Bug: Mounted asgi app unable to connect via websocket

[GH-maggio]

Description

While mounting a asgi app (marimo), websocket connections cannot be accepted.

environment: Win10, WSL2 - Rockylinux 9.5

URL to code causing the issue

https://github.com/GH-maggio/hello_marimo

MCVE

from litestar import Litestar, asgi
import marimo

server = marimo.create_asgi_app().with_app(
    path="/notebook1", root="./notebooks/notebook1.py"
)


get_notebook = asgi(path="/notebooks", is_mount=True)(server.build())


app = Litestar(
    route_handlers=[
        get_notebook,
    ]
)

Steps to reproduce

1. uv run litestar run
2. navigate via web browser to http://127.0.0.1:8000/notebooks/notebook1
3. See error in terminal

Screenshots

NA

Logs

 ┌──────────────────────────────┬──────────────────────┐
 │ Litestar version             │ 2.14.0               │
 │ Debug mode                   │ Disabled             │
 │ Python Debugger on exception │ Disabled             │
 │ CORS                         │ Disabled             │
 │ CSRF                         │ Disabled             │
 │ OpenAPI                      │ Enabled path=/schema │
 │ Compression                  │ Disabled             │
 └──────────────────────────────┴──────────────────────┘
 INFO:     Started server process [1394]
 INFO:     Waiting for application startup.
 INFO:     Application startup complete.
 INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
 INFO:     127.0.0.1:56740 - "GET / HTTP/1.1" 200 OK
 INFO:     127.0.0.1:56740 - "GET /favicon.ico HTTP/1.1" 404 Not Found
 INFO:     127.0.0.1:56750 - "GET /notebook1/ HTTP/1.1" 200 OK
 INFO:     ('127.0.0.1', 35512) - "WebSocket /notebook1/ws/?session_id=s_u9diol" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed
 INFO:     127.0.0.1:56750 - "GET /notebook1/public-files-sw.js/ HTTP/1.1" 404 Not Found
 INFO:     ('127.0.0.1', 35518) - "WebSocket /notebook1/ws/?session_id=s_u9diol" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed
 INFO:     ('127.0.0.1', 35520) - "WebSocket /notebook1/ws/?session_id=s_u9diol" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed
 INFO:     ('127.0.0.1', 35530) - "WebSocket /notebook1/ws/?session_id=s_u9diol" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed
 INFO:     ('127.0.0.1', 35538) - "WebSocket /notebook1/ws/?session_id=s_u9diol" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed
 INFO:     127.0.0.1:56750 - "GET /notebook2/ HTTP/1.1" 200 OK
 INFO:     ('127.0.0.1', 35544) - "WebSocket /notebook2/ws/?session_id=s_ujsdgw" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed
 INFO:     127.0.0.1:56750 - "GET /notebook2/public-files-sw.js/ HTTP/1.1" 404 Not Found
 INFO:     ('127.0.0.1', 44400) - "WebSocket /notebook2/ws/?session_id=s_ujsdgw" 403
 INFO:     connection rejected (403 Forbidden)
 INFO:     connection closed

Litestar Version

pyproject.toml

[project]
name = "hello-marimo"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.13"
dependencies = [
"fastapi[standard]>=0.115.7",
"litestar[standard]>=2.14.0",
"marimo>=0.10.19",
]

Platform

• Linux
• Mac
• Windows
• Other (Please specify in the description above)

[euri10]

I cant reproduce there's a bunch of statics that arent shipped, can you give a simpler MCVE ? :

INFO:     Started server process [696455]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:57296 - "GET /notebook1/ HTTP/1.1" 200 OK
INFO:     127.0.0.1:57296 - "GET /assets/gradient-yHQUC_QB.png/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57308 - "GET /assets/Lora-VariableFont_wght-B2ootaw-.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57312 - "GET /assets/PTSans-Regular-CxL0S8W7.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57322 - "GET /assets/PTSans-Bold-D9fedIX3.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57328 - "GET /assets/FiraMono-Regular-BTCkDNvf.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57336 - "GET /assets/FiraMono-Medium-DU3aDxX5.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57296 - "GET /assets/index-DfLILvXO.css/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57308 - "GET /assets/noise-60BoTA8O.png/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57312 - "GET /assets/index-C_IDebto.js/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57336 - "GET /assets/FiraMono-Bold-CLVRCuM9.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57312 - "GET /public-files-sw.js/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:57348 - "GET /manifest.json/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55268 - "GET /notebook1/ HTTP/1.1" 200 OK
INFO:     127.0.0.1:55268 - "GET /assets/gradient-yHQUC_QB.png/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55272 - "GET /assets/Lora-VariableFont_wght-B2ootaw-.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55278 - "GET /assets/PTSans-Regular-CxL0S8W7.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55288 - "GET /assets/PTSans-Bold-D9fedIX3.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55304 - "GET /assets/FiraMono-Regular-BTCkDNvf.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55306 - "GET /assets/index-DfLILvXO.css/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55268 - "GET /assets/FiraMono-Medium-DU3aDxX5.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55268 - "GET /assets/FiraMono-Bold-CLVRCuM9.ttf/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55306 - "GET /assets/index-C_IDebto.js/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55278 - "GET /assets/noise-60BoTA8O.png/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55272 - "GET /public-files-sw.js/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:55314 - "GET /manifest.json/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:59600 - "GET /notebook1/ HTTP/1.1" 200 OK
INFO:     127.0.0.1:59600 - "GET /assets/gradient-yHQUC_QB.png/ HTTP/1.1" 404 Not Found
INFO:     127.0.0.1:59602 - "GET /assets/Lora-VariableFont_wght-B2ootaw-.ttf/ HTTP/1.1" 404 Not Found

[GH-maggio]

@euri10 slimmed down the MVCE, if possible can you see the FastAPI version that is in the repo:

uv run fastapi dev

      INFO   Will watch for changes in these directories: ['/home/cmaggio/repos/hello_marimo']
      INFO   Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
      INFO   Started reloader process [1535] using WatchFiles
      INFO   Started server process [1537]
      INFO   Waiting for application startup.
      INFO   Application startup complete.
      INFO   127.0.0.1:42232 - "GET /notebooks/notebook1/ HTTP/1.1" 200
      INFO   ('127.0.0.1', 42240) - "WebSocket /notebooks/notebook1/ws?session_id=s_oddu05"
      INFO   connection open
      INFO   127.0.0.1:42232 - "POST /notebooks/notebook1/api/kernel/instantiate HTTP/1.1" 200
      INFO   127.0.0.1:42232 - "GET /notebooks/notebook1/public-files-sw.js HTTP/1.1" 200

[GH-maggio]

I'm not sure if the root path is being passed to the ASGI marimo app /notebooks/notebook1

[euri10]

add a / to your ws route seems to solve it
seems like an issue with starlette and forward slash ?

@router.websocket("/ws/")
async def websocket_endpoint(
    websocket: WebSocket,
) -> None:
    """
    responses:
        200:
            description: Websocket endpoint
    """

[euri10]

usually with ws when you get a 403 it's a pretty good indication the route is not found, hence how I found the reason.

more on starlette trailing slashes: encode/starlette#869

[GH-maggio]

@euri10 thanks for investigating, wasn't aware litestar used starlette as the backend.

[euri10]

@euri10 thanks for investigating, wasn't aware litestar used starlette as the backend.

mmmm litestar was using starlette a while ago, though now it's completely seperated

you wouldn't have that bug using litestar as a base, just saying