harness-engineering
Latest stories covering harness-engineering, with summaries and links to related coverage.
1 stories · 1 reports · page 1 of 1
Connected topics
Harness Engineering 101: The LLM is the brain, the harness is the body
tool
4 min read
Harness Engineering 101 is a September 2026 book and GitHub repository by Isuru Wijesiri that explains how to build AI agents from first principles. It defines a large language model as the “brain” and the harness as the programmatic “body” that builds, maintains, and protects the conversation history sent to a stateless LLM API. Each API turn sends the full conversation as a JSON array and receives generated text in return. The book develops a roughly 300-line Python toy agent across 16 chapters, with each chapter presenting a failure, a minimal patch, and the patch’s cost. Topics include tool calling, repeated agent loops, caching, context-window limits, subagents, steering, asynchronous background work, debugging, guardrails, coding agents, retrieval-augmented generation, MCP, skills, hooks, model routing, worktree isolation, operating modes, retries, rate limits, and streaming. Wijesiri says the same ideas are used in One Code, an open-source coding agent built on pi. The repository includes Markdown book sources, runnable toy-harness stages, diagram generators, and mdBook configuration. The prose is licensed under CC BY-NC-ND 4.0, while Python harness and diagram code is MIT-licensed. The book can be built locally with mdBook and viewed through a live-reloading server.