What a harness is and how to build one with Claude Agent SDK

Everybody is saying, “It’s not the model, it’s the harness,” but almost nobody stops to explain what a harness actually is. So I did. I built one live on the show: a Sentry bug-debugging harness for my company ChatPRD, using the Claude Agent SDK, a custom terminal UI built with the Ink library, and opinionated adapters for Sentry, Linear, GitHub, and Vercel. The harness handles evidence gathering, root-cause analysis, and follow-up artifact creation, all without me needing to type “dear agent, please fix this bug” ever again. I also walk through the architecture, share the code structure, and give you the exact process I used so you can build your own harness for any repetitive, structured workflow in your business.

What you’ll learn:
• What a harness actually is
• When to build a harness versus when to stick with a general-purpose tool like Claude Code or Codex
• How to encode specific permissions into a harness
• The three components every harness needs
• How I used GPT-5.5 and Claude Opus to build the harness code itself (and where they both initially resisted)
• How to structure the artifacts your harness produces so the whole team can use the output
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In this episode, we cover:
(00:00) What is an AI harness?
(03:19) When to build a harness
(04:33) Why Claire picked bug triage
(06:00) Why not just use Claude Code?
(07:48) Demo: The custom harness interface
(11:04) Architecture: runs, tasks, tools, and artifacts
(13:44) Building it with Codex and Claude
(15:08) Code map and file layout
(16:51) A look at the code
(19:18) The live investigation result
(21:01) How to build your own harness
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Tools referenced:
• Claude Agent SDK (Anthropic): code.claude.com/docs/en/agent-sdk/overview
• Claude Sonnet 4.6 (model used inside the harness): anthropic.com/news/claude-sonnet-4-6
• Claude Opus (used to build the harness): anthropic.com/claude/opus
• GPT-5.5 (Codex, used to build the harness): openai.com/index/introducing-gpt-5-5
• Ink (terminal UI library for Node.js): github.com/vadimdemedes/ink
• Sentry (error monitoring): sentry.io
• Linear (project management): linear.app
• GitHub: github.com
• Vercel: vercel.com
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Where to find Claire Vo:
ChatPRD: chatprd.ai
Website: clairevo.com
LinkedIn: linkedin.com/in/clairevo
X: x.com/clairevo
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