Building a Coding Agent from Scratch: Harness Design and Tool Orchestration
dl_weekly · x · 2026-08-08
This article explores how to build a coding agent framework in Python from scratch, diving into the underlying mechanics of tools like Claude Code.
- The Harness Matters: In the Terminal-Bench experiment, changing only the agent harness (keeping the same model) jumped a coding agent from 30th place into the top 5, proving engineering is more critical than just swapping models.
- Core Components: The breakdown covers essential modules including the agent loop, durable execution, permission sandboxing, context engineering, and online AI evals.
- Lessons Learned: The author initially tried to one-shot the entire harness via prompt, but it fell apart when adding skills and memory. Building a robust agent requires strategic architecture rather than just stitching code blocks together.
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