Based on this issue #35632 raised i believe the issue is not with whitespace header validation since the logic works fine coz the server detects the malformed request and queues a 400 reply as expected.
The problem is how the server closes the connection afterwards. It queues the reply and closes the socket right away sometimes before it has finished reading everything the client had sent. So when socket is closed while there's still unread data sitting in the kernel's receive buffer, Windows responds with a TCP RST. That reset just throws away the reply that was already on its way out, so the client never gets to read the 400 and instead sees the connection aborted.
So my LLM Buddie and I realized that we should change how the server closes a connection after a parse/size error so the client always gets a chance to read the response first by queuing the 400/413 response with Connection: close.
Then, waits until the response has actually left the send buffer. Then, Half-close only the write side (shutdown(SHUT_WR/SD_SEND)) so the client can still finish reading while the server stops sending. Afterwards, Keep draining whatever the client is still sending until it hits EOF or a socket error and only force the connection closed after a 1s fallback timeout, for a client that never finishes.
