HTTP Headers

Origin-Agent-Cluster header

HTTP Header

The HTTP Origin-Agent-Cluster response header requests that the associated Document be placed in an origin-keyed agent cluster. This means that operating system resources (such as the operating system process) used for evaluating the document will only be shared with other documents from the same origin.

As a result, a document that uses many resources is less likely to negatively affect the performance of documents from different origins.

Syntax

Origin-Agent-Cluster: <boolean>

Directives

<boolean>: ?1 signifies that the associated Document should be placed in an origin-keyed agent cluster. This means that operating system resources (for example, the operating system process) used to evaluate the document will only be shared with other documents from the same origin. Values other than ?1 are disregarded.

Example

Origin-Agent-Cluster: ?1

How to Modify Header using Requestly

Requestly is a powerful Chrome extension that allows you to modify HTTP headers, including the Origin-Agent-Cluster header. This is useful when you want to test origin isolation behavior and understand how your site is grouped into agent clusters during development or debugging. Steps to Modify the Origin-Agent-Cluster 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 Origin-Agent-Cluster.
- In the “Header Value” field, enter ?1 to opt in to origin-keyed agent clusters.
Set the URL condition: Specify the URL or pattern where this header should apply (for example, https://your-website.com/*).
Save the rule.

Once configured, Requestly will inject the Origin-Agent-Cluster header into all matching responses. For example, using Origin-Agent-Cluster: ?1 instructs the browser to isolate the origin into its own agent cluster.

You might need to modify the Origin-Agent-Cluster header to test memory isolation, cross-origin behavior, or compatibility with features that rely on origin-level separation, without changing your production server configuration. This helps you validate isolation behavior before deploying changes live.