The New GTM Preview Mode

Google Tag Manager just had a BIG update—its preview mode got a major overhaul. And if you’ve taken a look at it, you’re probably full of questions.

What has changed? Will this affect my tracking? Is this actually a better system?

With GTM’s old preview mode entirely gone and replaced, you may feel like the pressure is on to learn the new preview mode. It’s especially important to be comfortable with it if you reference any outdated tutorials that use the old preview mode.

So what are the most important changes in GTM’s new preview mode?

All of these have important implications, so we’ll be discussing what these mean for you as a trackers. Let’s take a quick tour of the new setup. We’ll also give you some of our top tips to make full use of GTM preview mode’s new features.

If you learn the new system well, it will serve you far better than the old preview mode.

As always, we have a lot to cover. Let’s dive in!

What’s New?

With all of the new and rebranded features, we’ll need to be able to navigate the new system. Let’s start by learning how to use our new GTM preview mode, which will demonstrate the first big change.

Preview Mode Gets Its Own Tab

If you open your Google Tag Manager account, you’ll find the GTM interface is unchanged. Everything should look and feel the same until you click the Preview button in your topbar.

Google Tag Manager workspace with preview button highlighted

Instead of opening your website with a GTM console across the bottom, you’ll be directed to a new tab with Tag Assistant and asked to provide the URL for the site you want to debug.

With this update, the Google Tag Assistant has adopted a new role. Instead of a browser extension, the Tag Assistant is now a tool independent of cookies or extensions. It is now the central hub to debug all your different Tags.

So how do we use the new GTM preview mode? Well, we’ll set up our Tag Assistant, of course.

Entering the URL for the site that you want to attach or connect to the Tag Assistant. You can also include the debug signal in the URL. I would leave that checked, then click on Start.

Google Tag Assistant in a new tab and GTM Preview Mode popup with URL field and Start button highlighted

Next, Tag Assistant will open your site in a new tab. It’s an extra step at first, but you will have an option later for the Tag Manager to save your URLs so that you can select them later.

Now we see this little window down at the bottom of our site that says Debugger connected.

Website in GTM preview mode with Debugger connected popup and site icon highlighted

If you go back to the Tag Assistant tab, you will find confirmation that your Tag Assistant is connected to your site. With everything connected, you can click Continue to start debugging your Tags.

Tag Assistant Connected popup with continue button highlighted and website opened in new tab

You’ll find here that everything in the new preview window looks about the same as it did in the old preview console. Everything’s just bigger, and of course, it’s in a separate window. This is the feature that is a huge perk for GTM users.

For example, you can pull these tabs into separate monitors. If you have multiple screens, this makes it much easier to see everything that is going on both on your site and in your debugger. You can also just split the two windows on one screen, if your monitor is big enough. This is also especially handy for mobile debugging.

???? Top Tip: Debug for Mobile Users Mobile developers, rejoice! Keeping the debugger in a separate tab is a huge improvement when debugging for mobile users. The mobile view of your site is so small that the old preview console would obscure most of the page. This made both navigating and debugging way harder. But now with our debugger tucked neatly away in a different tab, we can see all that information quite nicely now.

Website in GTM preview mode in mobile view

Closing GTM Preview Mode

And with this new setup, there’s one more question: how do we exit preview mode from a separate window?

In the Tag Assistant window, you can simply click on the X in the top left corner.

Tag Assistant window with exit X highlighted

The Tag Assistant will confirm that you want to exit preview mode, which you can confirm by clicking Stop debugging. You can also check the box for Keep the domain [] enabled for debugging in order to reconnect anytime later.

Tag Assistant Stop debugging popup with Stop debugging button highlighted

After this, any and all tabs you were previewing should disconnect from your debugger. These should all reload and go back to normal.

Debugger Records Events Across Different Pageviews

If you check out the Tag Assistant debugger, you’ll notice some changes in the events menu on the left. In the old preview mode, we had a simple list of events tracked on the current pageview.

Now, we see different pageviews in these dropdown menus. Under each dropdown is a list of all the events recorded for that pageview. (You can expand and condense these by clicking the arrow next to the pageview.)

Tag Assistant window with pageview events in events menu highlighted

What makes this change so special? This record of multiple pageviews helps us get around some major inconveniences that we had with the old GTM preview mode.

For example, say you have a Tag that is triggered when a user clicks on a button that navigates them to a new page. With the old preview mode, you wouldn’t be able to see the Tag fire in your preview console because the events would disappear when you were sent to the new page. 

There were some workarounds, like opening the link in a new tab, but they were tedious and inelegant. With the new GTM preview mode, all of your events and Tags will be visible even if you navigate to a new page.

This is a huge improvement over the old GTM preview mode. Now, we can debug Tags across multiple pages. The Tag Assistant records all the pageviews (and their events) during your entire preview session, so you’ll always know what’s going on across your different pages.

???? Top Tip: Copy Text from the Data Layer In the old GTM preview mode, you couldn’t copy text from your preview console. This made it harder to quickly and accurately build Tags and variables that relied on information from the Variables and Data Layer tabs. Now, you can copy out of the preview mode and paste all kinds of values that you formerly needed to type out manually. Neat!

Event Data Layer in Tag Assistant with some of the script highlighted for copying

New Event Field: API Call

You’ll notice that the events for each pageview are numbered. Otherwise, most of these events and their data fields will look very familiar—at first.

Just like in the old preview console, you can check out the variables, the Data Layer, and any errors that may have occurred. But you’ll also see something new. At the top, there’s a field named API Call.

Tag Assistant window with API Call field highlighted

Formerly, we’ve referred to the timeline of events in the waterfall menu as an “events menu” (at least in our videos and blog posts). When a page loads, it shows which items have loaded and in what order. These items are essentially our API Calls.

The API Call field describes the script that loads for each call. In many cases, these will be Data Layer pushes that include the event names in the script.

For many GTM users, this isn’t a huge change. It’s really just extra information on how your tracking code itself is functioning. That said, I imagine our future guides will refer to an event in GTM as an API Call. It’s certainly helpful to have a definitive term for each script event.

Debug Multiple Containers at Once

If you’ve been peeking at my events menu in my debugger, you may have noticed that I have two API Calls for loading a GTM container. This isn’t a bug.

Tag Assistant events menu with two Container Loaded events highlighted

On this demo site, I’ve installed Google Tag Manager multiple times using both the Google Tag Manager for WordPress plugin by Thomas Geiger and a manual installation in my theme header file. With these separate installations, I’ve installed a couple of different GTM containers, each with a completely different set of tags for different tracking (and demonstration) purposes.

If you have multiple GTM containers installed on your site, then debugging just got a lot simpler. At the top of your Tag Assistant window, you have a drop down menu where you can choose any of your GTM or Gtag implementations to debug.

Tag Assistant GTM container dropdown highlighted with three container options

With the new dropdown menu, you can switch effortlessly between any of your Google tracking implementations without leaving preview mode. Thanks to this feature, you can run through some potential user actions and check your tags in all of your containers at once.

???? Top Tip: Debug Cross-Domain Tracking in One Window If you have Google Tag Manager installed on two linked domains, you can debug your cross-domain tracking all in one window. Initially, the domain will not be recognized and you won’t have a log of what’s happening on your second domain. But if you go to the Domains screen on your Tag Assistant, you can connect your two domains to start tracking them together. Then, you’d be able to go seamlessly from one domain to the next.

Debug in Multiple Tabs with the Tag Assistant Extension

Now that Google has released the new GTM preview mode under the name “Tag Assistant,” what does this mean for the Tag Assistant browser extension?

If you’ve been using Google Tag Manager or Google’s Global Site Tag for any length of time, you should already know about the Tag Assistant browser extension. This extension was always super helpful for debugging any Google tags.

Since GTM’s new preview mode is also called the Tag Assistant, you might be fearing that our beloved little extension has been made obsolete. On the contrary—it has now become part of a complex but awesome debugging ecosystem.

If you ever tried debugging on multiple tabs at once on the old system (maybe when tracking button clicks, for example?), you know that you had a separate preview console at the bottom of each tab. This meant flipping between tabs to check each tab’s individual Tags.

With your Tag Assistant extension enabled, go ahead and try opening up multiple tabs in preview mode. Everything shows up in your single Tag Assistant window! But how do you tell each tab’s events apart?

Check out this colorful set of icons—a blue diamond, a tiger emoji, and a yellow square. Each of these signals that the event is from a specific tab. You can now distinguish events from separate tabs all inside one Tag debugger system.

Tag Assistant with pageviews from multiple tabs highlighted, icons are a yellow square, a tiger emoji, and a blue diamond

To know which tab each icon belongs to, just open one of the tabs and check out the Debugger connected popup in the bottom right corner. The icon for that tab will display on the left side of the popup’s text. Nifty.

Website in GTM preview mode with blue diamond tab identifier icon highlighted

The icon shapes and colors are totally randomized and don’t mean anything except for which tab it identifies. So if one of your tabs has a green circle and another has a red square, don’t worry—it doesn’t mean that your system is broken or faulty. 

And while the Tag Assistant extension certainly makes the debugging process easier, you’re certainly not required to install it in order to debug your Tag system. This means that you don’t have to worry about being locked into the Chrome ecosystem if you use a different browser. And speaking of browsers… 

No More Cookies! Use Any Browser You Want

If you’re a Safari user, this may be your favorite update. GTM’s new preview mode has completely done away with cookies.

Why is this important? The old GTM preview mode relied on third-party cookies in order to track down your events. (This is what also limited your Summary to include only events from a single pageview.)

Now, lots of browsers block all third-party cookies by default, such as Safari and Firefox. You can also adjust the setting of most other browsers, including Chrome and Edge, to block third-party cookies.

This was a huge pain for GTM users on browsers that block third-party cookies. There were some workarounds, but it was a big inconvenience. In the new GTM preview mode, this hurdle has been resolved. The preview mode is also just more stable without the reliance on cookies.

???? Note: For now, you may still need to disable any third-party ad blockers or popup blockers to use the new GTM preview mode. Otherwise, your debugger may not connect correctly to your website.

Debugger popup could not connect to website, so disable third-party ad blockers


Overall, there are plenty of new things to get used to. You may love some of the changes and hate others. There will probably be some adjustments and debugging from Google over the next couple of weeks, but this is essentially the new GTM preview mode.

If you can get through some of the visual changes, you’ll find that most of the functionality is preserved. And thanks to events preserved across pageviews, the ability to debug multiple containers, and the elimination of third-party cookies, some functions are better than ever.

I wish you well in your new relationship with GTM preview mode, and I’d love to hear about your experience with it. What are your favorite new features? What do you miss about the old preview mode? Have you discovered any other changes that I missed here? Leave a comment below!

About the author: Julian Juenemann

Julian started and grew venture-backed startups with his unique 'data first' approach to Online Marketing. He then founded to help marketers, like him, the data-driven way of digital marketing.

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