective_low_priority_routes_version = routes_version
+        return self._effective_low_priority_routes
+
     def _build_effective_context(
         self, route: BaseRoute
     ) -> _EffectiveRouteContext | None:
@@ -1533,6 +1578,11 @@ class _IncludedRouter(BaseRoute):
                 original_route=route,
                 include_context=self.include_context,
             )
+        if isinstance(route, _FrontendRouteGroup):
+            return _EffectiveRouteContext(
+                original_route=route,
+                starlette_route=route.with_prefix(self.include_context.prefix),
+            )
         if isinstance(route, routing.Route):
             starlette_route: BaseRoute = routing.Route(
                 self.include_context.path_for(route),
@@ -1720,6 +1770,294 @@ def _iter_routes_with_context(
             yield route, None
 
 
+def _normalize_frontend_path(path: str) -> str:
+    if not path:
+        raise AssertionError("A frontend path cannot be empty")
+    if not path.startswith("/"):
+        raise AssertionError("A frontend path must start with '/'")
+    if path != "/":
+        path = path.rstrip("/")
+    return path
+
+
+def _join_frontend_paths(prefix: str, path: str) -> str:
+    if not prefix:
+        return path
+    if path == "/":
+        return prefix
+    return prefix + path
+
+
+def _frontend_path_specificity(path: str) -> int:
+    if path == "/":
+        return 0
+    return len(path)
+
+
+def _get_resolved_absolute_path(path: str | os.PathLike[str]) -> str:
+    return os.path.realpath(os.fspath(path))
+
+
+class _FrontendStaticFiles(StaticFiles):
+    def __init__(
+        self,
+        *,
+        directory: str | os.PathLike[str],
+        fallback: Literal["auto", "index.html", "404.html"] | None,
+        check_dir: bool = True,
+    ) -> None:
+        self.fallback = fallback
+        if check_dir and not os.path.isdir(directory):
+            raise RuntimeError(
+                f"Frontend directory '{directory}' does not exist. "
+                f"Resolved absolute path: '{_get_resolved_absolute_path(directory)}'"
+            )
+        super().__init__(
+            directory=directory,
+            html=True,
+            check_dir=check_dir,
+            follow_symlink=False,
+        )
+        if check_dir and fallback in {"index.html", "404.html"}:
+            self._check_fallback_file(fallback)
+
+    def _check_fallback_file(self, fallback: str) -> None:
+        _, stat_result = self.lookup_path(fallback)
+        if stat_result is None or not stat.S_ISREG(stat_result.st_mode):
+            raise RuntimeError(
+                f"Frontend fallback file '{fallback}' does not exist in "
+                f"directory '{self.directory}'. Resolved absolute directory: "
+                f"'{self._get_resolved_directory()}'"
+            )
+
+    def _get_resolved_directory(self) -> str:
+        assert self.directory is not None
+        return _get_resolved_absolute_path(self.directory)
+
+    def get_path(self, scope: Scope) -> str:
+        path = _get_fastapi_scope(scope).get(_FASTAPI_FRONTEND_PATH_KEY, "")
+        assert isinstance(path, str)
+        return os.path.normpath(os.path.join(*path.split("/")))
+
+    async def get_response(self, path: str, scope: Scope) -> Response:
+        if scope["method"] not in ("GET", "HEAD"):
+            raise HTTPException(status_code=405)
+
+        try:
+            full_path, stat_result = await run_in_threadpool(self.lookup_path, path)
+        except PermissionError:
+            raise HTTPException(status_code=401) from None
+        except OSError as exc:
+            if exc.errno == errno.ENAMETOOLONG:
+                raise HTTPException(status_code=404) from None
+            raise exc
+        except ValueError:
+            raise HTTPException(status_code=404) from None
+
+        if stat_result and stat.S_ISREG(stat_result.st_mode):
+            return self.file_response(full_path, stat_result, scope)
+
+        if stat_result and stat.S_ISDIR(stat_result.st_mode):
+            index_path = os.path.join(path, "index.html")
+            full_path, stat_result = await run_in_threadpool(
+                self.lookup_path, index_path
+            )
+            if stat_result is not None and stat.S_ISREG(stat_result.st_mode):
+                if not scope["path"].endswith("/"):
+                    url = URL(scope=scope)
+                    url = url.replace(path=url.path + "/")
+                    return RedirectResponse(url=url)
+                return self.file_response(full_path, stat_result, scope)
+
+        if self.fallback == "404.html" or (
+            self.fallback == "auto" and self._fallback_file_exists("404.html")
+        ):
+            return await self._fallback_response("404.html", scope, status_code=404)
+
+        if (
+            self.fallback == "index.html"
+            or (self.fallback == "auto" and self._fallback_file_exists("index.html"))
+        ) and _is_frontend_navigation_request(scope):
+            return await self._fallback_response("index.html", scope, status_code=200)
+
+        raise HTTPException(status_code=404)
+
+    def _fallback_file_exists(self, fallback: str) -> bool:
+        _, stat_result = self.lookup_path(fallback)
+        return stat_result is not None and stat.S_ISREG(stat_result.st_mode)
+
+    async def _fallback_response(
+        self, fallback: str, scope: Scope, *, status_code: int
+    ) -> Response:
+        full_path, stat_result = await run_in_threadpool(self.lookup_path, fallback)
+        if stat_result is None or not stat.S_ISREG(stat_result.st_mode):
+            raise RuntimeError(
+                f"Frontend fallback file '{fallback}' does not exist in "
+                f"directory '{self.directory}'. Resolved absolute directory: "
+                f"'{self._get_resolved_directory()}'"
+            )
+        return self.file_response(
+            full_path, stat_result, scope, status_code=status_code
+        )
+
+
+def _iter_accept_media_types(accept: str) -> Iterator[tuple[str, float]]:
+    for raw_value in accept.split(","):
+        message = email.message.Message()
+        message["content-type"] = raw_value.strip()
+        q = message.get_param("q")
+        quality = 1.0
+        if isinstance(q, str):
+            try:
+                quality = float(q)
+            except ValueError:
+                pass
+        yield (
+            f"{message.get_content_maintype()}/{message.get_content_subtype()}",
+            quality,
+        )
+
+
+def _is_frontend_navigation_request(scope: Scope) -> bool:
+    route_path = get_route_path(scope)
+    final_segment = route_path.rsplit("/", 1)[-1]
+    if os.path.splitext(final_segment)[1]:
+        return False
+    request = Request(scope)
+    wildcard_accepted = False
+    html_rejected = False
+    for media_type, quality in _iter_accept_media_types(
+        request.headers.get("accept", "")
+    ):
+        if media_type in {"text/html", "application/xhtml+xml"}:
+            if quality == 0:
+                html_rejected = True
+            else:
+                return True
+        elif media_type == "*/*" and quality != 0:
+            wildcard_accepted = True
+    return wildcard_accepted and not html_rejected
+
+
+class _FrontendRoute(BaseRoute):
+    def __init__(
+        self,
+        path: str,
+        *,
+        directory: str | os.PathLike[str],
+        fallback: Literal["auto", "index.html", "404.html"] | None = "auto",
+        check_dir: bool = True,
+    ) -> None:
+        if fallback not in {"auto", "index.html", "404.html", None}:
+            raise AssertionError(
+                "fallback must be 'auto', 'index.html', '404.html', or None"
+            )
+        self.path = _normalize_frontend_path(path)
+        self.methods = {"GET", "HEAD"}
+        self.app = _FrontendStaticFiles(
+            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]:
+        if scope["type"] != "http":
+            return Match.NONE, {}
+        frontend_path = self._get_frontend_path(get_route_path(scope))
+        if frontend_path is None:
+            return Match.NONE, {}
+        child_scope = {_FASTAPI_SCOPE_KEY: {_FASTAPI_FRONTEND_PATH_KEY: frontend_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 == "/":
+            return route_path.lstrip("/")
+        if route_path == self.path:
+            return ""
+        prefix = self.path + "/"
+        if route_path.startswith(prefix):
+            return route_path[len(prefix) :]
+        return None
+
+    async def handle(self, scope: Scope, receive: Receive, send: Send) -> None:
+        await self.app(scope, receive, send)
+
+    def url_path_for(self, name: str, /, **path_params: Any) -> URLPath:
+        raise NoMatchFound(name, path_params)
+
+
+class _FrontendRouteGroup(BaseRoute):
+    def __init__(self) -> None:
+        self.routes: list[_FrontendRoute] = []
+
+    def add_frontend_route(
+        self,
+        path: str,
+        *,
+        directory: str | os.PathLike[str],
+        fallback: Literal["auto", "index.html", "404.html"] | None = "auto",
+        check_dir: bool = True,
+    ) -> None:
+        self.routes.append(
+            _FrontendRoute(
+                path,
+                directory=directory,
+                fallback=fallback,
+                check_dir=check_dir,
+            )
+        )
+
+    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)
+        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
+        for route in self.routes:
+            match, child_scope = route.matches(scope)
+            if match == Match.FULL:
+                if full is None or _frontend_path_specificity(
+                    route.path
+                ) > _frontend_path_specificity(full[1].path):
+                    full = (child_scope, route)
+            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 full is not None:
+            child_scope, route = full
+            return Match.FULL, child_scope, route
+        if partial is not None:
+            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)
+        if match == Match.NONE or route is None:
+            raise HTTPException(status_code=404)
+        _update_scope(scope, child_scope)
+        await route.handle(scope, receive, send)
+
+    def url_path_for(self, name: str, /, **path_params: Any) -> URLPath:
+        raise NoMatchFound(name, path_params)
+
+
 class APIRouter(routing.Router):
     """
     `APIRouter` class, used to group *path operations*, for example to structure
@@ -2032,6 +2370,8 @@ class APIRouter(routing.Router):
         self.generate_unique_id_function = generate_unique_id_function
         self.strict_content_type = strict_content_type
         self._routes_version = 0
+        self._low_priority_routes: list[BaseRoute] = []
+        self._frontend_routes: _FrontendRouteGroup | None = None
 
     def _mark_routes_changed(self) -> None:
         self._routes_version += 1
@@ -2093,6 +2433,150 @@ class APIRouter(routing.Router):
         super().add_websocket_route(path, endpoint, name=name)
         self._mark_routes_changed()
 
+    def frontend(
+        self,
+        path: Annotated[
+            str,
+            Doc(
+                """
+                The URL path prefix where the frontend build should be served.
+                """
+            ),
+        ],
+        *,
+        directory: Annotated[
+            str | os.PathLike[str],
+            Doc(
+                """
+                The directory containing the static frontend build output.
+                """
+            ),
+        ],
+        fallback: Annotated[
+            Literal["auto", "index.html", "404.html"] | None,
+            Doc(
+                """
+                The fallback file behavior for missing frontend paths.
+                """
+            ),
+        ] = "auto",
+        check_dir: Annotated[
+            bool,
+            Doc(
+                """
+                Check that the frontend directory exists when the app is created.
+                """
+            ),
+        ] = True,
+    ) -> None:
+        """
+        Serve a static frontend build as low-priority routes.
+
+        Use this for frontend tools that build static files into a directory,
+        such as `dist`. **FastAPI** path operations are checked first, and
+        the frontend files are checked only if no normal route matched.
+
+        A typical project could look like this:
+
+        ```text
+        .
+        ├── pyproject.toml
+        ├── app
+        │   ├── __init__.py
+        │   └── main.py
+        └── dist
+            ├── index.html
+            └── assets
+                └── app.js
+        ```
+
+        Then in `app/main.py`:
+
+        ```python
+        from fastapi import APIRouter, FastAPI
+
+        app = FastAPI()
+        router = APIRouter()
+        router.frontend("/", directory="dist")
+        app.include_router(router)
+        ```
+        """
+        normalized_path = _normalize_frontend_path(path)
+        if self._frontend_routes is None:
+            self._frontend_routes = _FrontendRouteGroup()
+            self._low_priority_routes.append(self._frontend_routes)
+        self._frontend_routes.add_frontend_route(
+            _join_frontend_paths(self.prefix, normalized_path),
+            directory=directory,
+            fallback=fallback,
+            check_dir=check_dir,
+        )
+        self._mark_routes_changed()
+
+    async def app(self, scope: Scope, receive: Receive, send: Send) -> None:
+        assert scope["type"] in ("http", "websocket", "lifespan")
+
+        if "router" not in scope:
+            scope["router"] = self
+
+        if scope["type"] == "lifespan":
+            await self.lifespan(scope, receive, send)
+            return
+
+        partial: tuple[BaseRoute, Scope] | None = None
+        for route in self.routes:
+            match, child_scope = route.matches(scope)
+            if match == Match.FULL:
+                scope.update(child_scope)
+                await route.handle(scope, receive, send)
+                return
+            if match == Match.PARTIAL and partial is None:
+                partial = (route, child_scope)
+
+        if partial is not None:
+            route, child_scope = partial
+            scope.update(child_scope)
+            await route.handle(scope, receive, send)
+            return
+
+        route_path = get_route_path(scope)
+        if scope["type"] == "http" and self.redirect_slashes and route_path != "/":
+            redirect_scope = dict(scope)
+            if route_path.endswith("/"):
+                redirect_scope["path"] = redirect_scope["path"].rstrip("/")
+            else:
+                redirect_scope["path"] = redirect_scope["path"] + "/"
+
+            for route in self.routes:
+                match, _ = route.matches(redirect_scope)
+                if match != Match.NONE:
+                    redirect_url = URL(scope=redirect_scope)
+                    response = RedirectResponse(url=str(redirect_url))
+                    await response(scope, receive, send)
+                    return
+
+        (
+            low_priority_match,
+            low_priority_scope,
+            low_priority_route,
+            low_priority_context,
+        ) = self._match_low_priority(scope)
+        if low_priority_match != Match.NONE and low_priority_route is not None:
+            _update_scope(scope, low_priority_scope)
+            if low_priority_context is not None:
+                _get_fastapi_scope(scope)[_FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY] = (
+                    low_priority_context
+                )
+                original_route = low_priority_context.original_route
+                if isinstance(original_route, APIRoute):
+                    scope["route"] = original_route
+                    await original_route.handle(scope, receive, send)
+                    return
+            await low_priority_route.handle(scope, receive, send)
+            return
+
+        await self.default(scope, receive, send)
+
     async def handle(self, scope: Scope, receive: Receive, send: Send) -> None:
         included_router = _get_scope_included_router(scope)
         if (
@@ -2113,6 +2597,60 @@ class APIRouter(routing.Router):
             return match, child_scope
         return Match.NONE, {}
 
+    def _iter_low_priority_routes(
+        self,
+    ) -> Iterator[BaseRoute | _EffectiveRouteContext]:
+        yield from self._low_priority_routes
+        for route in self.routes:
+            if isinstance(route, _IncludedRouter):
+                yield from route.effective_low_priority_routes()
+
+    def _match_low_priority(
+        self, scope: Scope
+    ) -> tuple[Match, Scope, BaseRoute | None, _EffectiveRouteContext | None]:
+        full: tuple[Scope, BaseRoute, _EffectiveRouteContext | None] | None = None
+        partial: tuple[Scope, BaseRoute, _EffectiveRouteContext | None] | None = None
+        for candidate in self._iter_low_priority_routes():
+            route: BaseRoute
+            if isinstance(candidate, _EffectiveRouteContext):
+                route_context: _EffectiveRouteContext | None = candidate
+                original_route = candidate.original_route
+                if isinstance(original_route, APIRoute):
+                    fastapi_scope = _get_fastapi_scope(scope)
+                    previous_context = fastapi_scope.get(
+                        _FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY, _SCOPE_MISSING
+                    )
+                    fastapi_scope[_FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY] = route_context
+                    try:
+                        match, child_scope = original_route.matches(scope)
+                    finally:
+                        _restore_fastapi_scope_key(
+                            scope,
+                            _FASTAPI_EFFECTIVE_ROUTE_CONTEXT_KEY,
+                            previous_context,
+                        )
+                    route = original_route
+                else:
+                    match, child_scope = candidate.matches(scope)
+                    route = candidate.starlette_route or original_route
+            else:
+                route_context = None
+                match, child_scope = candidate.matches(scope)
+                route = candidate
+            if match == Match.FULL:
+                if full is None:
+                    full = (child_scope, route, route_context)
+            elif match == Match.PARTIAL:
+                if partial is None:
+                    partial = (child_scope, route, route_context)
+        if full is not None:
+            child_scope, route, route_context = full
+            return Match.FULL, child_scope, route, route_context
+        if partial is not None:
+            child_scope, route, route_context = partial
+            return Match.PARTIAL, child_scope, route, route_context
+        return Match.NONE, {}, None, None
+
     def route(
         self,
         path: str,

tests/test_frontend.py

@@ -0,0 +1,858 @@
+import errno
+import os
+import runpy
+from pathlib import Path
+
+import anyio
+import pytest
+from fastapi import APIRouter, FastAPI, HTTPException, Request, WebSocket
+from fastapi.testclient import TestClient
+from starlette.exceptions import HTTPException as StarletteHTTPException
+from starlette.responses import PlainTextResponse, Response
+from starlette.routing import BaseRoute, Match, NoMatchFound, Route
+
+
+def write_file(path: Path, content: str) -> None:
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(content)
+
+
+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)
+
+    response = TestClient(app).get("/app")
+
+    assert response.status_code == 200
+    assert response.text == "app"
+
+
+def test_apirouter_frontend_with_router_prefix_and_frontend_subpath(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "asset.txt", "asset")
+    router = APIRouter(prefix="/internal")
+    router.frontend("/ui", directory=dist)
+    app = FastAPI()
+    app.include_router(router, prefix="/prefix")
+
+    response = TestClient(app).get("/prefix/internal/ui/asset.txt")
+
+    assert response.status_code == 200
+    assert response.text == "asset"
+
+
+def test_frontend_fallback_rejects_invalid_fallback(tmp_path: Path):
+    dist = tmp_path / "dist"
+    dist.mkdir()
+    app = FastAPI()
+
+    with pytest.raises(AssertionError, match="fallback"):
+        app.frontend("/", directory=dist, fallback="invalid")  # type: ignore[arg-type]  # ty: ignore[invalid-argument-type]
+
+
+def test_index_fallback_ignores_invalid_q_value(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(
+        "/dashboard/settings", headers={"accept": "text/html; q=wat"}
+    )
+
+    assert response.status_code == 200
+    assert response.text == "app shell"
+
+
+def test_frontend_static_files_lookup_errors(monkeypatch, tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+    frontend_routes = app.router._frontend_routes
+    assert frontend_routes is not None
+    static_files = frontend_routes.routes[0].app
+
+    def raise_permission_error(path: str):
+        raise PermissionError
+
+    monkeypatch.setattr(static_files, "lookup_path", raise_permission_error)
+    response = TestClient(app).get("/asset.txt")
+    assert response.status_code == 401
+
+    def raise_value_error(path: str):
+        raise ValueError
+
+    monkeypatch.setattr(static_files, "lookup_path", raise_value_error)
+    response = TestClient(app).get("/asset.txt")
+    assert response.status_code == 404
+
+    def raise_name_too_long(path: str):
+        raise OSError(errno.ENAMETOOLONG, "name too long")
+
+    monkeypatch.setattr(static_files, "lookup_path", raise_name_too_long)
+    response = TestClient(app).get("/asset.txt")
+    assert response.status_code == 404
+
+    def raise_os_error(path: str):
+        raise OSError(5, "other")
+
+    monkeypatch.setattr(static_files, "lookup_path", raise_os_error)
+    with pytest.raises(OSError):
+        TestClient(app).get("/asset.txt")
+
+
+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)
+    route_group = app.router._frontend_routes
+    assert route_group is not None
+
+    match, child_scope = route_group.matches({"type": "websocket", "path": "/"})
+    assert match == Match.NONE
+    assert child_scope == {}
+
+    with pytest.raises(StarletteHTTPException) as exc_info:
+        anyio.run(
+            route_group.with_prefix("/app").handle,
+            {"type": "http", "path": "/missing", "method": "GET"},
+            None,
+            None,
+        )
+    assert exc_info.value.status_code == 404
+
+    with pytest.raises(NoMatchFound):
+        route_group.url_path_for("frontend")
+    with pytest.raises(NoMatchFound):
+        route_group.routes[0].url_path_for("frontend")
+
+
+def test_included_low_priority_routes_cache_is_reused():
+    async def low_priority_endpoint(request: Request):
+        return PlainTextResponse("low")
+
+    router = APIRouter()
+    router._low_priority_routes.append(Route("/low", low_priority_endpoint))
+    router._mark_routes_changed()
+    app = FastAPI()
+    app.include_router(router, prefix="/prefix")
+    included_router = next(
+        route
+        for route in app.router.routes
+        if hasattr(route, "effective_low_priority_routes")
+    )
+
+    first = included_router.effective_low_priority_routes()  # ty: ignore[call-non-callable]
+    second = included_router.effective_low_priority_routes()  # ty: ignore[call-non-callable]
+    response = TestClient(app).get("/prefix/low")
+
+    assert first is second
+    assert response.status_code == 200
+    assert response.text == "low"
+
+
+def test_low_priority_api_route_handles_with_context():
+    app = FastAPI()
+
+    async def endpoint(request: Request) -> Response:
+        return PlainTextResponse(request.scope["path_params"]["item_id"])
+
+    route = app.router.route_class("/low/{item_id}", endpoint=endpoint, methods=["GET"])
+    app.router._low_priority_routes.append(route)
+    app.router._mark_routes_changed()
+
+    response = TestClient(app).get("/low/abc")
+
+    assert response.status_code == 200
+    assert response.text == "abc"
+
+
+def test_included_low_priority_api_route_handles_with_context():
+    router = APIRouter()
+
+    async def endpoint(request: Request) -> Response:
+        return PlainTextResponse(request.scope["path_params"]["item_id"])
+
+    route = router.route_class("/low/{item_id}", endpoint=endpoint, methods=["GET"])
+    router._low_priority_routes.append(route)
+    router._mark_routes_changed()
+    app = FastAPI()
+    app.include_router(router, prefix="/prefix")
+
+    response = TestClient(app).get("/prefix/low/abc")
+
+    assert response.status_code == 200
+    assert response.text == "abc"
+
+
+def test_normal_route_partial_match_returns_before_frontend(tmp_path: Path):
+    class PartialRoute(BaseRoute):
+        def matches(self, scope):
+            return Match.PARTIAL, {}
+
+        async def handle(self, scope, receive, send):
+            response = PlainTextResponse("partial", status_code=405)
+            await response(scope, receive, send)
+
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "frontend")
+    app = FastAPI()
+    app.router.routes.append(PartialRoute())
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/anything")
+
+    assert response.status_code == 405
+    assert response.text == "partial"
+
+
+def test_normal_route_partial_match_wins_before_frontend(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "api", "frontend")
+    app = FastAPI()
+
+    @app.get("/api")
+    def read_api():
+        return {"source": "api"}
+
+    app.frontend("/", directory=dist)
+
+    client = TestClient(app)
+
+    response = client.get("/api")
+    assert response.status_code == 200
+    assert response.json() == {"source": "api"}
+
+    response = client.post("/api")
+    assert response.status_code == 405
+
+
+def test_basic_file_serving(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "assets" / "app.js", "console.log('ok')")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/assets/app.js")
+
+    assert response.status_code == 200
+    assert response.text == "console.log('ok')"
+    assert "etag" in response.headers
+    assert "last-modified" in response.headers
+
+
+def test_existing_api_route_wins_over_frontend(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "api" / "users", "frontend")
+    app = FastAPI()
+
+    @app.get("/api/users")
+    def read_users():
+        return {"source": "api"}
+
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/api/users")
+
+    assert response.status_code == 200
+    assert response.json() == {"source": "api"}
+
+
+def test_api_route_404_is_not_replaced_by_frontend_fallback(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "frontend")
+    app = FastAPI()
+
+    @app.get("/api/users")
+    def read_users():
+        raise HTTPException(status_code=404, detail="api missing")
+
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get("/api/users", headers={"accept": "text/html"})
+
+    assert response.status_code == 404
+    assert response.json() == {"detail": "api missing"}
+
+
+def test_index_fallback_for_navigation_request(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(
+        "/dashboard/settings", headers={"accept": "text/html"}
+    )
+
+    assert response.status_code == 200
+    assert response.text == "app shell"
+
+
+def test_index_fallback_parses_accept_parameters(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(
+        "/dashboard/settings", headers={"accept": "text/html; q=0.8"}
+    )
+
+    assert response.status_code == 200
+    assert response.text == "app shell"
+
+
+def test_index_fallback_ignores_q_zero_accept(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(
+        "/dashboard/settings", headers={"accept": "text/html; q=0.0"}
+    )
+
+    assert response.status_code == 404
+
+
+def test_index_fallback_respects_explicit_html_rejection_with_wildcard(
+    tmp_path: Path,
+):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(
+        "/dashboard/settings",
+        headers={"accept": "text/html; q=0, */*; q=1"},
+    )
+
+    assert response.status_code == 404
+
+
+def test_index_fallback_respects_explicit_xhtml_rejection_with_wildcard(
+    tmp_path: Path,
+):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(
+        "/dashboard/settings",
+        headers={"accept": "application/xhtml+xml; q=0, */*; q=1"},
+    )
+
+    assert response.status_code == 404
+
+
+@pytest.mark.parametrize(
+    ("path", "accept"),
+    [
+        ("/assets/missing.js", "*/*"),
+        ("/assets/missing.css", "text/css"),
+        ("/assets/missing.png", "image/png"),
+        ("/api/missing", "application/json"),
+        ("/users/jane.doe", "text/html"),
+    ],
+)
+def test_index_fallback_does_not_handle_asset_like_or_non_html_requests(
+    tmp_path: Path, path: str, accept: str
+):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get(path, headers={"accept": accept})
+
+    assert response.status_code == 404
+    assert response.text != "app shell"
+
+
+def test_404_fallback_handles_missing_assets(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "404.html", "missing")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback="404.html")
+
+    response = TestClient(app).get("/assets/missing.js")
+
+    assert response.status_code == 404
+    assert response.text == "missing"
+
+
+def test_auto_fallback_prefers_404_over_index(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    write_file(dist / "404.html", "missing")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/dashboard", headers={"accept": "text/html"})
+
+    assert response.status_code == 404
+    assert response.text == "missing"
+
+
+def test_auto_fallback_uses_index_when_404_is_missing(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/dashboard", headers={"accept": "text/html"})
+
+    assert response.status_code == 200
+    assert response.text == "app shell"
+
+
+def test_auto_fallback_returns_normal_404_without_fallback_files(tmp_path: Path):
+    dist = tmp_path / "dist"
+    dist.mkdir()
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/dashboard", headers={"accept": "text/html"})
+
+    assert response.status_code == 404
+    assert response.json() == {"detail": "Not Found"}
+
+
+def test_no_fallback_returns_normal_404(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app shell")
+    app = FastAPI()
+    app.frontend("/", directory=dist, fallback=None)
+
+    response = TestClient(app).get("/dashboard", headers={"accept": "text/html"})
+
+    assert response.status_code == 404
+    assert response.json() == {"detail": "Not Found"}
+
+
+def test_directory_index_and_redirect(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "about" / "index.html", "about")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+    client = TestClient(app)
+
+    redirect = client.get("/about", follow_redirects=False)
+    response = client.get("/about/")
+
+    assert redirect.status_code == 307
+    assert redirect.headers["location"] == "http://testserver/about/"
+    assert response.status_code == 200
+    assert response.text == "about"
+
+
+def test_path_validation_and_trailing_slash_normalization(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "asset.txt", "ok")
+    app = FastAPI()
+
+    with pytest.raises(AssertionError):
+        app.frontend("", directory=dist)
+    with pytest.raises(AssertionError):
+        app.frontend("app", directory=dist)
+
+    app.frontend("/app/", directory=dist)
+    response = TestClient(app).get("/app/asset.txt")
+
+    assert response.status_code == 200
+    assert response.text == "ok"
+
+
+def test_frontend_path_matching_uses_segment_boundaries(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI()
+    app.frontend("/app", directory=dist, fallback="index.html")
+
+    response = TestClient(app).get("/application", headers={"accept": "text/html"})
+
+    assert response.status_code == 404
+
+
+def test_multiple_frontends_use_longest_matching_prefix(tmp_path: Path):
+    site = tmp_path / "site"
+    admin = tmp_path / "admin"
+    write_file(site / "index.html", "site")
+    write_file(admin / "index.html", "admin")
+    app = FastAPI()
+    app.frontend("/", directory=site, fallback="index.html")
+    app.frontend("/admin", directory=admin, fallback="index.html")
+
+    response = TestClient(app).get("/admin/settings", headers={"accept": "text/html"})
+
+    assert response.status_code == 200
+    assert response.text == "admin"
+
+
+def test_apirouter_frontend_uses_include_prefix(tmp_path: Path):
+    dist = tmp_path / "admin"
+    write_file(dist / "index.html", "admin")
+    router = APIRouter()
+    router.frontend("/", directory=dist, fallback="index.html")
+    app = FastAPI()
+    app.include_router(router, prefix="/admin")
+
+    response = TestClient(app).get("/admin/settings", headers={"accept": "text/html"})
+
+    assert response.status_code == 200
+    assert response.text == "admin"
+
+
+def test_global_priority_across_included_routers(tmp_path: Path):
+    dist = tmp_path / "site"
+    write_file(dist / "index.html", "site")
+    site_router = APIRouter()
+    site_router.frontend("/", directory=dist, fallback="index.html")
+    api_router = APIRouter()
+
+    @api_router.get("/api/users")
+    def read_users():
+        return {"source": "api"}
+
+    app = FastAPI()
+    app.include_router(site_router)
+    app.include_router(api_router)
+
+    response = TestClient(app).get("/api/users", headers={"accept": "text/html"})
+
+    assert response.status_code == 200
+    assert response.json() == {"source": "api"}
+
+
+def test_nested_apirouter_frontend_uses_all_include_prefixes(tmp_path: Path):
+    dist = tmp_path / "admin"
+    write_file(dist / "index.html", "admin")
+    child_router = APIRouter()
+    child_router.frontend("/", directory=dist, fallback="index.html")
+    parent_router = APIRouter()
+    parent_router.include_router(child_router, prefix="/child")
+    app = FastAPI()
+    app.include_router(parent_router, prefix="/parent")
+
+    response = TestClient(app).get(
+        "/parent/child/settings", headers={"accept": "text/html"}
+    )
+
+    assert response.status_code == 200
+    assert response.text == "admin"
+
+
+def test_low_priority_cache_updates_after_route_added_to_included_router(
+    tmp_path: Path,
+):
+    dist = tmp_path / "site"
+    write_file(dist / "index.html", "site")
+    router = APIRouter()
+    router.frontend("/", directory=dist, fallback="index.html")
+    app = FastAPI()
+    app.include_router(router, prefix="/app")
+    client = TestClient(app)
+
+    frontend_response = client.get("/app/dashboard", headers={"accept": "text/html"})
+
+    @router.get("/dashboard")
+    def read_dashboard():
+        return {"source": "api"}
+
+    api_response = client.get("/app/dashboard", headers={"accept": "text/html"})
+
+    assert frontend_response.status_code == 200
+    assert frontend_response.text == "site"
+    assert api_response.status_code == 200
+    assert api_response.json() == {"source": "api"}
+
+
+def test_normal_route_slash_redirect_wins_before_frontend_redirect(tmp_path: Path):
+    dist = tmp_path / "site"
+    write_file(dist / "api" / "index.html", "frontend")
+    app = FastAPI()
+
+    @app.get("/api/")
+    def read_api():
+        return {"source": "api"}
+
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/api", follow_redirects=False)
+
+    assert response.status_code == 307
+    assert response.headers["location"] == "http://testserver/api/"
+
+    followed = TestClient(app).get("/api/")
+    assert followed.status_code == 200
+    assert followed.json() == {"source": "api"}
+
+
+def test_frontend_respects_root_path(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "assets" / "app.js", "console.log('ok')")
+    app = FastAPI()
+    app.frontend("/app", directory=dist)
+
+    response = TestClient(app, root_path="/proxy").get("/app/assets/app.js")
+
+    assert response.status_code == 200
+    assert response.text == "console.log('ok')"
+
+
+def test_websocket_route_wins_over_frontend(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "ws", "frontend")
+    app = FastAPI()
+
+    @app.websocket("/ws")
+    async def websocket_endpoint(websocket: WebSocket):
+        await websocket.accept()
+        await websocket.send_text("websocket")
+        await websocket.close()
+
+    app.frontend("/", directory=dist)
+
+    with TestClient(app).websocket_connect("/ws") as websocket:
+        data = websocket.receive_text()
+
+    assert data == "websocket"
+
+
+def test_head_requests_work(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "asset.txt", "ok")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).head("/asset.txt")
+
+    assert response.status_code == 200
+    assert response.text == ""
+    assert response.headers["content-length"] == "2"
+
+
+def test_unsupported_methods_return_405(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "asset.txt", "ok")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).post("/asset.txt")
+
+    assert response.status_code == 405
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/../secret.txt",
+        "/%2e%2e/secret.txt",
+        "/..%2fsecret.txt",
+        "/%5c..%5csecret.txt",
+        "/..%5csecret.txt",
+    ],
+)
+def test_path_traversal_cannot_escape_directory(tmp_path: Path, path: str):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    write_file(tmp_path / "secret.txt", "secret")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get(path)
+
+    assert response.status_code == 404
+    assert response.text != "secret"
+
+
+def test_symlink_outside_directory_is_not_served(tmp_path: Path):
+    dist = tmp_path / "dist"
+    dist.mkdir()
+    outside = tmp_path / "secret.txt"
+    outside.write_text("secret")
+    link = dist / "secret.txt"
+    try:
+        os.symlink(outside, link)
+    except (OSError, NotImplementedError):  # pragma: no cover
+        pytest.skip("symlinks are not supported")
+    app = FastAPI()
+    app.frontend("/", directory=dist)
+
+    response = TestClient(app).get("/secret.txt")
+
+    assert response.status_code == 404
+    assert response.text != "secret"
+
+
+def test_check_dir_true_fails_early_for_missing_directory(monkeypatch, tmp_path: Path):
+    app = FastAPI()
+    monkeypatch.chdir(tmp_path)
+
+    with pytest.raises(RuntimeError, match="does not exist") as exc_info:
+        app.frontend("/", directory="missing")
+
+    message = str(exc_info.value)
+    assert "'missing'" in message
+    assert str(tmp_path / "missing") in message
+
+
+def test_check_dir_false_allows_missing_directory_and_fails_on_request(tmp_path: Path):
+    app = FastAPI()
+    app.frontend("/", directory=tmp_path / "missing", check_dir=False)
+
+    with pytest.raises(RuntimeError, match="does not exist"):
+        TestClient(app).get("/asset.txt")
+
+
+def test_explicit_fallback_files_fail_clearly_when_missing(monkeypatch, tmp_path: Path):
+    dist = tmp_path / "dist"
+    dist.mkdir()
+    monkeypatch.chdir(tmp_path)
+    app = FastAPI()
+
+    with pytest.raises(RuntimeError, match="index.html") as exc_info:
+        app.frontend("/", directory="dist", fallback="index.html")
+
+    message = str(exc_info.value)
+    assert "directory 'dist'" in message
+    assert str(dist) in message
+
+    app = FastAPI()
+    app.frontend("/", directory="dist", fallback="404.html", check_dir=False)
+
+    with pytest.raises(RuntimeError, match="404.html") as exc_info:
+        TestClient(app).get("/missing.js")
+
+    message = str(exc_info.value)
+    assert "directory 'dist'" in message
+    assert str(dist) in message
+
+
+def test_frontend_routes_are_not_in_openapi(tmp_path: Path):
+    dist = tmp_path / "dist"
+    write_file(dist / "index.html", "app")
+    app = FastAPI()
+
+    @app.get("/api")
+    def read_api():
+        return {"ok": True}
+
+    app.frontend("/", directory=dist, fallback="index.html")
+
+    schema = TestClient(app).get("/openapi.json").json()
+
+    assert set(schema["paths"]) == {"/api"}
+
+    response = TestClient(app).get("/api")
+    assert response.status_code == 200
+    assert response.json() == {"ok": True}
+
+
+@pytest.mark.parametrize(
+    ("example", "files", "path", "status_code", "body"),
+    [
+        (
+            "tutorial001_py310.py",
+            {"asset.txt": "asset"},
+            "/asset.txt",
+            200,
+            "asset",
+        ),
+        (
+            "tutorial002_py310.py",
+            {"index.html": "index"},
+            "/dashboard",
+            200,
+            "index",
+        ),
+        (
+            "tutorial003_py310.py",
+            {"404.html": "missing"},
+            "/missing",
+            404,
+            "missing",
+        ),
+        (
+            "tutorial004_py310.py",
+            {"index.html": "index"},
+            "/app/dashboard",
+            200,
+            "index",
+        ),
+        (
+            "tutorial005_py310.py",
+            {"index.html": "index"},
+            "/dashboard",
+            404,
+            '{"detail":"Not Found"}',
+        ),
+        (
+            "tutorial006_py310.py",
+            {"asset.txt": "asset"},
+            "/asset.txt",
+            200,
+            "asset",
+        ),
+    ],
+)
+def test_docs_frontend_examples(
+    tmp_path: Path,
+    monkeypatch,
+    example: str,
+    files: dict[str, str],
+    path: str,
+    status_code: int,
+    body: str,
+):
+    dist = tmp_path / "dist"
+    for file, content in files.items():
+        write_file(dist / file, content)
+    monkeypatch.chdir(tmp_path)
+
+    namespace = runpy.run_path(
+        str(Path(__file__).parents[1] / "docs_src" / "frontend" / example)
+    )
+
+    app = namespace["app"]
+    assert isinstance(app, FastAPI)
+    response = TestClient(app).get(path, headers={"accept": "text/html"})
+    assert response.status_code == status_code
+    assert response.text == body
+
+
+def test_low_priority_routes_can_store_non_frontend_routes():
+    async def low_priority_endpoint(request):
+        return PlainTextResponse("low")
+
+    app = FastAPI()
+    app.router._low_priority_routes.append(Route("/low", low_priority_endpoint))
+    app.router._mark_routes_changed()
+
+    response = TestClient(app).get("/low")
+
+    assert response.status_code == 200
+    assert response.text == "low"
+
+
+def test_included_low_priority_routes_can_store_non_frontend_routes():
+    async def low_priority_endpoint(request):
+        return PlainTextResponse("low")
+
+    router = APIRouter()
+    router._low_priority_routes.append(Route("/low", low_priority_endpoint))
+    router._mark_routes_changed()
+    app = FastAPI()
+    app.include_router(router, prefix="/prefix")
+
+    response = TestClient(app).get("/prefix/low")
+
+    assert response.status_code == 200
+    assert response.text == "low"

uv.lock

@@ -4103,16 +4103,16 @@ wheels = [
 
 [[package]]
 name = "pydantic-settings"
-version = "2.14.2"
+version = "2.14.1"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "pydantic" },
     { name = "python-dotenv" },
     { name = "typing-inspection" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/5c/b5/8f48e906c3e0205276e8bd8cb7512217a87b2685304d64be27cad5b3019f/pydantic_settings-2.14.2.tar.gz", hash = "sha256:c19dd64b19097f1de80184f0cc7b0272a13ae6e170cbf240a3e27e381ed14a5f", size = 237700, upload-time = "2026-06-19T13:44:56.324Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/07/60/1d1e59c9c90d54591469ada7d268251f71c24bdb765f1a8a832cee8c6653/pydantic_settings-2.14.1.tar.gz", hash = "sha256:e874d3bec7e787b0c9958277956ed9b4dd5de6a80e162188fdaff7c5e26fd5fa", size = 235551, upload-time = "2026-05-08T13:40:06.542Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/77/c1/6e422f34e569cf8e18df68d1939c81c099d2b61e4f7d9621c8a77560799c/pydantic_settings-2.14.2-py3-none-any.whl", hash = "sha256:a20c97b37910b6550d5ea50fbcc2d4187defe58cd57070b73863d069419c9440", size = 61715, upload-time = "2026-06-19T13:44:55.02Z" },
+    { url = "https://files.pythonhosted.org/packages/ae/8d/f1af3832f5e6eb13ba94ee809e72b8ecb5eef226d27ee0bef7d963d943c7/pydantic_settings-2.14.1-py3-none-any.whl", hash = "sha256:6e3c7edfd8277687cdc598f56e5cff0e9bfff0910a3749deaa8d4401c3a2b9de", size = 60964, upload-time = "2026-05-08T13:40:04.958Z" },
 ]
 
 [[package]]