Chrome-facilitated testing

To prepare for third-party cookie deprecation, we will be providing Chrome-facilitated testing modes that allow sites to preview how site behavior and functionality work without third-party cookies. This guide provides an overview of the testing modes Chrome plans to provide and how to access experiment group labels.

Chrome browser in this context refers to a Chrome client: a Chrome installation on a device. Each individual user data directory constitutes a distinct client.

Experiment group: a set of Chrome browsers for which certain features are enabled, disabled or configured. In the context of Chrome-facilitated testing, a set of browsers for which labels are set.

Label: in this context, a request header value that is set for a browser which belongs to an experiment group. Each browser in an experiment group will remain in that group throughout the period of Chrome-facilitated testing, ensuring that the label for a browser remains consistent across testers.

We have offered two distinct modes:

  • Mode A: From November 2023, organizations testing the PS R&M APIs have been able to opt in to receive consistent labels on a subset of Chrome browsers to allow for coordinated testing across different testers.
  • Mode B: Starting from January 4th, 2024, Chrome globally disabled third-party cookies for a portion of Chrome browsers.

Both modes will continue until at least Q2 2024. Where third-party cookies are disabled in Mode B, they will remain disabled through the full phase out of third-party cookies.

We have worked with the CMA to ensure that these testing modes align with the testing framework (and timeline) for third parties as laid out in its guidance on industry testing. As a result, the CMA anticipates that the results from testing in these modes can be used in its assessment of the Privacy Sandbox. The CMA has indicated they are likely to put more weight on results from Experimental Design 2, which uses the Mode B labels and the Mode A control 1 labels. See the CMA's October 26 guidance for further information on Experimental Design 2.

We will also be sending this proposal through the usual Blink development process, where the technical design and the Chrome release milestone will be finalized. While this is the implementation we would like to ship, additional discussion and approval means these details are still subject to change. We will continue to update this page as the plans progress, and you can continue to provide feedback or questions.

Mode A: Labeled browser groups

Organizations participating in testing will be able to opt in to receiving a persistent set of labels for a subset of Chrome browsers, allowing for coordinated experiments across different ad techs on the same set of browsers. For example, if a browser falls into the label_only_3 experiment group (as shown in the following table) then all participating ad techs will be able to see the same label_only_3 label and coordinate accordingly: use the PS R&M APIs, but refrain from using third-party cookies. We expect participants in the page to ensure labels are forwarded to other participants to allow for consistent experimentation across the entire process of ad selection and measurement.

For example, this allows multiple participants to run Protected Audience auctions without third-party cookies across a consistent group of browsers. The auction seller participants would forward the observed label to buyers to facilitate coordinated testing.

The labels do not affect any functionality in those instances of Chrome, including the availability of third-party cookies. The labels provide the grouping for independent, coordinated experiments, but it's down to the participating parties to enforce the relevant parameters for the experiment. If you're testing the effect of removing third-party cookies, then each participant is responsible for excluding third-party cookie data for browsers with that label.

The aim is to have groups that are representative of normal Chrome traffic. That means both third-party cookies and the PS R&M APIs should be available, though some portion of users may have changed or disabled functionality via settings or extensions.

Labels will generally be persistent throughout a browsing session in Chrome, and across sessions. However, this is not guaranteed, as there are rare scenarios where entirely resetting a browser may also reset the current label.

We are planning to include 8.5% of Chrome Stable browsers for Mode A, and our initial proposal divides that population into nine groups. The smaller subgroups are intended to allow ad techs flexibility in combining labels to create their own experiments of varying sizes. Groups do not overlap.

Note that the control_1.* labels are intended to be used as "Control 1" as outlined in the CMA's guidance on industry testing, so testing participants should not use Topics API or run Protected Audiences auctions for this traffic. As the labels do not affect functionality, participants should not pass observed topics or run Protected Audience auctions when they detect the control_1.* group labels.

We welcome feedback as to whether this selection of groups meets the needs of participating organizations.

Label % of Stable traffic
control_1.1 0.25
control_1.2 0.25
control_1.3 0.25
control_1.4 0.25
label_only_1 1.5
label_only_2 1.5
label_only_3 1.5
label_only_4 1.5
label_only_5 1.5

Mode A label_only_ browser groups have been available since November 2023, and Mode A control_1_* groups were made available starting from January 4th, 2024. We will continue sending all Mode A and Mode B labels until third-party cookie phase-out in Q3 2024.

Mode B: Disable 1% of third-party cookies

Chrome disabled third-party cookies for approximately 1% of Chrome Stable browsers from January 4th, 2024 (and also in Dev, Canary, and Beta browsers during Q4 2023). Organizations testing the PS R&M APIs do not need to opt in for this mode, as it will be applied uniformly across the entire browser population. There is, of course, the possibility that some site features may be impacted if the site hasn't yet adopted an alternative solution, such as CHIPS or Related Website Sets.

Additionally, we plan to provide a small fraction of traffic within Mode B that has PS R&M APIs disabled. Other APIs, such as Related Website Sets, CHIPS, and FedCM, will not be disabled. We anticipate that this combination will be helpful to establish a baseline of performance for browsers without third-party cookies and without the PS R&M APIs.

As part of Mode B we will also provide labels for the affected browsers. The labels will be available at the same time as the APIs are disabled. We're proposing to divide the population into three treatment_1.* groups where third-party cookies are disabled, but PS R&M APIs are available, and one control_2 group where both third-party cookies and the PS R&M APIs are disabled.

To assist with debugging the Attribution Reporting API and Private Aggregation API integrations and to help testing participants better understand the noise impact, ARA debug reports and Private Aggregation debug reports will still be available for browsers in Mode B, as long as the user has not explicitly blocked third-party cookies. Debug reports won't be available in control_2, since PS R&M APIs aren't available in that slice. Debug reports will still be phased out along with third-party cookie phase out.

  • For Attribution Reporting API, since third-party cookies are disabled, the reporting origin won't be able to set the ar_debug cookie and should rely on setting the debug_key fields (for attribution-success reports) and the debug_reporting fields (for verbose reports) to opt-in or out of receiving debugging reports.
  • For Private Aggregation API, the reporting origin should rely on calling enableDebugMode() to control opt-in of receiving debugging reports. Companies should continue to consider how regulatory obligations may apply to use of Attribution Reporting API and Private Aggregation API, including debug reports.

Mode A continues to run and these groups are distinct from the Mode A groups, as in a user will either be in Mode A, Mode B, or neither. Testing participants should use the control_1.* traffic as a control group representing the status quo with third party cookies.

Label % of Stable traffic
treatment_1.1 0.25
treatment_1.2 0.25
treatment_1.3 0.25
control_2 0.25

Chrome has also restricted cookies for 20% of Chrome Canary, Dev, and Beta clients.

Label % of pre-Stable traffic
prestable_treatment_1 10%
prestable_control_2 10%

Inclusion in one of these experiment arms will have the same effect as for their Stable equivalents.

As with Mode A, the PS R&M APIs are not guaranteed to be available, as users can disable them from the Chrome Privacy and security settings. Likewise, third-party cookies are not guaranteed to be disabled for every member of the control_2 group, since users may access browser UI to allow third-party cookies for a site.

Experiment monitoring

Make sure to monitor the relative traffic volume of each treatment and control label. treatment_1.1 should have about the same amount of traffic as treatment_1.2 and treatment_1.3.

Pre-period labels

Until January 2024, we ran pre-periods for multiple experiment arms: a period of time to allow Chrome to accurately size and select statistically unbiased groups. These pre-periods ran for all arms that were scheduled to start in January: the Mode B arms and the Control_1.* arms. There is no need for developer or site action here—these pre-period arms will not experience any change in behavior or API availability—but you should be aware that you may see a preperiod label returned in some situations. While browsers receiving the preperiod label may transition to one of the experiment groups, this is not guaranteed, so it is recommended not to assume browsers with this label are guaranteed to be in the experiment.

An experiment arm is a subset of the population under study: in this case, one of the labeled groups.

For the duration of Mode A and Mode B we will be introducing a temporary Cookie-Deprecation value accessible via an opt-in HTTP header and JavaScript API, which will provide the label for the browser's applicable Mode A or B experiment group (as defined by the percentages above), if it falls into one.

Accessing labels involves accessing information stored on the user's device. In some jurisdictions (such as the EU and UK), we understand that this activity is analogous to the use of cookies and thus accessing labels likely requires end user consent. Before you begin requesting labels, we recommend that you seek legal advice as to whether this consent obligation applies to you.

To receive the Sec-Cookie-Deprecation request header, a site must first set the receive-cookie-deprecation cookie. This cookie must use the Partitioned attribute, which means that opt-in for receiving the header must be done per top-level site.

For example, if wants to receive the Sec-Cookie-Deprecation header on its resources embedded on, then must set the following cookie in that context.

Set-Cookie: receive-cookie-deprecation=1; Secure; HttpOnly; Path=/; SameSite=None; Partitioned;  Max-Age=15552000

The Secure, HttpOnly, SameSite, and Partitioned cookie attributes are mandatory. The other attributes: Domain, Path, Expires, and Max-Age may be set as best suits your needs, though Path=/ is a good default. The example here sets Max-Age=15552000 so that the cookie does not expire until after 180 days.

You may want to begin setting the receive-cookie-deprecation=1 cookie before the Chrome-facilitated testing period begins, to ensure that browsers in an experiment group include the Sec-Cookie-Deprecation request header as soon as it becomes available.

For example, assuming the browser is in the example_label_1 group, subsequent requests that include this cookie will also include the Sec-Cookie-Deprecation header.

Sec-Cookie-Deprecation: example_label_1

If the browser is not part of a group, no header will be sent. Labels are tied to the presence of the cookie, so if the cookie is deleted, blocked entirely, or blocked for the specific site, then labels will not be sent. As the Partitioned attribute is intended for continued use after third-party cookies are fully deprecated, this means Partitioned cookies may be set when third-party cookies are blocked.

Access the cookieDeprecationLabel JavaScript API

The Cookie-Deprecation value can also be accessed via the navigator.cookieDeprecationLabel.getValue() JavaScript API. This will return a promise which resolves to a string containing the applicable group label. For example, if the browser was in the example_label_1 group:

// Feature detect temporary API first
if ('cookieDeprecationLabel' in navigator) {
 // Request value and resolve promise
 navigator.cookieDeprecationLabel.getValue().then((label) => {
   // Expected output: "example_label_1"

If the browser is not part of a group, the API will either not be available or the value will be an empty string, so ensure you do feature detection.

The JavaScript API may be called regardless of the presence of the receive-cookie-deprecation cookie. However, if cookies are blocked completely or specifically for the site, the API will again either not be available or return an empty string.

As with any client-provided value, ensure that you sanitize and validate the value from the header or the JavaScript API before use.

Demo and testing

From Chrome 120 onwards, there are flags available to enable local developer testing of requesting and reading the labels.

The chrome://flags/#tpc-phase-out-facilitated-testing flag allows you to enable a selection of test labels. These labels are prefixed with fake_ to differentiate them from the real labels. Enabling the flag does not opt the browser into any of the experimental groups.

You can see the labels in action via

As enrollment is enforced for the Privacy Sandbox relevance and measurement APIs, you may need to override enforcement for local testing by using chrome://flags/#privacy-sandbox-enrollment-overrides and supplying the demo origin. Alternatively, include the following command-line flag if you are running Chrome from a terminal: --args --disable-features=EnforcePrivacySandboxAttestations


The flag drop-down includes multiple options. Testers will be primarily interested in the entries marked "Force" as these ensure that the experiment behavior is enabled regardless of other device configurations.

To test only experiment group labels, select "Enabled Force Control 1" or "Enabled Force LabelOnly". These will result in the browser sending the "fake_control_1.1" or "fake_label_only_1.1" labels.

In Chrome M120 or later you may also use the following entries.

To test third-party cookie blocking, select "Enabled Force Treatment". This will send the "fake_treatment_1.1" experiment group label, but also modify the cookie settings page and current cookie setting to block third-party cookies.

To test third-party cookie blocking without private ads APIs, select "Force Control 2". This will send the "fake_control_2" experiment group label, update the cookie settings page, block third-party cookies, and also suppress the new private ads APIs.

Note, there is currently an issue where the browser will remain with the new cookie setting page and setting that blocks third-party cookies even if you disable the flag. We are working to fix this problem, but in the meantime you can test these flag values in a separate Chrome data directory by launching Chrome with the --user-data-dir=<new dir> command line flag.


We use the "chrome-testing" label in the developer support repo on GitHub to manage questions. We welcome your feedback and discussion on the initial questions:

You can also raise new questions or discussions in the repo using the "Chrome-facilitated testing" template.