--- title: What the Heck Is an Agent Harness? | daily.dev description: An explanation of the terminology stack behind AI coding agents: models are the reasoning 'brain', agents are the software that gives models tool access, loops... canonical: https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: What the Heck Is an Agent Harness? | daily.dev og:description: An explanation of the terminology stack behind AI coding agents: models are the reasoning 'brain', agents are the software that gives models tool access, loops... og:url: https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv og:image: https://api.daily.dev/og/posts/DYY0wL9Dv.png og:image:alt: What the Heck Is an Agent Harness? og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # What the Heck Is an Agent Harness? **[Medium](https://daily.dev/sources/medium_js)** · 12 min read · 1 upvotes · 0 comments ## Summary An explanation of the terminology stack behind AI coding agents: models are the reasoning 'brain', agents are the software that gives models tool access, loops are the observe-decide-act cycle that lets agents keep working on a task, and the agent harness is the surrounding system that manages tools, permissions, context, and safety boundaries. Uses a chef-in-a-kitchen analogy to show how the same model can behave very differently depending on the harness built around it, and frames 'Harness Engineering' as a shift in focus from model intelligence to system design. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What is the difference between an AI model, an agent, and an agent harness? A model is the reasoning engine that decides what should happen, an agent is the software layer that gives the model tools to interact with the outside world, and the harness is the system that manages the environment, permissions, context, and rules around the model, including which tools it can access and what boundaries limit its actions. The harness turns raw model intelligence into safe, working agent behavior. _Developers building or evaluating coding agents can compare harness designs and agent tooling on daily.dev._ ### What is the observe-decide-act loop in an AI coding agent? It is the repeated cycle where an agent gathers information from the environment (observe), the model determines the next step (decide), and the agent executes that step using a tool (act), then feeds the result back to the model as new information. For example, a coding agent runs tests, sees failures, reads a file, edits code, and reruns tests until all tests pass. _Track how agent loop and tooling patterns evolve for building coding agents on daily.dev._ ## Similar posts on daily.dev - [Harness engineering for coding agent users](https://daily.dev/posts/harness-engineering-for-coding-agent-users-ujzvxz4ny) · Martin Fowler · 2 upvotes · 0 comments --- Tags: [#llm](https://daily.dev/tags/llm), [#ai-agents](https://daily.dev/tags/ai-agents), [#prompt-engineering](https://daily.dev/tags/prompt-engineering) [View this post on daily.dev](https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"What the Heck Is an Agent Harness?","url":"https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv"},"datePublished":"2026-08-17T21:01:13.963Z","dateModified":"2026-09-13T21:46:49.263Z","description":"An explanation of the terminology stack behind AI coding agents: models are the reasoning 'brain', agents are the software that gives models tool access, loops...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/becb81dd4fbfab30d62ec01ca23b16e4?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/becb81dd4fbfab30d62ec01ca23b16e4?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Medium","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Medium","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/medium","url":"https://daily.dev/sources/medium_js"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":1},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"llm,ai-agents,prompt-engineering","timeRequired":"PT12M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Medium","item":"https://daily.dev/sources/medium_js"},{"@type":"ListItem","position":3,"name":"What the Heck Is an Agent Harness?"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/what-the-heck-is-an-agent-harness--dyy0wl9dv#faq","mainEntity":[{"@type":"Question","name":"What is the difference between an AI model, an agent, and an agent harness?","acceptedAnswer":{"@type":"Answer","text":"A model is the reasoning engine that decides what should happen, an agent is the software layer that gives the model tools to interact with the outside world, and the harness is the system that manages the environment, permissions, context, and rules around the model, including which tools it can access and what boundaries limit its actions. 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