JSON:API Fiddle allows you to experiment with JSON:API with a running project / live database. It uses Codespaces, so it runs in the Browser - no install, config or risk to your local machine. Codespaces provides VSCode access to the project, so you can test the api, see / alter the code, and use the debugger.
This example includes not only api access, but a rules engine to ensure that updates obey business rules.