Progress Telerik Fiddler Classic is the original web debugging proxy: it logs HTTP and HTTPS traffic between an app and a server, then lets you inspect each request, build new ones, and mock responses while you work out why something fails.
Progress Telerik Fiddler Classic works as a proxy that records every HTTP and HTTPS exchange made by a browser or desktop application. Start capturing, perform the action that misbehaves, and the session list fills with rows you can open one at a time to read headers, bodies, cookies, and timing. The composer tab sends hand-built requests, the AutoResponder returns mocked or redirected responses, and breakpoints hold a request or response so it can be edited before it continues. Sessions can be saved, filtered, and archived, which makes an intermittent bug reproducible later. No code changes are needed to see what the application actually did.
Progress Telerik Fiddler Classic is used for the debugging questions that logs cannot answer: whether a header was really sent, why a token was rejected, which request is slowest, or what a third-party API returned during a failure. Testers mock responses to check error states, developers verify payloads before writing parsing code, and support staff capture a customer's session to reproduce a fault without guessing. Sessions can be archived and reopened, and the statistics and timeline views point at the requests that dominate page load. It operates above the level of a bandwidth utility such as NetWorx, which measures how much data moves rather than what each request contained.
Progress Telerik Fiddler Classic earns its place because it shows the truth rather than a summary. When two systems disagree about what was sent, the captured exchange settles it, and the fix usually follows in minutes instead of a day of speculation. AutoResponder rules make it possible to test how an interface handles a slow, empty, or failed response without waiting for a real outage, and breakpoints allow a request to be altered in flight to prove where a problem starts. The composer turns a captured call into a clean, repeatable request for a colleague or a future regression check. Saving a session into an archive means the evidence outlives the browser tab, which is exactly what a bug report usually lacks. That record is often the fastest route from a vague complaint to a confirmed cause.
Key capabilities and highlights.
Progress Telerik Fiddler Classic records every request and response passing through the proxy, including encrypted sessions once its certificate is trusted. Traffic from browsers, desktop clients, and other devices on the network appears in one list as it happens.
Opening a session reveals headers, bodies, cookies, raw bytes, and timing in separate tabs, with dedicated views for JSON, XML, and images. That level of detail is what turns a vague report of a broken page into a specific field or status code.
The composer sends hand-crafted HTTP and HTTPS requests with any method, header, or body, so a single endpoint can be tested in isolation. It is the quickest way to confirm whether the server or the calling application is at fault.
AutoResponder rules in Progress Telerik Fiddler Classic match a request and return a local file, a prepared response, or a redirect instead of contacting the server. Teams use it to simulate error codes, slow responses, and missing assets, which is far cheaper than engineering those failures for real.
Breakpoints can pause a request before it leaves or a response before it returns, allowing headers, cookies, and bodies to be edited by hand. Stepping through traffic this way shows exactly where a chain of calls starts to fail.
Progress Telerik Fiddler Classic offers filters that narrow a busy session to specific hosts, processes, or response codes, and the QuickExec command line applies common actions by typing a short instruction. Both features matter once a page generates hundreds of requests in a few seconds.
Sessions collected by Progress Telerik Fiddler Classic can be saved to a file and reopened later, which preserves evidence for a bug that only appears occasionally. Statistics and timeline views break down where the time goes, highlighting the slowest requests on a page.
Custom rules can be written in the built-in scripting environment to rewrite, block, or log traffic programmatically, and extensions add extra inspectors and tools. That makes the proxy adaptable to unusual protocols and repeatable test scenarios that would otherwise require a bespoke client.
Browse historical releases and previous builds.
Common questions about Progress Telerik Fiddler Classic.
It is a proxy that records HTTP and HTTPS traffic so you can see exactly what an application sent and received. Typical uses include diagnosing failed API calls, checking headers and tokens, mocking slow or broken responses, and capturing a session that can be reopened later.
Yes, encrypted sessions can be read once the tool's certificate is trusted by the browser or application generating the traffic. Without that trust step, requests still pass through the proxy but their contents remain unreadable.
Yes. AutoResponder rules can match a request by pattern and answer it with a local file, a stored response, or a redirect, so error states and slow endpoints can be tested without touching the real service.
Yes, the composer tab builds HTTP and HTTPS requests with any method, header, or body, and it can copy an existing session as a starting point. That makes it easy to reproduce a single call outside the application that failed.
Sessions can be saved to an archive file and reopened at any time, which is useful when a fault appears only occasionally or when a colleague needs the original evidence. Filters and statistics make those saved sessions easier to analyze later.
Progress Telerik Fiddler Classic is the original web debugging proxy: it logs HTTP and HTTPS traffic between an app and a server, then lets you inspect each request, build new ones, and mock responses while you work out why something fails.
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