The Repl.it code execution API allows users to send code for execution in a secure, reliable, and cost-effective manner, suitable for various applications such as in-browser coding environments, online courses, and real-time interviews. The API provides two interfaces: an HTTP interface for executing code without establishing a persistent connection, and a WebSocket interface for creating a stateful environment where users can execute code with variable access. A JavaScript library is also available to simplify the process of connecting to the service and running code. Plans and limits are in place to ensure fair usage, including a limit on concurrent connections and runs. Users can sign up for the service, generate a secret token, and start executing code using either interface. The Repl.it API has been retired as of October 2022 due to resource constraints.