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Build a Mini Coding Agent Harness in JavaScript

A tiny Node coding-agent harness, built one runnable stage at a time

Build a Mini Coding Agent Harness in JavaScript
CRAIG WEST

Build a coding agent the way you actually learn: one runnable stage at a time. You will start with a chatbot at Stage 3, give it file tools, memory and lifecycle hooks, then turn the whole thing into a testable ReAct loop — with no framework, no agent SDK and one runtime dependency. By the end you will own a small Node.js coding-agent harness you can explain, test and extend.

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About

About the Book

A tiny Node coding-agent harness built one runnable stage at a time.

Most coding-agent books start with the sandbox. They teach you to build the container, the permission model and the tool registry before the program ever says a word. It makes a tidy diagram and a slow lesson — you wait hours before anything talks.

This book does the opposite.

  • By Stage 3 you have a chatbot.
  • By Stage 7 it can read your files.
  • By Stage 9 it can run commands.
  • By Stage 12 it has a real ReAct loop, and by the end it has nine tools, a workspace sandbox, persistent memory, nine lifecycle hooks, an offline demo and a test suite.

Why start with a chatbot?

Because every stage should feel like a superpower, and because you should meet the failure modes in the order a real user does:

- A bad key is a Stage 2 problem.

- A model that forgets the previous turn is a Stage 4 problem.

- A model that writes the wrong file is a Stage 8 problem.

- A command that hangs is a Stage 9 problem.

- A model that loops forever is a Stage 12 problem.

Build the loop first and you hide all of them behind a harness you do not yet understand.

What you get

- **17 stages**, each a complete, runnable Node project

- **9 tools**: list, read, create, write, edit, delete, run commands, remember, recall

- **9 lifecycle hooks** that can observe, rewrite or block a run

- A workspace sandbox that refuses escapes — including symlinks and shell commands

- A testable ReAct loop with step limits and a truncation guard

- An **offline demo** that proves the safety guards with no API key

- An offline test suite and a 20-item acceptance checklist

- A **185-question architecture Q&A** and a function-by-function code reference

- Teacher material: run sheets, a live-demo script, the factory analogy, misconceptions and a rubric

What it deliberately is not

No framework. No agent SDK. No database. One runtime dependency (`chalk`). And no false claims: the sandbox is a teaching policy, not a security boundary — the book says so repeatedly, because overclaiming safety is worse than naming its limits.

Who it is for

Developers who can read modern JavaScript — modules, `async`/`await`, `fetch`, classes. You do not need to have built an agent before. Bring Node.js 20+ and a terminal. The live chapters use an OpenRouter key; the tests and the demo need none.

The promise

By the last page, every coding agent you meet will look like a variation on a program you can read. You will recognise the loop, the tools, the context budget, the safety layers and the extension points — and you will know exactly where the design stops being safe.

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Author

About the Author

CRAIG WEST

Contents

Table of Contents

Build a Mini Coding Agent in JavaScript — Table of Contents

Table of Contents

Build a Mini Coding Agent in JavaScript

A tiny Node coding-agent harness, built one runnable stage at a time.

By Craig West · 17 runnable stages · 9 tools · 9 lifecycle hooks · 1 runtime dependency

  1. Preface
  2. Before you start
  3. Part 0 The idea before the code
  4. Phase A A chatbot that talks
    1. Stage 0 Scaffold
    2. Stage 1 Terminal language (ui.js)
    3. Stage 2 Config and the model client
    4. Stage 3 The simplest chat loop
    5. Stage 4 Conversation history
    6. Stage 5 The system prompt
  5. Phase B Give the chatbot hands
    1. Stage 6 Path policy
    2. Stage 7 Read-only file tools
    3. Stage 8 Sandbox and write tools
    4. Stage 9 Command execution
  6. Phase C Memory and observability
    1. Stage 10 Long-term memory
    2. Stage 11 Lifecycle hooks
  7. Phase D The agent loop and polish
    1. Stage 12 The ReAct loop
    2. Stage 13 Offline demo
    3. Stage 14 Sessions, trimming and CLI polish
  8. Phase E Hardening and documentation
    1. Stage 15 The full test suite
    2. Stage 16 Documentation and final acceptance
  9. Appendices Protocol, safety, bounds, debugging, glossary and reading map
    1. A The message protocol
    2. B The safety model in one page
    3. C Every bound and cap
    4. D Debugging by symptom
    5. E Glossary
    6. F Reading the finished code
  10. Part II The teacher's handbook
  11. Part III The student workbook
  12. Part IV Deep dives on harness engineering
  13. Part V Reference
    1. A Configuration reference
    2. B CLI and exit-code reference
    3. C Tool response reference
    4. D Hook payload reference
    5. E File and API map
    6. F Final acceptance checklist
    7. G unction reference
    8. H Architecture Q&A
  14. About the author

Build order: each stage is a complete, runnable Node project. By Stage 3 the program is a chatbot; by Stage 12 it is a testable ReAct loop with tools, memory, hooks and a workspace sandbox.

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