aio-libs/aiohttp · error · WSHandshakeError

Invalid window size

Error message

Invalid window size

What it means

WSHandshakeError raised by ws_ext_parse() during the CLIENT side (isserver=False) of the permessage-deflate negotiation when the server's Sec-WebSocket-Extensions response advertises a client_max_window_bits value outside the valid 9-15 range. zlib cannot use wbits=8 and values above 15 are illegal, so aiohttp rejects the handshake rather than accepting a broken compression agreement. It is a hard handshake failure, not a warning.

Source

Thrown at aiohttp/_websocket/helpers.py:117

                    # Compress wbit 8 does not support in zlib
                    # If compress level not support,
                    # CONTINUE to next extension
                    if compress > 15 or compress < 9:
                        compress = 0
                        continue
                if match.group(1):
                    notakeover = True
                # Ignore regex group 5 & 6 for client_max_window_bits
                break
            else:
                if match.group(6):
                    compress = int(match.group(6))
                    # Group5 must match if group6 matches
                    # Compress wbit 8 does not support in zlib
                    # If compress level not support,
                    # FAIL the parse progress
                    if compress > 15 or compress < 9:
                        raise WSHandshakeError("Invalid window size")
                if match.group(2):
                    notakeover = True
                # Ignore regex group 5 & 6 for client_max_window_bits
                break
        # Return Fail if client side and not match
        elif not isserver:
            raise WSHandshakeError("Extension for deflate not supported" + ext.group(1))

    return compress, notakeover


def ws_ext_gen(
    compress: int = 15, isserver: bool = False, server_notakeover: bool = False
) -> str:
    # client_notakeover=False not used for server
    # compress wbit 8 does not support in zlib
    if compress < 9 or compress > 15:
        raise ValueError(

View on GitHub (pinned to c0ef574e29)

Solutions

1. Disable compression on the client to bypass the broken negotiation: await session.ws_connect(url, compress=0).
2. Wrap ws_connect in try/except WSHandshakeError and fall back to a non-compressed connection (retry with compress=0).
3. Fix the server/gateway so client_max_window_bits, if echoed, is in the 9-15 range or omitted entirely.
4. Report the non-compliant Sec-WebSocket-Extensions header to the server maintainer; capture the raw header to confirm the offending value.

Example fix

# before
ws = await session.ws_connect('wss://buggy.example/feed')  # WSHandshakeError: Invalid window size

# after
try:
    ws = await session.ws_connect('wss://buggy.example/feed')
except aiohttp.WSHandshakeError:
    ws = await session.ws_connect('wss://buggy.example/feed', compress=0)

from aiohttp import WSHandshakeError

try:
    ws = await session.ws_connect(url)
except WSHandshakeError as exc:
    # server advertised an invalid client_max_window_bits; retry uncompressed
    ws = await session.ws_connect(url, compress=0)

When it happens

Trigger: The server returns a header like 'Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits=8' (or 16, or any value <9 or >15) and the client called session.ws_connect(url) with compression enabled (compress=15 default). ws_ext_parse parses group(6) from _WS_EXT_RE, sees the out-of-range value, and raises.

Common situations: Talking to a buggy/non-RFC-7692 server or gateway that mangles the deflate extension params; a custom server echoing back the client's offered window bits incorrectly; testing against a mock server that hard-codes an invalid wbits. The error appears at await session.ws_connect(...) time.

Related errors

• Extension for deflate not supported{ext}
• Compress wbits must between 9 and 15, zlib does not support
• Invalid response status
• Invalid upgrade header
• Invalid connection header

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/d12a94b0d124567a.json. Report an issue: GitHub.