GitHub - hoppscotch/hoppscotch: Open source API development ecosystem - https://hoppscotch.io (open-source alternative to Postman, Insomnia)
Open source API development ecosystem Built with ❤︎ by contributors by contributors. Lightweight: Crafted with minimalistic UI design. Fast: Send requests and get/copy responses in real-time. Customizable themes are synced with cloud / local session. WebSocket: Establish full-duplex communication channels over a single TCP connection.
This is the open-source alternative to Postman that you're looking for.