Hoppscotch : A free, online, open source API request builder
Quicktype : Quicktype generates types and helper code for reading JSON in C#, Swift, JavaScript, Flow, Python, TypeScript, Go, Rust, Objective-C, Kotlin, C++ and ...
Bluepine is a DSL for defining API Schema/Endpoint with the capabilities to generate Open API (v3) spec, validate API request and serialize object for API response based on single schema definition.