Sniffy

Sniffy is a free, open-source HTTP and HTTPS debugging proxy for Windows, macOS, and Linux. It helps developers and testers inspect traffic, replay requests, mock API responses, and pause requests or responses at breakpoints to edit them before continuing.

The desktop app includes a traffic inspector, a request composer with cURL import and export, and rules for rewriting, redirecting, blocking, and mocking traffic. It also lets you inspect WebSocket conversations and streaming responses, including SSE and gRPC, and export captured sessions as HAR or JSON.

JavaScript plugins let you customize requests, responses, WebSocket messages, and streaming messages. Write plugins in the built-in editor, limit them to selected URLs, and save an enabled plugin to reload it while you work. Plugin settings, persistent storage, and logs support reusable debugging workflows.

For server deployments and automation, Sniffy also provides a headless mode with a REST management API and WebSocket events. Sniffy is released under the Apache License 2.0.