Common Scripting Examples

Updated on Sep 29, 2026

This article collects practical examples that use the Session object (oSession) inside the OnBeforeRequest and OnBeforeResponse script hooks to inspect and modify traffic. Use them as a starting point for your own scripts.

OnBeforeRequest runs before a request is sent to the server, so only the request (oSession.oRequest) exists yet. OnBeforeResponse runs after a response is received, so both the request and the response (oSession.oResponse) are available—but any change you make to the request at that point is no longer sent to the server, since the server has already replied.

These examples are adapted from the equivalent FiddlerScript examples for Fiddler Classic, translated to plain C# and to the OnBeforeRequest/OnBeforeResponse hooks used by Fiddler Everywhere. Use the script editor's autocomplete (type . after oSession) to explore the members available on Session in your installed version.

Add a Request Header

csharp

void OnBeforeRequest(Session oSession)
{
    // Runs when a client request is received, before it is sent to the server.
    oSession.oRequest["NewHeaderName"] = "New header value";
}

Remove a Response Header

csharp

void OnBeforeResponse(Session oSession)
{
    oSession.oResponse.headers.Remove("Set-Cookie");
}

Replace Text in an HTML Response

Decode the response so it is no longer compressed or chunked, then search and replace within the body:

csharp

void OnBeforeResponse(Session oSession)
{
    // Runs when a server response is received, before it is returned to the client.
    if (oSession.HostnameIs("example.com") && oSession.oResponse.headers.ExistsAndContains("Content-Type", "text/html"))
    {
        oSession.utilDecodeResponse();
        oSession.utilReplaceInResponse("<title>", "<u>");
        oSession.utilReplaceInResponse("</title>", "</u>");
    }
}

Change the Path of a Request on the Same Server

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.PathAndQuery == "/version1.css")
    {
        oSession.PathAndQuery = "/version2.css";
    }
}

Redirect All Requests for a Host to a Different Host

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.HostnameIs("www.example.com"))
    {
        oSession.hostname = "test.example.com";
    }
}

Redirect All Requests for a Host and Port to a Different Host and Port

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.host == "www.example.com:8080")
    {
        oSession.host = "test.example.com:9090";
    }
}

Redirect a Host, Including HTTPS Tunnels

Changing hostname redirects plain HTTP traffic. To also redirect HTTPS traffic to the same host, the CONNECT tunnel itself must be retargeted:

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.HTTPMethodIs("CONNECT") && oSession.PathAndQuery == "www.example.com:443")
    {
        oSession.PathAndQuery = "beta.example.com:443";
    }

    if (oSession.HostnameIs("www.example.com"))
    {
        oSession.hostname = "beta.example.com";
    }
}

Retarget a Host Without Changing the Host Header

This simulates an entry in the operating system's HOSTS file: the request is sent to a different IP address, but the Host header the server sees is unchanged.

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.HostnameIs("subdomain.example.com"))
    {
        oSession.bypassGateway = true; // Skip any configured upstream proxy for this request.
        oSession["x-overrideHost"] = "128.123.133.123"; // DNS name or IP address of the target server.
    }
}

Retarget a Single URL, Changing the Host Header

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.url == "www.example.com/live.js")
    {
        oSession.url = "dev.example.com/workinprogress.js";
    }
}

Remove Cookies from Outgoing Requests

csharp

void OnBeforeRequest(Session oSession)
{
    oSession.oRequest.headers.Remove("Cookie");
}

Flag Responses That Contain a Search Term

csharp

void OnBeforeResponse(Session oSession)
{
    if (oSession.oResponse.headers.ExistsAndContains("Content-Type", "text/html") && oSession.utilFindInResponse("searchfor", false) > -1)
    {
        // Add a custom header so the match is easy to spot in the Inspector.
        oSession.oResponse["X-Contains-Searchfor"] = "true";
    }
}

Remove All `<div>` Tags from an HTML Response

csharp

void OnBeforeResponse(Session oSession)
{
    if (oSession.oResponse.headers.ExistsAndContains("Content-Type", "html"))
    {
        oSession.utilDecodeResponse();
        var oBody = System.Text.Encoding.UTF8.GetString(oSession.responseBodyBytes);

        var oRegEx = new System.Text.RegularExpressions.Regex(
            "<div[^>]*>(.*?)</div>",
            System.Text.RegularExpressions.RegexOptions.Singleline | System.Text.RegularExpressions.RegexOptions.IgnoreCase);
        oBody = oRegEx.Replace(oBody, "");

        oSession.utilSetResponseBody(oBody);
    }
}

Spoof the User-Agent Header

csharp

void OnBeforeRequest(Session oSession)
{
    oSession.oRequest["User-Agent"] = "Googlebot/2.X (+http://www.googlebot.com/bot.html)";
}

Request Content in a Specific Language

csharp

void OnBeforeRequest(Session oSession)
{
    oSession.oRequest["Accept-Language"] = "he";
}

Block Requests That Match a Condition

csharp

void OnBeforeRequest(Session oSession)
{
    if (oSession.uriContains(".css"))
    {
        oSession.oRequest.FailSession(404, "Blocked", "Fiddler Everywhere blocked this CSS file.");
    }
}