Privacy Sandbox: What It Was and Why Google Retired It

Privacy Sandbox is a Google initiative, announced in August 2019, that proposed a set of browser APIs to support advertising without cross-site tracking. It existed to let Chrome remove third-party cookies without collapsing the ad-funded web. Google abandoned that goal and retired most of the initiative’s advertising APIs on 17 October 2025.

Why Privacy Sandbox Existed

Safari and Firefox had already blocked third-party cookies, and Chrome planned to follow. The problem was that ad targeting, remarketing, and conversion measurement all depended on those cookies. Privacy Sandbox was Google’s attempt to rebuild each of those use cases as a browser API that processed data on the device instead of shipping it to third parties.

The first attempt failed publicly. Federated Learning of Cohorts (FLoC) grouped users into behavioural cohorts computed in the browser. Mozilla, Apple, Brave, and Vivaldi all declined to implement it, and critics argued it created new fingerprinting surfaces rather than closing old ones. Google withdrew FLoC in early 2022 and replaced it with the Topics API.

The Main Advertising APIs

Three APIs carried the advertising workload, each replacing one cookie-based job.

Topics API handled interest-based targeting. Chrome kept a human-curated public taxonomy of roughly 469 topics. Each week (an “epoch”) the browser computed the user’s top five topics from browsing history, entirely on device. A caller received up to three topics, one from each of the previous three epochs:

const topics = await document.browsingTopics();

Five percent of the time, the topic returned for an epoch was drawn at random from the full taxonomy instead of the user’s actual interests. That noise was deliberate, and it capped how confidently anyone could profile a single user.

Protected Audience (originally FLEDGE) handled remarketing by running the ad auction on the device, so the advertiser never learned the user’s browsing history directly.

Attribution Reporting API handled conversion measurement. An ad link declared a reporting endpoint with the attributionsrc attribute:

<a href="https://advertiser.example/product"
   attributionsrc="https://adtech.example/register-source">Shop now</a>

It produced two report types. Event-level reports tied one click or view to coarse, noised conversion data. Aggregatable reports were encrypted and processed by an aggregation service into noisy summary reports suited to campaign-level totals.

Privacy Sandbox vs Third-Party Cookies

Privacy Sandbox never replaced third-party cookies, and the deadline moved four times. Google committed in 2021 to removing them by the end of 2022, then slipped to 2024, then 2025.

In July 2024 Google reversed the plan entirely. Instead of a phase-out, Chrome would show users a one-time choice prompt. On 22 April 2025 Google dropped even that prompt. Third-party cookies remain enabled by default in Chrome today.

That reversal removed the reason to adopt the Sandbox APIs. Ad tech had been asked to rebuild its stack for a deprecation that never arrived, and adoption stayed low.

What Happened to Privacy Sandbox

On 17 October 2025 Google announced it was retiring ten Privacy Sandbox technologies across Chrome and Android:

Topics, Protected Audience, and Attribution Reporting
Private Aggregation, including Shared Storage, and the Aggregation Service
SelectURL, Related Website Sets, and IP Protection
On-Device Personalization, Protected App Signals, and SDK Runtime on Android

Google cited low adoption and a reassessment of the technologies’ expected value. Regulators had also raised competition concerns throughout, including the UK Competition and Markets Authority, the European Commission, and the US Department of Justice.

Chrome deprecated the APIs in version 144 in January 2026. Chrome 150, released on 30 June 2026, removed the related settings entries and began rejecting API calls. Chrome 152 removes the underlying implementation, leaving a lightweight stub in its place.

What Survived

Privacy Sandbox is not entirely gone. Three technologies continue to ship and are supported by default:

CHIPS gives a third-party cookie its own partitioned jar per top-level site. Default since Chrome 114.
FedCM supports federated sign-in without handing an identifying email to the relying party. Shipped in Chrome 108.
Private State Tokens remain in place for fraud and bot detection.

Several related protections also stayed, including the Storage Access API (default from Chrome 119), storage and network state partitioning, bounce tracking mitigations, and User-Agent reduction. Google has said feedback from teams that built on the Attribution Reporting API will inform work on an interoperable attribution standard at the W3C.

The practical lesson is about dependency. The retired APIs only ever worked in Chrome, which is why a single vendor decision could erase them. Campaign tags travel in the URL itself, so a link built with a UTM builder reports identically in Safari, Firefox, an in-app browser, or an email client, and no browser release can withdraw it.

Frequently Asked Questions

Is the Topics API still available?

No. Topics was among the ten technologies Google retired on 17 October 2025. Chrome deprecated it in version 144 in January 2026 and stopped honoring calls in version 150. Any implementation guide that tells you to call document.browsingTopics() for live targeting is out of date.

Is the Attribution Reporting API the same as attribution reporting?

No, and the names cause real confusion. The Attribution Reporting API was a specific retired Chrome API that produced noised, privacy-constrained conversion reports. Attribution reporting is the ongoing marketing practice of showing which channels and campaigns earned credit for conversions. The practice is unaffected by the API’s removal.

Why did Google retire Privacy Sandbox?

Google cited low adoption and a reassessment of the expected value of the technologies. The deeper cause was the July 2024 decision to keep third-party cookies, which removed the deadline that would have forced anyone to migrate. Sustained competition scrutiny from the UK CMA, the European Commission, and the US Department of Justice added pressure throughout.

Do marketers need to do anything about Privacy Sandbox now?

Remove any Sandbox API calls still in your tags, since they no longer return data in current Chrome versions. Beyond that, nothing needs replacing, because almost nobody had migrated. The measurement pressures that predate the Sandbox are unchanged: Safari and Firefox still block third-party cookies, and consent rules still apply everywhere.

For campaign tracking that does not depend on any browser API, start with the UTM parameters guide at linkutm.