Using Fiddler for Mobile App Traffic Capture, Mocking, and Breakpoint Debugging

Mobile testers often need to capture packets, mock data, and throttle traffic; Fiddler is a powerful auxiliary tool that many testers favor, though it can present some issues during use.

When the proxy is set up, all captured requests may appear as the gray "Tunnelto 443" message because the request uses HTTPS; this requires enabling additional HTTPS settings.

Open Tools → Options → HTTPS in Fiddler and check "Capture HTTPS CONNECTs", "Decrypt HTTPS traffic", and "Ignore server‑certificate errors", then click OK (the first time you’ll be prompted to trust the Fiddler certificate).

To capture HTTPS requests from a mobile device, download the FiddlerRootCertificate onto the phone, install it (you may need to set a lock‑screen password), and configure the phone’s browser to point to the PC’s IP and port.

On iPhone, after installing the certificate you must manually trust it via Settings → General → About → Certificate Trust Settings , enable the Fiddler certificate, and restart Fiddler to see HTTPS traffic.

In Fiddler’s UI, the left pane lists requests (sortable by column), and you can save selected sessions as a .saz file, replay them, or inspect detailed request and response information, including raw headers, auth data, and cookies.

For mocking, select a request, open the AutoResponder, add a rule, and choose a matching method (string, exact, or regex). You can redirect the request to a local file, add delays, or change status codes, then save the rule to intercept and return custom responses.

Breakpoints can be set globally via Rules → Automatic Breakpoint → Before Requests , allowing you to modify any request before it leaves the client; disable the breakpoint when finished. Individual breakpoints can be added via the command line using bpu <URL> for before‑request or bpafter <URL> for after‑response.

Overall, Fiddler provides a comprehensive set of features for mobile app testing, from traffic capture and inspection to mocking and breakpoint debugging, making it a valuable tool for developers and QA engineers.