"Server disconnected without sending a response" — a keep-alive race¶

An HTTP client that polls or otherwise makes repeated requests with idle gaps sometimes fails, intermittently, with something like:

• Python / httpx: httpx.RemoteProtocolError: Server disconnected without sending a response.
• Dart / package:http: ClientException: Connection closed before full header was received

The request never reaches the server. It fails at the transport layer, and it's timing-dependent — the same call works most of the time.

Cause (not your code, not a server bug)¶

HTTP clients keep a connection pool and reuse keep-alive connections. Servers close idle keep-alive connections after a timeout (uvicorn defaults to ~5s). If your next request happens to reuse a connection the server just closed, the send fails. Polling loops with backoff are the classic trigger: the gap between polls grows past the server's keep-alive timeout, so the pooled connection is dead by the time you reuse it.

Nothing is actually wrong — it's inherent to keep-alive + connection pooling.

Fix: retry idempotent requests on transient transport errors¶

A retry simply opens a fresh connection and succeeds. Retry only the transport-level failures; never swallow real HTTP status errors (4xx/5xx).

Python (httpx) — note httpx.HTTPStatusError is not a TransportError, so it still propagates:

for attempt in range(3):
    try:
        r = await client.get(url)
        r.raise_for_status()
        return r.json()
    except httpx.TransportError:          # RemoteProtocolError, ConnectError, …
        if attempt == 2:
            raise
        await asyncio.sleep(0.5 * (attempt + 1))

Dart (package:http):

// retry on http.ClientException / SocketException / TimeoutException