Mount path is applied twice

[aabrodskiy]

First Check

• I added a very descriptive title here.
• This is not a Q&A. I am sure something is wrong with NiceGUI or its documentation.
• I used the GitHub search to find a similar issue and came up empty.

Example Code

fastapi_app = FastAPI()

ui.run_with(
    app=fastapi_app,
    mount_path="/tstudio",
    favicon="⚒",
    reconnect_timeout=10,
)

Description

It was working in 2.17.0 (and starlette 0.45.3, fastapi 0.115.12), but now (with 2.24.2 and starlette 0.48.0, fastapi 0.118.0) I get the errors below.

NiceGUI is applying the mount_path twice in proxying. It could be that the underlying starlette or fastapi changed the way the apply the root path and NiceGUI just applying once again on top.

Nothing changed around it in the k8s configuration, so it must be some recent change in either starlette or nicegui or fastapi. Unfortunately I had to upgrade them all at once, so can't point to either of them now...

>> [WARNING: 2025-10-14 18:36:23,281: nicegui: nicegui.py:156 - _exception_handler_404()]: http://some.domain/tstudio/tstudio/_nicegui/2.24.2/static/nicegui.css not found <<
>> [WARNING: 2025-10-14 18:36:23,283: nicegui: nicegui.py:156 - _exception_handler_404()]: http:/some.domain/tstudio/tstudio/_nicegui/2.24.2/static/socket.io.min.js not found 

NiceGUI Version

2.24.2

Python Version

3.13

Browser

Chrome

Operating System

macOS

Additional Context

No response

[ibirothe]

Just a thought: Could that be related to app.py line 220 where "/" is stripped from the path?

nicegui/nicegui/app/app.py

Line 220 in bf00c7d

@self.get(url_path.rstrip('/') + '/{path:path}') # NOTE: prevent double slashes in route pattern

I am keen to work on a fix. @falkoschindler Feel free to assign me.

[evnchn]

@ibirothe Thanks for the report.

First off, any ways to recreate this locally without all the k8s and stuff? I can't manage to do it....

Second, for verifying whether a line of code is the culprit, I must share git blame. It really is your friend.

For example, the code snippet you attached above, by git blame we can see it is changed in bc17b1b, which is in v2.22.0 onwards.

So you can then check if your code worked in v2.21.1 and broke on v2.22.0.

• If so it narrows things down to whatever we changed in https://github.com/zauberzeug/nicegui/releases/tag/v2.22.0
• If not then you can bisect your way into the breaking version and we can look at the changelog there.

Do keep me updated, and add oil!