mitmproxy
mitmproxy.org
An open-source interactive HTTPS proxy for intercepting, inspecting, modifying, and replaying HTTP and HTTPS traffic from any application — desktop, mobile, CLI, or IoT device.
Why it is useful
Essential when you need to understand what an app actually sends over the network, especially mobile apps or third-party tools without documentation. You can rewrite responses in real time, record sessions to replay later, and script complex traffic modification with Python. Far more capable than browser DevTools for debugging non-browser traffic or reverse-engineering undocumented APIs.