Bugfix: Fix no context in catchall route

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 - [#3805](https://github.com/plotly/dash/pull/3805) Fix FastAPI POST routes deadlock caused by middleware consuming request body. Fixes [#3801](https://github.com/plotly/dash/issues/3801).
+- [#3819](https://github.com/plotly/dash/pull/3819) Fix `RuntimeError: No active request in context` when a non-Dash path falls through to the FastAPI catch-all route. Fixes [#3812](https://github.com/plotly/dash/issues/3812).
 
 ## [4.2.0] - 2026-06-01 - *The Freedom Update*
 

dash/backends/_fastapi.py

@@ -334,8 +334,12 @@ def _setup_catchall(self):
         try:
             dash_app = get_app()
 
-            async def catchall(_request: Request):
-                return Response(content=dash_app.index(), media_type="text/html")
+            async def catchall(request: Request):
+                token = set_current_request(request)
+                try:
+                    return Response(content=dash_app.index(), media_type="text/html")
+                finally:
+                    reset_current_request(token)
 
             # pylint: disable=protected-access
             self.add_url_rule("{path:path}", catchall, methods=["GET"])

tests/backend_tests/test_preconfig_backends.py

@@ -334,3 +334,22 @@ async def echo_post(request: Request):
     )
     assert resp.status_code == 200
     assert resp.json() == {"echo": {"hello": "world"}}
+
+
+def test_fastapi_catchall_request_context(dash_duo):
+    """Test that non-Dash paths falling through to the catch-all route work.
+
+    Regression test for https://github.com/plotly/dash/issues/3812
+    The catch-all route renders ``dash_app.index()``, which needs a request
+    context; without it the request raised ``RuntimeError: No active request in
+    context`` and returned a 500.
+    """
+    import requests
+
+    app = Dash(__name__, backend="fastapi")
+    app.layout = html.Div("Dash is running")
+
+    dash_duo.start_server(app)
+
+    resp = requests.get(f"{dash_duo.server_url}/some/non-dash/path", timeout=5)
+    assert resp.status_code == 200