Vercel Academy course teaches building an AI coding agent harness from scratch

Vercel Academy published an 11-module course that builds TeensyCode, a working AI coding agent harness in TypeScript, from an empty tool loop up to sandbox backends, subagents and a web surface. The course adds tools step by step, starting with read and grep and then bash behind execute-level safety gates, and covers a Sandbox interface with local, in-memory (just-bash) and cloud implementations, context pruning with bounded tool output and cache control, and Explorer and Executor subagents with isolated context. Modules 1 through 6 are build-along, Module 7 is concept and analysis on sandbox lifecycle and durable workflows, and the capstone runs the harness against a real project; prerequisites are TypeScript, Node.js 22+ or Bun, and an AI_GATEWAY_API_KEY.