unclecode/crawl4ai · error · TypeError

Hook {hook_type} must return an instance of webdriver.Chrome

Error message

Hook {hook_type} must return an instance of webdriver.Chrome or None.

What it means

Error "Hook {hook_type} must return an instance of webdriver.Chrome or None." thrown in unclecode/crawl4ai.

Source

Thrown at crawl4ai/legacy/crawler_strategy.py:175

        if kwargs.get("cookies"):
            for cookie in kwargs.get("cookies"):
                self.driver.add_cookie(cookie)

    def set_hook(self, hook_type: str, hook: Callable):
        if hook_type in self.hooks:
            self.hooks[hook_type] = hook
        else:
            raise ValueError(f"Invalid hook type: {hook_type}")

    def execute_hook(self, hook_type: str, *args):
        hook = self.hooks.get(hook_type)
        if hook:
            result = hook(*args)
            if result is not None:
                if isinstance(result, webdriver.Chrome):
                    return result
                else:
                    raise TypeError(
                        f"Hook {hook_type} must return an instance of webdriver.Chrome or None."
                    )
        # If the hook returns None or there is no hook, return self.driver
        return self.driver

    def update_user_agent(self, user_agent: str):
        self.options.add_argument(f"user-agent={user_agent}")
        self.driver.quit()
        self.driver = webdriver.Chrome(service=self.service, options=self.options)
        self.driver = self.execute_hook("on_user_agent_updated", self.driver)

    def set_custom_headers(self, headers: dict):
        # Enable Network domain for sending headers
        self.driver.execute_cdp_cmd("Network.enable", {})
        # Set extra HTTP headers
        self.driver.execute_cdp_cmd("Network.setExtraHTTPHeaders", {"headers": headers})

    def _ensure_page_load(self, max_checks=6, check_interval=0.01):

View on GitHub (pinned to 7e80152142)

Solutions

1. Make the hook return the webdriver.Chrome instance it configured, or None.
2. Do not return arbitrary values from driver hooks.

Example fix

def hook(driver):
    driver.execute_cdp_cmd(...)
    return driver

When it happens

Trigger: Thrown at crawl4ai/legacy/crawler_strategy.py:175 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of unclecode/crawl4ai@7e80152142 (2026-08-14). Data as JSON: /api/errors/11aa02c2baefeb43. Report an issue: GitHub.