unclecode/crawl4ai · error · TypeError

Hook must return an instance of webdriver.Chrome or None.

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.

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.

Appendix: 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):

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