RuntimeError instead of ClientDisconnected when client quickly disconnects in websockets-sansio

[MrNaif2018]

When testing the new 0.44.0 release I accidentally found another bug.
Basically my websocket endpoint, inside on_connect of starlette's WebSocketEndpoint runs some checks against db to verify if it's an existing object and I can start redis pub/sub for this websocket.
And if a client opens a tab, the code runs validation, client closes the tab, then the code finishes validation, calls websocket.accept(), but uvicorn raises RuntimeError:

2026-04-06 12:02:24.495047 UTC [error] Exception in ASGI application
 [uvicorn.error]
Traceback (most recent call last):
  File "/app/.venv/lib/python3.12/site-packages/uvicorn/protocols/websockets/websockets_sansio_impl.py", line 343, in run_asgi
    result = await self.app(self.scope, self.receive, self.send)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/api/bootstrap.py", line 101, in __call__
    await Starlette.__call__(self, scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 416, in _sentry_patched_asgi_app
    return await middleware(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/asgi.py", line 178, in _run_asgi3
    return await self._run_app(scope, receive, send, asgi_version=3)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/asgi.py", line 347, in _run_app
    raise exc from None
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/asgi.py", line 336, in _run_app
    return await self.app(
           ^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/applications.py", line 107, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 151, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/opentelemetry/instrumentation/asgi/__init__.py", line 811, in __call__
    await self.app(scope, otel_receive, otel_send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 151, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/opentelemetry/instrumentation/fastapi/__init__.py", line 307, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/prometheus_fastapi_instrumentator/middleware.py", line 127, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/api/ioc/starlette.py", line 48, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 103, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/cors.py", line 79, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/api/bootstrap.py", line 69, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 103, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 303, in _sentry_exceptionmiddleware_call
    await old_call(self, scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 63, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/app/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/starlette.py", line 164, in _create_span_call
    return await old_call(app, scope, receive, send, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/starlette/routing.py", line 716, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/starlette/routing.py", line 736, in app
    await route.handle(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/starlette/routing.py", line 364, in handle
    await self.app(scope, receive, send)
  File "/app/.venv/lib/python3.12/site-packages/starlette/endpoints.py", line 68, in dispatch
    await self.on_connect(websocket)
  File "<dishka_func_on_connect>", line 3, in dishka_func_on_connect
    return await func_on_connect(*args, **kwargs, container=await container.get(AsyncContainer, ''), redis_pool=await container.get(Redis, ''), auth_service=await container.get(AuthService, ''))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/api/views/websocket.py", line 76, in on_connect
    await websocket.accept()
  File "/app/.venv/lib/python3.12/site-packages/starlette/websockets.py", line 110, in accept
    await self.send({"type": "websocket.accept", "subprotocol": subprotocol, "headers": headers})
  File "/app/.venv/lib/python3.12/site-packages/starlette/websockets.py", line 76, in send
    await self._send(message)
  File "/app/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 39, in sender
    await send(message)
  File "/app/.venv/lib/python3.12/site-packages/opentelemetry/instrumentation/asgi/__init__.py", line 1026, in otel_send
    await send(message)
  File "/app/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/asgi.py", line 329, in _sentry_wrapped_send
    return await send(event)
           ^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.12/site-packages/uvicorn/protocols/websockets/websockets_sansio_impl.py", line 448, in send
    raise RuntimeError(
RuntimeError: Expected ASGI message 'websocket.send' or 'websocket.close', but got 'websocket.accept'.

It fails here:

uvicorn/uvicorn/protocols/websockets/websockets_sansio_impl.py

Line 448 in edb54c4

raise RuntimeError(

Here's a minimal reproducer script:

Details

"""
Reproducer for: websockets-sansio raises RuntimeError when client disconnects
before websocket.accept.

Run:
    python reproduce_issue.py

Expected (broken — websockets-sansio):
    ERROR:    Exception in ASGI application
    RuntimeError: Expected ASGI message 'websocket.send' or 'websocket.close',
                  but got 'websocket.accept'.
"""

from __future__ import annotations

import asyncio

import httpx
from starlette.endpoints import WebSocketEndpoint
from starlette.routing import WebSocketRoute
from starlette.websockets import WebSocket

import uvicorn


class SlowAcceptEndpoint(WebSocketEndpoint):
    encoding = "text"

    async def on_connect(self, websocket: WebSocket) -> None:
        # Simulate slow async work (auth, DB lookup, etc.)
        # The client will disconnect during this sleep.
        await asyncio.sleep(0.5)
        await websocket.accept()

    async def on_receive(self, websocket: WebSocket, data: str) -> None:
        await websocket.send_text(f"echo: {data}")


app = WebSocketRoute("/ws", SlowAcceptEndpoint)


async def test_ws_impl(ws: str) -> bool:
    """Returns True if the implementation handled disconnect gracefully."""
    print(f"\n{'=' * 60}")
    print(f"Testing ws={ws!r}")
    print("=" * 60)

    config = uvicorn.Config(
        app=app,
        host="127.0.0.1",
        port=8765,
        ws=ws,
        log_level="info",
    )
    server = uvicorn.Server(config)
    serve_task = asyncio.create_task(server.serve())

    while not server.started:
        await asyncio.sleep(0.05)

    # Send a WebSocket upgrade request, then disconnect via timeout
    async with httpx.AsyncClient() as client:
        try:
            await client.get(
                "http://127.0.0.1:8765/ws",
                headers={
                    "upgrade": "websocket",
                    "connection": "upgrade",
                    "sec-websocket-version": "13",
                    "sec-websocket-key": "dGhlIHNhbXBsZSBub25jZQ==",
                },
                timeout=0.2,
            )
        except httpx.ReadTimeout:
            pass  # Expected — we just want to trigger the upgrade then disconnect

    # Give the server time to hit the bug
    await asyncio.sleep(1.0)

    server.should_exit = True
    await serve_task
    return True


async def main() -> None:
    for ws in ("websockets", "websockets-sansio"):
        await test_ws_impl(ws)


if __name__ == "__main__":
    asyncio.run(main())

Claude suggests this fix:

But I'm not so sure about that

[MrNaif2018]

cc @Kludex
Sorry for the ping, should I attempt to create a PR with fix for this one, or you will be able to? To not have 2 different PRs again 😅