The LLM is the brain. The harness is the body.

A blog series about building AI agents from first principles, published as a book.

📖 Read it online: https://isuruwijesiri.com/harness-engineering-101/

An LLM API is stateless. Every turn, you send the entire conversation as a JSON array and get text back. A harness is the program that builds, maintains, and protects that array. Everything the field calls "agents" is a set of patches to that one loop, and each patch exists because something concrete broke. This series develops that one idea across 16 chapters, growing a ~300-line toy agent in plain Python as it goes.

The patterns here are developed in full in One Code, an open-source coding agent built on pi.

The rendered book reads best online at the link above. If you'd rather read the source here on GitHub, follow the chapters in order below. The full introduction is on the book's landing page.

Every chapter has the same shape: the failure, the minimal patch, what the patch costs.

Chapter The failure it patches

1 It's Just a JSON Array "How do I even talk to this thing?"

2 The Brain: A Next-Token Black Box "Why does it behave like that?"

3 Tools: JSON Mapped to Functions The brain can't touch the world

4 The Agent Loop One tool call isn't enough

5 Caching: Why Order Is Load-Bearing Resending the array gets expensive

Part II — Running Long (the array under pressure)

Chapter The failure it patches

6 Context Is a Budget, Not a Bag The window fills up

7 Subagents: Fork the Context One array can't hold all the work

8 Steering the Running Loop The brain drifts mid-task

9 Background Work and Time The loop is synchronous; the world isn't

Part III — The Ecosystem (naming what you already understand)

Chapter The failure it patches

10 Every Framework Is a Wrapper Around Chapter 1 Abstraction anxiety

11 Extending the Body: MCP, Skills, Deferred Loading, Hooks Capabilities don't fit in the array

12 Debugging the Array You can't fix the prompt you can't see

Part IV — Trust, Domains, and Data

Chapter The failure it patches

13 Reflexes and Guardrails The loop will do something dumb

14 Case Study: Coding Agents How specialized does the body need to be?

15 RAG Was a Harness Pattern All Along A thousand names for one idea

Epilogue

Build Your Own — the complete toy harness, annotated

Appendix — Advanced Topics

Standalone pieces. Read them when you need them.

Path What it holds

src/*.md The chapters and appendices (the book source)

src/SUMMARY.md The book's table of contents

src/harness/ The runnable toy harness — one file per stage

src/diagrams/ Diagram generators and their generated PNGs

book.toml mdBook configuration

scripts/sync-to-blog.sh Builds the book and mounts it into the blog for serving

Building the book locally

cargo install mdbook mdbook-mermaid # or: brew install mdbook (+ mdbook-mermaid binary) mdbook serve --open # live-reloading preview at http://localhost:3000 mdbook build # static site into ./book

mdbook-mermaid renders the inline mermaid diagrams; without it those blocks show as raw code.

cd src/diagrams && python3 gen_all.py # Python 3.10+, matplotlib

License

This repository is dual-licensed: