Warning Header

Table of Content

The Warning Header is a general header in HTTP (Hyper Text Transfer Protocol) used to carry additional information about the status or transformation of the message body. It can provide extra details about the server's operation which weren't reflected in the status code.


Warning: <warn-code> <warn-agent> <warn-text> [<warn-date>]

The warn-code is a three-digit integer, warn-agent is a token reflecting the warning's sender, warn-text is a quoted string containing the warning text, and optional warn-date in HTTP-date format represents when the warning was created.


  • Warn-code: A three-digit code that indicates the specific warning.
  • Warn-agent: Represents the server that added the warning header.
  • Warn-text: Provides a brief explanation about the warning.
  • Warn-date: Optional; indicates the time when the Warning header was added in HTTP-date format.


Warning: 199 Example-Server "This is an example warning"
Warning: 110 Example-Server "Response is stale"

Browser Compatibility

Browser Compatibility
Chrome Not Supported
Firefox Not Supported
Safari Not Supported
Opera Not Supported
Edge Not Supported

The HTTP Warning Header is not directly exposed to the API for manipulation or interception in the above listed browsers and it is typically handled by the browser's networking stack. It's also important to note that the Warning header field is generally not used by modern HTTP clients and APIs for security reasons, as its content is not reliably secured.

How to modify Warning header

ModHeader is a Chrome extension used to modify request and response headers. With ModHeader, one can add, modify or remove an HTTP request header.

Here's how to use it:

  1. Install the ModHeader extension from the Chrome Web Store.
  2. After successful installation, click on the ModHeader icon in your browser toolbar.
  3. In the ModHeader tab, click on 'Request Headers'.
  4. Type Warning into the Name field.
  5. Input the desired value in the Value field. Example: 110 Any_Server "Response is stale"
  6. Save the changes.

However, as mentioned earlier, modern browsers do not use the Warning header and modifying it might not provide any practical utility from the client's side.