HTTP Headers

Observe Browsing Topics header

HTTP Header

The HTTP Observe-Browsing-Topics response header is used to mark topics of interest, which are determined from the URL of a calling site (specifically, the site where an ad tech <iframe> is embedded). These topics are marked as observed in the response to a request initiated by a feature that utilizes the Topics API. The browser will then use these observed topics to determine the most relevant topics for the current user in subsequent epochs.

For additional information, refer to Using the Topics API.

Syntax

Observe-Browsing-Topics: ?1

Directives

?1: A character sequence declaring that topics of interest inferred from a calling site’s URL (i.e., the site where the ad tech <iframe> is embedded) are marked as observed. The browser will subsequently use those topics to calculate topics of interest for a user for future epochs.

Example

How to Modify Header using Requestly

Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the Observe-Browsing-Topics header. This is helpful when you want to test how your site interacts with the browser’s Topics API during development or debugging. Steps to Modify the Observe-Browsing-Topics Header:

Install and open the Requestly Chrome extension. You can find it on the Chrome Web Store.
Create a new rule: Click on “Create Rule” and choose “Modify Headers” from the list of available rule types.
Add a new header modification:
- Under “Action”, select “Add” or “Override”.
- In the “Header Name” field, enter Observe-Browsing-Topics.
- In the “Header Value” field, enter your preferred value (for example, ?1 to opt in).
Set the URL condition: Specify the URL or pattern where this header change should apply (for example, https://your-website.com/*).
Save the rule.

Once set up, Requestly will inject the Observe-Browsing-Topics header into all matching responses. For example, using Observe-Browsing-Topics: ?1 signals that the page wants to receive browsing topics from the browser.

You might need to modify the Observe-Browsing-Topics header to test how your application behaves with the Topics API enabled, without changing your server configuration. This allows you to validate ad tech integrations, privacy-related behavior, or feature experiments before rolling them out to production.