TandemCode

In a coding interview you think aloud while someone watches your code take shape. TandemCode lets two people practice that together: one room, one editor with live cursors, a chat, and one verdict that both see at the same moment.

The React app edits a Monaco editor over a Yjs document. A FastAPI server relays the editor frames between the two browsers without keeping a copy, records them for replay, and rate-limits every socket. Design rooms add a shared whiteboard in the same document.

A run becomes a pending submission that a separate runner claims and judges inside a throwaway container with no network, a read-only disk and gVisor in production. Postgres notifies the room when the verdict lands, and an accepted run can be timed on larger inputs to estimate how it scales. The API, runner and database run on one AWS Lightsail host with Docker Compose and Caddy, and the web app is on Vercel.