Modifying a Request or Response

Modifying a Request or Response

To make custom changes to web requests and responses, use FiddlerScript to add rules to Fiddler's OnBeforeRequest or OnBeforeResponse function. Which function is appropriate depends on the objects your code uses: OnBeforeRequest is called before each request, and OnBeforeResponse is called before each response. Note:

  • It is not possible to access the response objects inside OnBeforeRequest as they have not yet been created.
  • It is possible to use objects from the request inside OnBeforeResponse; however, any changes you make to those objects will not be seen by the server, as it has already received the request.

Add a request header

    oSession.oRequest["NewHeaderName"] = "New header value";

Delete a response header


Change a request for one page to a different page on the same server

    if (oSession.PathAndQuery=="/version1.css") {

Point all requests for one server to the same port on a different server

    if (oSession.HostnameIs("")) {

Point all requests for one port to a different port on a different server

    if ("") {"";

Point all requests for one server to a different server, including HTTPS tunnels

    // Redirect traffic, including HTTPS tunnels
    if (oSession.HTTPMethodIs("CONNECT") && (oSession.PathAndQuery == "")) { 
        oSession.PathAndQuery = ""; 

    if (oSession.HostnameIs("")) oSession.hostname = ""; 

Simulate the Windows HOSTS file, by pointing one Hostname to a different IP address. (Retargets without changing the request's Host header)

    // All requests for should be directed to the development server at
    if (oSession.HostnameIs("")){
    oSession.bypassGateway = true;                   // Prevent this request from going through an upstream proxy
    oSession["x-overrideHost"] = "";  // DNS name or IP address of target server

Retarget requests for a single page to a different page, potentially on a different server. (Retargets by changing the request's Host header)

    if (oSession.url=="") {
      oSession.url = "";

Prevent upload of HTTP Cookies


Decompress and unchunk a HTTP response, updating headers if needed

    // Remove any compression or chunking from the response in order to make it easier to manipulate

Search and replace in HTML.

    if (oSession.HostnameIs("") && oSession.oResponse.headers.ExistsAndContains("Content-Type","text/html")){

Case insensitive Search of response HTML.

    if (oSession.oResponse.headers.ExistsAndContains("Content-Type", "text/html") && oSession.utilFindInResponse("searchfor", false)>-1){
      oSession["ui-color"] = "red";

Remove all DIV tags (and content inside the DIV tag)

    // If content-type is HTML, then remove all DIV tags
    if (oSession.oResponse.headers.ExistsAndContains("Content-Type", "html")){
      // Remove any compression or chunking
      var oBody = System.Text.Encoding.UTF8.GetString(oSession.responseBodyBytes);

      // Replace all instances of the DIV tag with an empty string
      var oRegEx = /<div[^>]*>(.*?)<\/div>/gi;
      oBody = oBody.replace(oRegEx, "");

      // Set the response body to the div-less string

Pretend your browser is the GoogleBot webcrawler

    oSession.oRequest["User-Agent"]="Googlebot/2.X (+";

Request Hebrew content


Deny .CSS requests

    if (oSession.uriContains(".css")){
     oSession.oRequest.FailSession(404, "Blocked", "Fiddler Classic blocked CSS file");

Simulate HTTP Basic authentication (Requires user to enter a password before displaying web content.)

    if ((oSession.HostnameIs("")) && 
    // Prevent IE's "Friendly Errors Messages" from hiding the error message by making response body longer than 512 chars.
    var oBody = "<html><body>[Fiddler] Authentication Required.<BR>".PadRight(512, ' ') + "</body></html>";
    // Build up the headers
    oSession.oResponse.headers.HTTPResponseCode = 401;
    oSession.oResponse.headers.HTTPResponseStatus = "401 Auth Required";
    oSession.oResponse["WWW-Authenticate"] = "Basic realm=\"Fiddler Classic (just hit Ok)\"";
    oResponse.headers.Add("Content-Type", "text/html");

Respond to a request with a file loaded from the \Captures\Responses folder (Can be placed in OnBeforeRequest or OnBeforeResponse function)

    if (oSession.PathAndQuery=="/version1.css") {
      oSession["x-replywithfile"] ="version2.css";
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