What is the _gtm_debug cookie?
_gtm_debug is created by the Google Tag Manager (GTM) script when an editor opens the site with Preview mode enabled directly from tagmanager.google.com. It does not appear for regular site visitors — only in the browser session used to test the container.
It is saved in sessionStorage rather than a classic cookie, so it only persists in that specific tab for the duration of the debug session.
What data it stores
The value signals that the browser is in a GTM preview session, enabling the debug console that shows which tags, triggers, and variables fire for each event.
It contains no personal data, no long-term tracking identifiers, and is not sent to third-party servers for profiling — it is used purely locally, in the editor's browser, to power the GTM debug panel.
What it is used for
Its purpose is strictly technical: it lets a developer or marketer verify that tags in the container (Google Analytics, Ads, Meta Pixel, etc.) fire correctly before publishing changes live.
For the site owner, this helps troubleshoot GTM configuration without affecting real visitors, who never see this entry.
Does it require consent?
CookieFix classifies _gtm_debug as strictly necessary (functional), since it only appears during internal testing sessions, does not track visitors, and serves no statistics or marketing purpose.
- No consent is required under ePrivacy/GDPR, as it has no tracking role toward the general public.
- Recommended: still list it in the cookie policy, noting it only appears in GTM preview mode.
- Site owners must still make sure the actual tags fired through the container (analytics, marketing) are correctly classified and blocked until consent.
How to block or delete _gtm_debug
Visitors can clear this sessionStorage entry by clearing the site's browsing data or simply closing the tab/browser, which automatically ends the debug session.
Since it has no impact on real users, it does not need to be blocked by a CMP like CookieFix — automatic blocking should target the actual tags (analytics, marketing) fired through GTM, not this internal debug entry.
Frequently asked questions
It's a sessionStorage entry created by Google Tag Manager only when someone previews the container in Preview/Debug mode, not for regular visitors.
No, it only exists in the browser session used for testing and collects no data about the general audience.
No, it's considered strictly necessary/functional, but other tags fired through GTM must be assessed separately.
It lasts for the browser session only and is automatically removed when the tab or window is closed.