SeleniumHQ/selenium · error · WebDriverException

The command's generator function did not exit when expected!

Error message

The command's generator function did not exit when expected!

What it means

Internal assertion in _deserialize_result. BiDi command objects are generator-based: _serialize_command does next(command) for the request, and _deserialize_result does command.send(result) expecting the generator to terminate (StopIteration) and return its parsed value. If the generator yields again instead of returning, this error fires - meaning the command implementation is structurally wrong.

Source

Thrown at py/selenium/webdriver/remote/websocket_connection.py:174

        return id(_callback)

    on = add_callback

    def remove_callback(self, event, callback_id):
        event_name = event.event_class
        if event_name in self.callbacks:
            for callback in self.callbacks[event_name]:
                if id(callback) == callback_id:
                    self.callbacks[event_name].remove(callback)
                    return

    def _serialize_command(self, command):
        return next(command)

    def _deserialize_result(self, result, command):
        try:
            _ = command.send(result)
            raise WebDriverException("The command's generator function did not exit when expected!")
        except StopIteration as exit:
            return exit.value

    def _start_ws(self):
        def on_open(ws):
            self._started = True

        def on_message(ws, message):
            self._process_message(message)

        def on_error(ws, error):
            logger.debug(f"error: {error}")
            ws.close()

        def run_socket():
            if self.url.startswith("wss://"):
                self._ws.run_forever(sslopt={"cert_reqs": CERT_NONE}, suppress_origin=True)
            else:

View on GitHub (pinned to aa36b38e69)

Solutions

1. Report upstream - this indicates a defect in the command implementation, not your inputs.
2. If you wrote a custom command, ensure its generator yields exactly once then returns the parsed result.
3. Pin/upgrade the selenium version where the command works correctly.

Example fix

# custom command generator - before (yields twice)
# def cmd():
#     payload = yield {...}
#     extra = yield {...}   # wrong
#     return parse(payload)

# after (yield once, then return)
def cmd():
    payload = yield {"method": "x", "params": {}}
    return parse(payload)

try:
    result = conn.execute(cmd)
except WebDriverException as e:
    if "generator function did not exit" in str(e):
        log.error("Defective BiDi command implementation: %r", cmd)
        raise

When it happens

Trigger: A BiDi command class whose generator has more than one yield, or a custom/monkeypatched command that does not `return` after consuming the result. Not triggered by ordinary inputs.

Common situations: Almost exclusively a Selenium-internal defect, or a user who subclassed/extended the BiDi command generators incorrectly.

Related errors

• interval must be a positive number
• Pattern must be an instance of UrlPattern. Received: '${patt
• Pattern must be an instance of UrlPattern. Received:'${patte
• Pattern must be an instance of String. Received:'${pattern}'
• Pass in a CaptureScreenshotParameters object. Received: ${ca

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/5e3bb8d6ac7c7146. Report an issue: GitHub.