HTTP Headers

Sec-Browsing-Topics header

HTTP Header

The HTTP Sec-Browsing-Topics request header transmits the selected topics for the current user along with the associated request. This information is utilized by an ad technology platform to select a personalized advertisement to display.

If the calling site has not successfully completed the privacy sandbox enrollment process to include the Topics API, any attempt to create or modify the Sec-Browsing-Topics header will fail silently, and any existing Sec-Browsing-Topics header will be removed.

For additional details, see Using the Topics API.

Header type	Request header
Forbidden request header	Yes (Sec- prefix)

Syntax

Sec-Browsing-Topics: <array-of-observed-topics>

Directives

This header contains a JSON object, which is an array of up to three objects. Each object represents the current user’s selected topics for the last three epochs and includes the following properties:

configVersion: A string that identifies the algorithm (excluding the model portion) used to compute the topic.
modelVersion: A string that indicates the model employed to categorize a string (such as a web page’s hostname) into specific topic identifiers.
taxonomyVersion: A string that specifies the version of the taxonomy used.
topic: A number representing the topic’s ID, which the browser can use to retrieve the topic from the taxonomy.
version: The configVersion, modelVersion, and taxonomyVersion values, joined together with colons (:) in between them.

Example

How to Modify Header using Requestly

Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the Sec-Browsing-Topics header. This is useful when you want to test how your application handles browsing topics sent by the browser as part of the Topics API during development or debugging. Steps to Modify the Sec-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 Sec-Browsing-Topics.
- In the “Header Value” field, enter a test value (for example, <array-of-observed-topics>).
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 configured, Requestly will inject the Sec-Browsing-Topics header into all matching requests. For example, using Sec-Browsing-Topics: <array-of-observed-topics> lets you simulate topic data being sent to your server.

You might need to modify the Sec-Browsing-Topics header to test how your backend, ad tech integrations, or analytics systems handle topic data, without changing your actual server setup. This helps you validate behavior and ensure everything works as expected before deploying to product