aio-libs/aiohttp · error · HTTPBadRequest

No WebSocket UPGRADE hdr: {headers.get(hdrs.UPGRADE)} Can "

Error message

No WebSocket UPGRADE hdr: {headers.get(hdrs.UPGRADE)}
 Can "Upgrade" only to "WebSocket".

What it means

Error "No WebSocket UPGRADE hdr: {headers.get(hdrs.UPGRADE)} Can "Upgrade" only to "WebSocket"." thrown in aio-libs/aiohttp.

Source

Thrown at aiohttp/web_ws.py:275

    async def prepare(self, request: BaseRequest) -> AbstractStreamWriter:
        # make pre-check to don't hide it by do_handshake() exceptions
        if self._payload_writer is not None:
            return self._payload_writer

        protocol, writer = self._pre_start(request)
        payload_writer = await super().prepare(request)
        assert payload_writer is not None
        self._post_start(request, protocol, writer)
        await payload_writer.drain()
        return payload_writer

    def _handshake(
        self, request: BaseRequest
    ) -> tuple["CIMultiDict[str]", str | None, int, bool]:
        headers = request.headers
        if "websocket" != headers.get(hdrs.UPGRADE, "").lower().strip():
            raise HTTPBadRequest(
                text=(
                    f"No WebSocket UPGRADE hdr: {headers.get(hdrs.UPGRADE)}\n Can "
                    '"Upgrade" only to "WebSocket".'
                )
            )

        if not request._message.upgrade:
            raise HTTPBadRequest(
                text=f"No CONNECTION upgrade hdr: {headers.get(hdrs.CONNECTION)}"
            )

        # find common sub-protocol between client and server
        protocol: str | None = None
        if hdrs.SEC_WEBSOCKET_PROTOCOL in headers:
            req_protocols = [
                str(proto.strip())
                for proto in headers[hdrs.SEC_WEBSOCKET_PROTOCOL].split(",")
            ]

View on GitHub (pinned to c0ef574e29)

When it happens

Trigger: Thrown at aiohttp/web_ws.py:275 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/c065460ba867d6f6.json. Report an issue: GitHub.