What Changed
Chrome began switching on the main Privacy Sandbox APIs in July 2023 with Chrome 115, planning to reach almost all browsers by mid-August.1 On September 7, 2023, Google announced that the APIs were generally available, meaning they worked by default for most Chrome users without flags or trials. A small share of browsers was held back for testing, and users got new "Ad privacy" settings to choose which APIs to allow.2
The launch covered six APIs. Topics has its own entry; two others were the replacements marketers would have felt most directly.
Protected Audience was built for retargeting and custom audiences. Instead of an ad network following a person across sites with a cookie, a site would ask the browser to add the person to an interest group, such as people who viewed a product. The browser keeps the groups itself, and no site can read them. When the person visits a publisher, the browser runs the ad auction on the device and shows the winning ad in an isolated frame, so the advertiser doesn't learn where the person has been.3
Attribution Reporting was built for conversion measurement. Rather than matching a click to a sale with a cookie, the browser sends reports. In the most detailed type, a click could carry only 3 bits of information about the conversion, an integer from 0 to 7, and a view only 1 bit; reports arrive between an hour and 30 days later, with random noise added. Aggregated summary reports give totals across many people instead.4
Why It Mattered
For marketers, the launch showed what a web without third-party cookies would look like in Chrome: retargeting run inside the browser rather than through ad networks' own data, and conversion reporting that was delayed, coarse and partly modelled. Ad tech companies now had the tools to test that against cookies, which they did through 2024 alongside the 1% cookie test.
Few relied on them. With cookies still working in Chrome, and the APIs supported by no other browser, there was little reason to switch. Once Google kept third-party cookies, it retired most of the APIs in October 2025, citing "their low levels of adoption".5
Notes
- “Shipping the Privacy Sandbox Relevance and Measurement APIs,” Privacy Sandbox, Google, accessed October 12, 2026, https://privacysandbox.google.com/blog/privacy-sandbox-launch. Published July 2023, last updated December 21, 2023. ↩
- Anthony Chavez, “Privacy Sandbox for the Web Reaches General Availability,” Privacy Sandbox, Google, September 7, 2023, https://privacysandbox.google.com/blog/privacy-sandbox-for-the-web-reaches-general-availability. ↩
- “Protected Audience API Overview,” Privacy Sandbox, Google, accessed October 17, 2026, https://privacysandbox.google.com/private-advertising/protected-audience. ↩
- “Constraints on Aggregation Reporting Data for Attribution Reporting,” Privacy Sandbox, Google, accessed October 17, 2026, https://privacysandbox.google.com/private-advertising/attribution-reporting/constraints. Covers event-level report limits for clicks and views. ↩
- Anthony Chavez, “Update on Plans for Privacy Sandbox Technologies,” Privacy Sandbox, Google, October 17, 2025, https://privacysandbox.google.com/blog/update-on-plans-for-privacy-sandbox-technologies. ↩