Guide · Advertising and data
Third-Party Cookies: What Chrome's First Test Means
Third-party cookies are now switched off by default for about 1% of Chrome users, a test that Google began on 4 January 2024. For most agencies, nothing breaks this week. But the test is the first real step toward a web where ad tracking across sites no longer works in the biggest browser. This guide explains what changed, who is affected and what to do now.
What Chrome changed on 4 January
Google calls the feature Tracking Protection. It limits cross-site tracking by blocking websites from using third-party cookies by default. Google began the test on 4 January 2024 and said it could take several days to reach the full 1% of Chrome browsers.
Chrome picks the users at random. People in the group are told when they open Chrome on a computer or an Android phone. Chrome counts each browser profile folder as its own client, so one person may be counted more than once.
This is a test, not the end of third-party cookies. The other 99% of Chrome users keep them for now.
Sources: Google, The next step toward phasing out third-party cookies in Chrome; Google Privacy Sandbox, Third-party cookies restricted by default for 1% of Chrome users
What a third-party cookie is, and why it matters
A cookie is a small file a website saves in your browser. A first-party cookie comes from the site you are visiting. It keeps you logged in and remembers your basket.
A third-party cookie comes from a different company whose code runs on that page, such as an ad network or an analytics tool. Because the same company sits on many sites, it can follow one browser from page to page. That is how many retargeting ads and cross-site reports work.
Google also notes that sites use these cookies for things beyond ads, such as sign-in and embedded content. That is why a block can break some pages.
Sources: Google, The next step toward phasing out third-party cookies in Chrome; Google Privacy Sandbox, Third-party cookies restricted by default for 1% of Chrome users
What users and developers will see
In the test group, an eye icon shows in the address bar on a site that tries to use third-party cookies. A click opens a switch that lets the person allow them again for that site. Chrome can also offer this itself when it detects a broken page, for example when someone refreshes the same page again and again.
Developers get warnings too. Browsers outside the test see DevTools warnings about cross-site cookie issues. Browsers inside the test see errors when a third-party cookie is blocked. To try it yourself, use Chrome 121 or newer and turn on the flag named test-third-party-cookie-phaseout.
Google also asks site owners to report anything that breaks. It offers a deprecation trial for non-advertising uses that need more time, which takes an embedded service out of the 1% test.
Sources: Google, The next step toward phasing out third-party cookies in Chrome; Google Privacy Sandbox, Third-party cookies restricted by default for 1% of Chrome users
When could it reach everyone
Google has said it plans to phase out third-party cookies for all Chrome users in the second half of 2024. Its developer page says the ramp to 100% starts in the third quarter of 2024.
That date is not firm. Google ties it to concerns raised by the UK Competition and Markets Authority, the CMA, which reviews the Privacy Sandbox plan. Plan for the change, but do not bet a budget on one exact day.
Sources: Google, The next step toward phasing out third-party cookies in Chrome; Google Privacy Sandbox, Third-party cookies restricted by default for 1% of Chrome users
What it means for tracking, ads and first-party data
Area Likely effect in the 1% group Retargeting Cookie-based audiences cannot follow these users across sites Conversion tracking Tags that depend on third-party cookies may miss sales Embedded tools Chat, video and widgets that need a cookie across sites may fail First-party data Not affected, because it comes from your own site Treat that table as a reading of how the block works, not a measured result. The test is too new for solid numbers.
First-party data is the clear winner. This means data people give you on your own site, such as email sign-ups, purchases and account details. You collect it with consent, you own it, and a browser block does not remove it.
Sources: Google, The next step toward phasing out third-party cookies in Chrome; Google Privacy Sandbox, Third-party cookies restricted by default for 1% of Chrome users
What an agency should do this month
- Audit every tag and tool on your sites. List which ones set or read third-party cookies.
- Open your key pages in Chrome 121 with the test flag on. Look for broken sign-in, forms, payments and embeds.
- Report any breakage to Google through the link in its Privacy Sandbox post.
- Ask your ad partners how they plan to measure results without these cookies.
- Start growing first-party data now, with clear consent and a plain reason for each field.
- Check your consent banner and privacy notice. Cookie rules differ by country, so ask a lawyer where you are unsure.
None of this has a deadline today. But a check now is cheap, and the same list will help you whichever date Chrome finally picks.
Sources: Google, The next step toward phasing out third-party cookies in Chrome; Google Privacy Sandbox, Third-party cookies restricted by default for 1% of Chrome users
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