> ## Documentation Index
> Fetch the complete documentation index at: https://docs.requestly.com/llms.txt
> Use this file to discover all available pages before exploring further.
# How to Modify Cookies Using Requestly Interceptor
Modifying cookies is useful when you're testing authentication flows, simulating different user states, or tweaking behavior without changing backend code. Requestly Interceptor makes this easy by letting you **intercept HTTP requests/responses** and **manipulate cookies** on the fly.
## Use Cases
* Testing login/logout behavior
* A/B testing variants
* Simulating admin/user roles
* Bypassing feature flags
## Modify Cookies in Requestly Interceptor
This is useful for **sending a custom cookie value** to the server.
Cookies are sent as a single string in the `Cookie` header, separated by semicolons.
Go to the **HTTP Rules** section, click on `+ New Rule` , and select `Modify Headers` from the list of options.
Give your rule a clear name like **Set Custom Cookie**. Optionally, add a short description so you can easily identify its purpose later.
Define where the rule should be applied by specifying criteria such as **URL**, **Host**, or **Path**. You can choose from options like **Regex**, **Contains**, **Wildcard**, or **Equals**, or use **Advance Filters** for more precise conditions.
For more details on source conditions, [**read this guide**.](/http-interception/http-rules/advanced-usage/advance-targeting)
In the **Modification Type**, select **Request Headers** since we're modifying outgoing request headers.\
Under the **Request Headers** tab, click **Add Header** and set the following:
* **Action**: Add or Override
* **Header Name**: `Cookie`
* **Value**: `username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC`
This tells the browser to attach the specified cookie string in all matching requests.
Click **Save** to store the rule. After saving, make sure to **toggle it ON** to activate it.
This is useful to **change or inject cookies** coming from the server (e.g., simulate logged-in state).
Response Headers modification done by Requestly Interceptor are not visible in Browsers devtool but they are actually modified
Go to the **HTTP Rules** section, click `+ New Rule`, and choose **Modify Headers** from the list.
Give your rule a meaningful name like **Override Set-Cookie**.\
Add a short description to clarify that this rule modifies cookies sent from the server, useful for simulating logged-in sessions or testing different cookie values
Define where the rule should be applied by specifying criteria such as **URL**, **Host**, or **Path**. You can choose from options like **Regex**, **Contains**, **Wildcard**, or **Equals**, or use **Advance Filters** for more precise conditions.
For more details on source conditions, [**read this guide**.](/http-interception/http-rules/advanced-usage/advance-targeting)
Choose **Response Headers** under **Modification Type**, since we're altering headers coming from the server.
Then under the **Response Headers** tab:
* **Action**: Modify
* **Header Name**: `Set-Cookie`
* **Value**: `username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC`
This will override the original `Set-Cookie` header sent by the server and inject your custom value instead.
Click **Save** to store your rule and **toggle it ON** to enable it.
Use `document.cookie` in the DevTools Console to check current cookies, or inspect request headers in the **Network tab**.