Only a minor issue. Does not cause problems in production applications.

Terminal

ERROR: Exception in ASGI application
Traceback (most recent call last):
File "/usr/local/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
result = await app( # type: ignore[func-returns-value]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in call
return await self.app(scope, receive, send)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/tux/tremolo/example.py", line 12, in app
await send({
File "/usr/local/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 517, in send
raise RuntimeError(msg % message_type)
RuntimeError: Expected ASGI message 'http.response.body', but got 'http.response.start'.

Browser

< HTTP/1.1 200 OK
< date: Thu, 06 Nov 2025 02:37:23 GMT
< server: uvicorn
< content-type: text/plain
< transfer-encoding: chunked
<

• transfer closed with outstanding read data remaining
• closing connection #0
  curl: (18) transfer closed with outstanding read data remaining

Code

async def app(scope, receive, send):
    assert scope['type'] == 'http'

    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            (b'content-type', b'text/plain'),
        ]
    })
    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            (b'content-type', b'text/plain'),
        ]
    })

Running uvicorn 0.38.0 with CPython 3.12.11 on Linux

Thanks.