--- title: How to Build a Custom Agent Harness | daily.dev description: Building useful AI agents requires a harness — the scaffolding that connects a model to tools, context, and the environment. LangChain's `create_agent`... canonical: https://daily.dev/posts/how-to-build-a-custom-agent-harness-md9xqtbur twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: How to Build a Custom Agent Harness | daily.dev og:description: Building useful AI agents requires a harness — the scaffolding that connects a model to tools, context, and the environment. LangChain's `create_agent`... og:url: https://daily.dev/posts/how-to-build-a-custom-agent-harness-md9xqtbur og:image: https://api.daily.dev/og/posts/mD9xQTbuR.png og:image:alt: How to Build a Custom 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. # How to Build a Custom Agent Harness **[LangChain](https://daily.dev/sources/langchain)** · 5 min read · 0 upvotes · 0 comments ## Summary Building useful AI agents requires a harness — the scaffolding that connects a model to tools, context, and the environment. LangChain's `create_agent` primitive provides a minimalistic base that implements the core agent loop and exposes middleware as the primary customization mechanism. Middleware hooks into the loop before/after model and tool calls, enabling deterministic business logic, tool lifecycle management, custom state tracking, and stream handling. Prebuilt middleware covers common patterns, while custom middleware handles bespoke needs. The concept of 'task-harness fit' — how well the harness matches the actual demands of the task — is presented as the key determinant of agent quality. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [How Middleware Lets You Customize Your Agent Harness](https://daily.dev/posts/how-middleware-lets-you-customize-your-agent-harness-4kz9umlkd) · LangChain · 1 upvotes · 0 comments - [The Agent Harness: What It Is and Two Ways to Build One](https://daily.dev/posts/the-agent-harness-what-it-is-and-two-ways-to-build-one-zvqv3odcc) · InfoQ · 1 upvotes · 0 comments --- Tags: [#python](https://daily.dev/tags/python), [#ai-agents](https://daily.dev/tags/ai-agents), [#langchain](https://daily.dev/tags/langchain) [View this post on daily.dev](https://daily.dev/posts/how-to-build-a-custom-agent-harness-md9xqtbur) ```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":"How to Build a Custom Agent Harness","url":"https://daily.dev/posts/how-to-build-a-custom-agent-harness-md9xqtbur","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/how-to-build-a-custom-agent-harness-md9xqtbur"},"datePublished":"2026-07-08T19:22:56.744Z","dateModified":"2026-07-08T19:23:46.871Z","description":"Building useful AI agents requires a harness — the scaffolding that connects a model to tools, context, and the environment. 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