fix: handle `with_dynamic_directory` mounted at sub-path (#8322) by mscolnick · Pull Request #8434 · marimo-team/marimo

mscolnick · 2026-02-23T16:04:49Z

fix: handle with_dynamic_directory mounted at sub-path

When mounting a marimo ASGI app at a sub-path (e.g., app.mount("/server2", server.build())), the DynamicDirectoryMiddleware failed to match requests because Starlette's Mount keeps the mount prefix in scope["path"] while also setting scope["root_path"].

For example, with with_dynamic_directory(path="/apps", ...) mounted at /server2, a request to /server2/apps/notebook/ would have path="/server2/apps/notebook/" and root_path="/server2". The middleware
checked path.startswith("/apps/") which failed because of the /server2 prefix.

Additionally, the app_builder was called with the filesystem path as base_url instead of a proper URL path.

Changes

Strip root_path from scope["path"] before matching base_path, so
the middleware works regardless of where it's mounted
Keep fallback for frameworks that fully strip the path (not just prefix)
Compute proper URL-based base_url for sub-apps instead of filesystem
paths

When mounting a marimo ASGI app at a sub-path (e.g., `app.mount("/server2", server.build())`), the `DynamicDirectoryMiddleware` failed to match requests because Starlette's `Mount` keeps the mount prefix in `scope["path"]` while also setting `scope["root_path"]`. For example, with `with_dynamic_directory(path="/apps", ...)` mounted at `/server2`, a request to `/server2/apps/notebook/` would have `path="/server2/apps/notebook/"` and `root_path="/server2"`. The middleware checked `path.startswith("/apps/")` which failed because of the `/server2` prefix. Additionally, the `app_builder` was called with the filesystem path as `base_url` instead of a proper URL path. ## Changes - Strip `root_path` from `scope["path"]` before matching `base_path`, so the middleware works regardless of where it's mounted - Keep fallback for frameworks that fully strip the path (not just prefix) - Compute proper URL-based `base_url` for sub-apps instead of filesystem paths - Update `my_server.py` smoke test with two mounted servers (root + `/server2`) Closes #8322

vercel · 2026-02-23T16:04:54Z

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Pull request overview

Fixes DynamicDirectoryMiddleware routing when a marimo ASGI app is mounted under a sub-path by accounting for scope["root_path"], and corrects the base_url passed into dynamically-built notebook apps so it’s a URL path (not a filesystem path).

Changes:

Strip root_path from scope["path"] to correctly match base_path under Starlette/FastAPI mounts.
Compute and pass a URL-path base_url to app_builder for dynamic directory apps.
Add regression tests covering sub-mount behavior (HTTP, redirects, assets, websockets) and update the custom smoke-test server example.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
`marimo/_server/asgi.py`	Adjusts dynamic-directory request matching for mounted apps and fixes `base_url` computation for sub-app creation.
`tests/_server/test_asgi.py`	Adds regression coverage for dynamic directories when the ASGI app is mounted at a sub-path.
`marimo/_smoke_tests/custom_server/my_server.py`	Expands the manual smoke-test script to include root mount + sub-path mount scenarios.

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Copilot · 2026-02-23T16:09:55Z

marimo/_server/asgi.py

        cache_key = str(marimo_file)
        if cache_key not in self._app_cache:
            LOGGER.debug(f"Creating new app for {cache_key}")
+            # Compute the URL base path for this notebook.
+            # This is used for template rendering (e.g., OpenGraph URLs).
+            try:
+                relative_notebook = str(
+                    marimo_file.relative_to(self.directory)
+                )
+                if relative_notebook.endswith(".py"):
+                    relative_notebook = relative_notebook[:-3]
+                # Compute the URL prefix for this notebook. When
+                # root_path already ends with base_path (because the
+                # parent mount includes it), avoid doubling the prefix.
+                if root_path.endswith(self.base_path):
+                    url_prefix = root_path


relative_notebook = str(marimo_file.relative_to(self.directory)) will use OS-specific path separators (e.g., backslashes on Windows), which can leak into notebook_base_url and produce invalid URL paths. Use a POSIX/URL-safe form (e.g., Path(...).as_posix() or replace os.sep with "/") before concatenating into a URL.

Copilot · 2026-02-23T16:09:55Z

tests/_server/test_asgi.py

+    the parent framework strips the prefix from scope["path"] and adds it to
+    scope["root_path"], causing the middleware to never match.


The class docstring says the parent mount "strips the prefix from scope['path']", but the issue description for #8322 (and this test setup) is about frameworks keeping the mount prefix in scope['path'] while also setting scope['root_path']. Please update the docstring to reflect the actual behavior being tested to avoid confusion for future maintainers.

Suggested change

the parent framework strips the prefix from scope["path"] and adds it to

scope["root_path"], causing the middleware to never match.

the parent framework keeps the mount prefix in scope["path"] while also

setting scope["root_path"], causing the middleware's path matching logic

to not behave as originally expected.

Copilot · 2026-02-23T16:09:56Z

marimo/_smoke_tests/custom_server/my_server.py

+# Key test: dynamic directory at "/" relative to mount = "/server2/" absolute
+server2 = (
+    marimo.create_asgi_app()
+    # Dynamic directory at sub-path: /apps becomes /server2/apps in FastAPI
+    .with_dynamic_directory(
+        path="/apps", directory=str(dirname / "altair_examples")


dirname / "smoke_tests" does not exist in marimo/_smoke_tests (the smoke test notebooks live directly under dirname). As written, with_dynamic_directory(..., directory=str(dirname / "smoke_tests")) will fail at runtime. Point this at an existing directory (likely str(dirname)) or add the missing folder to the repo.

Copilot · 2026-02-23T16:09:56Z

marimo/_smoke_tests/custom_server/my_server.py

+    # Dynamic directory at sub-path: /apps becomes /server2/apps in FastAPI
+    .with_dynamic_directory(
+        path="/apps", directory=str(dirname / "altair_examples")


with_dynamic_directory(path="/", ...) is likely not reachable: DynamicDirectoryMiddleware normalizes base_path with rstrip("/"), turning "/" into an empty string, and the request-matching logic is guarded by if self.base_path .... Use path="" for a root mount (consistent with with_app(path="", ...)) or adjust the middleware to explicitly treat an empty base_path as root.

marimo/_server/asgi.py

mscolnick requested review from Light2Dark and Copilot February 23, 2026 16:04

Copilot started reviewing on behalf of mscolnick February 23, 2026 16:05 View session

fixes

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mscolnick added the bug Something isn't working label Feb 23, 2026

Light2Dark reviewed Feb 23, 2026

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marimo/_server/asgi.py Outdated Show resolved Hide resolved

Light2Dark requested a review from dmadisetti February 23, 2026 18:30

fix: use removesuffix for cleaner .py suffix removal

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mscolnick merged commit f3bea81 into main Feb 26, 2026
49 of 84 checks passed

mscolnick deleted the ms/dynamic-dir branch February 26, 2026 16:56

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fix: handle `with_dynamic_directory` mounted at sub-path (#8322)#8434

fix: handle `with_dynamic_directory` mounted at sub-path (#8322)#8434
mscolnick merged 4 commits intomainfrom
ms/dynamic-dir

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		the parent framework strips the prefix from scope["path"] and adds it to
		scope["root_path"], causing the middleware to never match.

-    the parent framework strips the prefix from scope["path"] and adds it to
-    scope["root_path"], causing the middleware to never match.
+    the parent framework keeps the mount prefix in scope["path"] while also
+    setting scope["root_path"], causing the middleware's path matching logic
+    to not behave as originally expected.

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mscolnick commented Feb 23, 2026 •

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