Fired when a web request is about to be made, to give the extension an opportunity to proxy it.

This event is closely modeled on the events defined in the webRequest API. Like those events, its addListener() function takes three arguments:

  • the listener that is called when the event is fired.
  • a RequestFilter object controlling which requests cause the event to fire.
  • an array of strings to control other aspects of the event's behavior.

The event is fired before any of the webRequest events for the same request.

When the event is fired, the listener is called with an object containing information about the request. The listener returns a proxy.ProxyInfo object representing a proxy to use (or an array of proxy.ProxyInfo objects, enabling the browser to fail over if a proxy is unreachable).

To use proxy.onRequest, an extension must have the "proxy" API permission and the host permission for the URLs of the requests that it intercepts, which means that the match patterns in the filter argument must be a subset of the extension's host permissions.


  listener,             //  function
  filter,               //  object
  extraInfoSpec         //  optional array of strings

Events have three functions:

addListener(listener, 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



The function that is called when this event occurs. The function is passed a single argument, which is a proxy.RequestDetails object containing details of the request.

The listener can return any of:

  • a proxy.ProxyInfo object.
  • an array of proxy.ProxyInfo objects.
  • a Promise that resolves to a ProxyInfo object.
  • a Promise that resolves to an array of ProxyInfo objects.

If the listener returns an array, or a Promise that resolves to an array, then all ProxyInfo objects after the first one represent failovers: if the proxy at position N in the array is not reachable when its ProxyInfo.failoverTimeout expires the browser will try the proxy at position N+1.

If there is an error specifying the proxy.ProxyInfo objects, then proxy.onError is called.


webRequest.RequestFilter. A set of filters that restricts the events that are sent to the listener.

extraInfoSpec Optional

array of string. Extra options for the event. Pass "requestHeaders" to include the request headers in the details object passed to the listener.

Browser compatibility

BCD tables only load in the browser


This code intercepts requests to <all_urls>, and proxies them if they are not for a top-level frame.

function shouldProxyRequest(requestInfo) {
  return requestInfo.parentFrameId !== -1;

function handleProxyRequest(requestInfo) {
  if (shouldProxyRequest(requestInfo)) {
    console.log(`Proxying: ${requestInfo.url}`);
    return { type: "http", host: "", port: 65535 };
  return { type: "direct" };

browser.proxy.onRequest.addListener(handleProxyRequest, {
  urls: ["<all_urls>"],

Example extensions