{
  "id": 15602923,
  "title": "Harness Engineering 101: The LLM is the brain, the harness is the body",
  "url": "https://urgent.news/2026/10/19/harness-engineering-101-the-llm-is-the-brain-the-harness-is-the-body",
  "topic": "ai",
  "section": "AI",
  "published": "2026-10-19T12:07:41.000Z",
  "source": {
    "name": "Hacker News",
    "slug": "hacker-news",
    "url": "https://github.com/IsuruMaduranga/harness-engineering-101"
  },
  "original_language": "en",
  "account": "The LLM serves as the brain, while the harness takes on the role of the body when constructing AI agents from the ground up. This distinction is emphasized in a blog series that is ultimately compiled into a book. The series can be accessed online at https://isuruwijesiri.com/harness-engineering-101/ and is also available for viewing on GitHub, with each chapter presented in a consistent format. Each chapter begins with an account of a specific failure, followed by a concise modification to rectify the issue, and a discussion on the associated costs.\n\nThe underlying principle of the series is that an LLM API is stateless, meaning that for every interaction, the entire conversation is submitted as a JSON array and the resulting text is returned. The harness is the program responsible for managing, updating, and safeguarding this array. All the components that are typically referred to as \"agents\" within the field are, in reality, a collection of modifications to this single loop, with each modification arising due to a concrete breakdown.\n\nThe series progresses across 16 chapters, progressively building a ~300-line toy agent using plain Python. These patterns are thoroughly developed in One Code, an open-source coding agent constructed on pi. The complete book is optimized for online reading at the provided link, although it can also be accessed directly from the GitHub repository. The introduction to the book is available on its landing page, and each chapter follows the same structure: the failure, the minimal patch, and the associated costs.\n\nIn terms of presentation, mdbook-mermaid is utilized to render inline mermaid diagrams; without this rendering tool, those blocks appear as raw code.",
  "summary": null,
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}