reCAPTCHA v2 Callback Mechanism: How Callbacks Work and How to Trigger Them

When a user completes a reCAPTCHA v2 challenge, Google calls a JavaScript callback function defined by the site. This callback typically enables a submit button, validates the form, or sends an AJAX request. After injecting a solved token from CaptchaAI, you need to trigger the same callback — otherwise the form won't recognize that the CAPTCHA was solved.

How callbacks work

The site defines a callback in the reCAPTCHA widget:

<div class="g-recaptcha"
     data-sitekey="6Le-SITEKEY"
     data-callback="onCaptchaSuccess"
     data-expired-callback="onCaptchaExpired">
</div>

<script>
function onCaptchaSuccess(token) {
  document.getElementById('submit-btn').disabled = false;
  document.getElementById('captcha-token').value = token;
}
</script>

When the user solves the CAPTCHA, Google's JavaScript calls onCaptchaSuccess(token). The function receives the token string as its only argument.

Finding the callback function

Method 1: Check data-callback attribute

// In browser console
const widget = document.querySelector('.g-recaptcha');
const callbackName = widget?.getAttribute('data-callback');
console.log('Callback:', callbackName);

Method 2: Check grecaptcha internals

Some sites use grecaptcha.render() with a callback option instead of the data-callback attribute:

// Search page source for grecaptcha.render
document.querySelectorAll('script:not([src])').forEach(s => {
  if (s.textContent.includes('grecaptcha.render')) {
    console.log(s.textContent.match(/callback\s*:\s*(\w+)/)?.[1]);
  }
});

Method 3: Intercept the callback registration

Run this in DevTools before the page loads (Sources → Snippets):

const origRender = grecaptcha.render;
grecaptcha.render = function(container, params) {
  console.log('Render callback:', params.callback);
  console.log('Expired callback:', params['expired-callback']);
  return origRender.apply(this, arguments);
};

Triggering the callback after token injection

Python (Selenium)

import requests
import time
from selenium import webdriver
from selenium.webdriver.common.by import By

API_KEY = "YOUR_API_KEY"
driver = webdriver.Chrome()
driver.get("https://example.com/login")

# Extract sitekey and callback
sitekey = driver.find_element(
    By.CSS_SELECTOR, ".g-recaptcha"
).get_attribute("data-sitekey")

callback = driver.find_element(
    By.CSS_SELECTOR, ".g-recaptcha"
).get_attribute("data-callback")

# Solve with CaptchaAI
resp = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY,
    "method": "userrecaptcha",
    "googlekey": sitekey,
    "pageurl": driver.current_url,
    "json": "1",
}).json()
task_id = resp["request"]

token = None
for _ in range(24):
    time.sleep(5)
    result = requests.get("https://ocr.captchaai.com/res.php", params={
        "key": API_KEY, "action": "get", "id": task_id, "json": "1"
    }).json()
    if result["status"] == 1:
        token = result["request"]
        break

# Inject token into textarea
driver.execute_script("""
    document.querySelector('textarea[name="g-recaptcha-response"]').value = arguments[0];
""", token)

# Trigger the callback
if callback:
    driver.execute_script(f"window['{callback}'](arguments[0]);", token)
    print(f"Triggered callback: {callback}")
else:
    # Fallback: try ___grecaptcha_cfg
    driver.execute_script("""
        try {
            var widgetId = Object.keys(___grecaptcha_cfg.clients)[0];
            var callback = ___grecaptcha_cfg.clients[widgetId].aa.l.callback;
            if (typeof callback === 'function') callback(arguments[0]);
        } catch(e) {}
    """, token)
    print("Triggered callback via ___grecaptcha_cfg")

JavaScript (Puppeteer)

const puppeteer = require('puppeteer');

// After solving and getting the token...
await page.evaluate((token, callbackName) => {
  // Set textarea value
  const textarea = document.querySelector(
    'textarea[name="g-recaptcha-response"]'
  );
  textarea.value = token;
  textarea.style.display = 'block'; // sometimes hidden

  // Trigger callback
  if (callbackName && typeof window[callbackName] === 'function') {
    window[callbackName](token);
    console.log(`Called ${callbackName}()`);
  } else {
    // Fallback: search grecaptcha config
    try {
      const clients = ___grecaptcha_cfg.clients;
      const widgetId = Object.keys(clients)[0];
      const cb = clients[widgetId]?.aa?.l?.callback;
      if (typeof cb === 'function') cb(token);
    } catch (e) {}
  }
}, token, callbackName);

Sites without data-callback

Some sites don't use a callback. Instead, they check grecaptcha.getResponse() when the form is submitted. For these sites, override the function:

driver.execute_script("""
    const token = arguments[0];
    document.querySelector('textarea[name="g-recaptcha-response"]').value = token;
    // Override getResponse to return the token
    if (typeof grecaptcha !== 'undefined') {
        grecaptcha.getResponse = function() { return token; };
    }
""", token)

# Then submit the form normally
driver.find_element(By.CSS_SELECTOR, "form").submit()

Expired callbacks

Some sites define data-expired-callback to disable the submit button when the token expires. If your token is close to expiration, this callback might fire and re-lock the form. Solve close to submission time to avoid this.

// Check for expired callback
const expiredCallback = document.querySelector('.g-recaptcha')
  ?.getAttribute('data-expired-callback');
console.log('Expired callback:', expiredCallback);

Troubleshooting

FAQ

What if there's no callback at all?

Some sites only check the token on the server side when the form is submitted. In that case, setting the g-recaptcha-response textarea value and submitting the form is sufficient.

Can the callback function name change between page loads?

Yes, on some sites. Always extract the callback name dynamically from the DOM rather than hardcoding it.

Does reCAPTCHA v3 use the same callback mechanism?

No. reCAPTCHA v3 uses grecaptcha.execute() which returns a Promise. There's no visible widget or data-callback attribute.

Solve reCAPTCHA v2 with proper callback handling using CaptchaAI

Get your API key at captchaai.com.

Related guides

• reCAPTCHA v2 Invisible: Trigger Detection
• Extracting reCAPTCHA Parameters from Page Source
• Handling Multiple CAPTCHAs on a Single Page