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Build Your Own AI Coding Agent Harness

AnnouncementProductNov 14, 2026

A new course teaches developers to build TeensyCode, a working AI coding agent harness written in TypeScript. Modules cover the agent loop with safety gates, tool design contracts, structured system prompts with AGENTS.md, sandbox abstraction with local and in-memory backends, context pruning, subagent delegation, and human-in-the-loop approval. Module 7 analyzes sandbox lifecycle using state machines and Vercel Workflow with sleep() for durable workflows. Prerequisites include TypeScript, async/await, and Node.js 22+ or Bun.

Why it matters

This article presents a hands-on course in which developers construct TeensyCode, an AI coding agent harness built in TypeScript from the ground up. Rather than treating an agent as a chatbot with tools, the course frames the harness as the surrounding system that makes an agent viable for real work: managing context that would otherwise grow without bound, gating dangerous shell commands, abstracting sandboxes behind a single interface, delegating work to subagents, and asking users when instructions are ambiguous. The teaching approach is causal — each capability is added because the previous stage broke, so learners see why every piece exists. Vercel appears in the sandbox lifecycle module, where durable workflows and state machines are analyzed conceptually rather than demonstrated. The piece matters because it shifts attention from model capabilities to engineering discipline, arguing that reliable coding agents depend on deliberate harness design: safety gates, structured prompts, sandbox lifecycle management, and verification contracts. For readers tracking applied AI, it offers a concrete map of the components production agent systems require.

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Evidence

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No score is assigned. Sources and their independence are shown in the citation chain below.

Citation chain · 1 source

01high
academy-front.vercel.shOfficialhttps://academy-front.vercel.sh/academy/build-ai-agent-harnessThe source is the course's own announcement, laying out its causal build-along structure and detailing the full module sequence, from the agent loop and tool contracts through sandbox lifecycle and human-in-the-loop design.
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Canonical: https://academy-front.vercel.sh/academy/build-ai-agent-harness