[5.x]: ChatGPT can't access internal pages

After upgrading to Craft 5 accessing internal pages of the site with ChatGPT no longer works, i.e. asking "Please analyse … " causes ChatGPT to report a 500 internal server error. ChatGPT can still access the site's home page though. And other models like Gemini and Claude can still access pages.

The client has reported that before the Craft 5 upgrade ChatGPT was able to access all pages fine.

Server logs show the following for ChatGPT requests:

HTTP_USER_AGENT: ChatGPT-Browser/1.0
REQUEST_METHOD: HEAD
web.ERROR: yii\web\BadRequestHttpException: Unable to verify your data submission
Trace:
#0 /vendor/craftcms/cms/src/web/Controller.php(221): yii\web\Controller->beforeAction()
#1 ...

Terminal curl tests from my local machine succeed though, e.g.

curl -I https://site-domain.com.au/internal-page -A "ChatGPT-Browser/1.0"

HTTP/2 200 
referrer-policy: no-referrer-when-downgrade
strict-transport-security: max-age=15552000
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-xss-protection: 1; mode=block
cache-control: no-store
x-robots-tag: all
link: <https://site-domain.com.au/internal-page>; rel='canonical'
x-powered-by: SEOmatic
content-type: text/html; charset=UTF-8
date: Tue, 19 Aug 2025 04:27:33 GMT
server: LiteSpeed
alt-svc: h3=":443"; ma=2592000, h3-29=":443"; ma=2592000, h3-Q050=":443"; ma=2592000, h3-Q046=":443"; ma=2592000, h3-Q043=":443"; ma=2592000, quic=":443"; ma=2592000; v="43,46"

Has something changed between Craft 4 and 5 (or one of the installed plugins) which would cause this?

Craft CMS version

5.8.12

PHP version

8.2.29

Operating system and version

No response

Database type and version

No response

Image driver and version

No response

Installed plugins and versions

ckeditor
cp-nav
craft-blitz
craft-freeform
craft-imageoptimize
craft-position-fieldtype
craft-retour
craft-seomatic
craft-sherlock
craft-sprig
environment-label
feed-me
field-manager
navigation
oembed
phpdotenv

Start with the logged request path in vendor/craftcms/cms/src/web/Controller.php at line 221 and compare the failing ChatGPT-Browser HEAD request with the successful curl request. Review the listed Craft 5 plugins for request-handling differences. Done means identifying why internal pages return a 500 for ChatGPT while the home page and other model requests continue to work.