SeleniumHQ/selenium · error · BrowserError

-32700

-32700

Error message

Client received invalid JSON

What it means

Raised as a BrowserError (code -32700, the JSON-RPC parse error code) by CdpConnection._reader_task() when a message received from the CDP WebSocket fails to parse as JSON. The reader calls json.loads(message); on JSONDecodeError it constructs a BrowserError with code -32700, the message 'Client received invalid JSON', and the raw message data. This indicates the browser or intermediary sent malformed data over the WebSocket.

Source

Thrown at py/private/cdp.py:466

        Dispatches responses to commands and events to listeners.
        """
        global devtools
        if devtools is None:
            raise RuntimeError("CDP devtools module not loaded. Call import_devtools() first.")
        while True:
            try:
                message = await self.ws.get_message()
            except WsConnectionClosed:
                # If the WebSocket is closed, we don't want to throw an
                # exception from the reader task. Instead we will throw
                # exceptions from the public API methods, and we can quietly
                # exit the reader task here.
                break
            try:
                data = json.loads(message)
            except json.JSONDecodeError:
                raise BrowserError(
                    {
                        "code": -32700,
                        "message": "Client received invalid JSON",
                        "data": message,
                    }
                )
            logger.debug("Received message %r", data)
            if "sessionId" in data:
                session_id = devtools.target.SessionID(data["sessionId"])
                try:
                    session = self.sessions[session_id]
                except KeyError:
                    raise BrowserError(
                        {
                            "code": -32700,
                            "message": "Browser sent a message for an invalid session",
                            "data": f"{session_id!r}",
                        }

View on GitHub (pinned to aa36b38e69)

Solutions

1. Verify the browser and Selenium/CDP endpoint versions are compatible — a version mismatch can cause unexpected message formats.
2. If behind a proxy or load balancer, check WebSocket frame integrity and buffering settings.
3. Add error handling around the CDP connection to catch BrowserError and reconnect if the connection becomes corrupt.
4. Enable debug logging to capture the raw message data field for diagnosis.

Example fix

# before
async with conn:
    doc = await dom.get_document()  # BrowserError surfaces from reader

# after
try:
    async with conn:
        doc = await dom.get_document()
except BrowserError as e:
    if e.code == -32700:
        logger.error('Invalid JSON from browser: %s', e.data)
        # reconnect or report
        raise

from selenium.webdriver.common.bidi.cdp import BrowserError
try:
    async with conn:
        await dom.get_document()
except BrowserError as e:
    if e.code == -32700 and 'invalid JSON' in str(e):
        logger.error('Malformed WebSocket frame: %s', e.data)
        # reconnect or abort
    else:
        raise

When it happens

Trigger: The WebSocket receives a message that is not valid JSON (e.g. truncated, binary, or protocol-violating text). json.loads raises JSONDecodeError, caught and re-raised as BrowserError. This happens in the background reader task and propagates to the caller.

Common situations: A buggy or non-conformant browser/devtools endpoint sending malformed frames; a proxy or intermediary corrupting WebSocket messages; a connection issue causing partial/truncated frames; using a devtools protocol version mismatch that sends unexpected message formats.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• {fn_name}() must be called in a connection context.
• {fn_name}() must be called in a session context.
• CDP devtools module not loaded. Call import_devtools() first
• Unable to find url to connect to from capabilities
• interval must be a positive number

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/d0b4ebff5ca8faac. Report an issue: GitHub.