This event is triggered before sending any HTTP data, but after all HTTP headers are available. This is a good place to listen if you want to modify HTTP request headers.

To have the request headers passed into the listener along with the rest of the request data, pass "requestHeaders" in the extraInfoSpec array.

To modify the headers, pass "blocking" in extraInfoSpec, then in your event listener, return an object with a property named requestHeaders, whose value is the set of request headers to send.

If you use "blocking", you must have the "webRequestBlocking" API permission in your manifest.json.

It is possible for add-ons to conflict here. If two add-ons listen to onBeforeSendHeaders for the same request, then the second listener will see modifications made by the first listener, and will be able to undo any changes made by the first listener. For example, if the first listener adds a Cookie header, and the second listener strips all Cookie headers, then the first listener's modifications will be lost. If you want to see the headers that are actually sent, without the risk that another add-on will subsequently alter them, use onSendHeaders, although you can't modify headers on this event.

Not all headers actually sent are always included in requestHeaders. In particular, headers related to caching (for example, Cache-Control, If-Modified-Since, If-None-Match) are never sent. Also, behavior here may differ across browsers.


  function(             // function
    details               // object
  ) {...},
  filter,               //  object
  extraInfoSpec         //  optional array of strings

This API is also available as browser.webRequest.onBeforeSendHeaders.*.

Events have three functions:

addListener(callback, filter, extraInfoSpec)
Adds a listener to this event.
Stop listening to this event. The listener argument is the listener to remove.
Check whether listener is registered for this event. Returns true if it is listening, false otherwise.

addListener syntax



Function that will be called when this event occurs. The function will be passed the following arguments:

object. Details of the request. This will include request headers if you have included "requestHeaders" in extraInfoSpec.

Returns: webRequest.BlockingResponse. If "blocking" is specified in the extraInfoSpec parameter, the event listener should return a BlockingResponse object, and can set its requestHeaders property.

webRequest.RequestFilter. A set of filters that restricts the events that will be sent to this listener.
array of string. Extra options for the event. You can pass any of the following values:
  • "blocking": make the request synchronous, so you can modify request headers
  • "requestHeaders": include the request headers in the details object passed to the listener

Additional objects


string. The ID of the request. Request IDs are unique within a browser session, so you can use them to relate different events associated with the same request.
string. Target of the request.
string. Standard HTTP method: for example, "GET" or "POST".
integer. Zero if the request happens in the main frame; a positive value is the ID of a subframe in which the request happens. If the document of a (sub-)frame is loaded (type is main_frame or sub_frame), frameId indicates the ID of this frame, not the ID of the outer frame. Frame IDs are unique within a tab.
integer. ID of the frame that contains the frame which sent the request. Set to -1 if no parent frame exists.
integer. ID of the tab in which the request takes place. Set to -1 if the request isn't related to a tab.
webRequest.ResourceType. The type of resource being requested: for example, "image", "script", "stylesheet".
number. The time when this event fired, in milliseconds since the epoch.
string. URL of the resource that triggered this request. Note that this may not be the same as the URL of the page into which the requested resource will be loaded. For example, if a document triggers a load in a different window through the target attribute of a link, or a CSS document includes an image using the url() functional notation, then this will be the URL of the original document or of the CSS document, respectively.
webRequest.HttpHeaders. The HTTP request headers that will be sent with this request.

Browser compatibility

Chrome Edge Firefox Firefox for Android Opera
Basic support Yes Yes 45.0 48.0 33

Compatibility notes


  • 'originUrl' is not supported.


  • 'originUrl' is supported from version 48.


  • 'originUrl' is not supported.


This code changes the "User-Agent" header so the browser identifies itself as Opera 12.16, but only when visiting pages under http://useragentstring.com/".

"use strict";

// This is the page for which we want to rewrite
// the User-Agent header.
var targetPage = "http://useragentstring.com/*";

// Set UA string to Opera 12
var ua = "Opera/9.80 (X11; Linux i686; Ubuntu/14.10) Presto/2.12.388 Version/12.16";

// Rewrite the User-Agent header to "ua".
function rewriteUserAgentHeader(e) {
  for (var header of e.requestHeaders) {
    if (header.name == "User-Agent") {
      header.value = ua;
  return {requestHeaders: e.requestHeaders};

// Add rewriteUserAgentHeader as a listener
// to onBeforeSendHeaders, only for the target page.
// Make it "blocking" so we can modify the headers.
  {urls: [targetPage]},
  ["blocking", "requestHeaders"]

Example add-ons


This API is based on Chromium's chrome.webRequest API. This documentation is derived from web_request.json in the Chromium code.

Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.

Document Tags and Contributors

 Contributors to this page: Makyen, wbamberg
 Last updated by: Makyen,