Sub-Agent Architecture for AI Coding Harnesses: When to Spawn, How to Route, What It Costs

Mon, 13 Apr 2026 10:00:00 +0800

This is Part 3 of the Harness Engineering series. Part 1 framed the thesis (Agent = Model + Harness). Part 2 went deep on CLAUDE.md, the single most important feedforward control. This article goes deep on a structural decision most teams get wrong: when to spawn a sub-agent, how to route it, and what it actually costs.

Sub-Agents on Bruce on AI Engineering

Sub-Agent Architecture for AI Coding Harnesses: When to Spawn, How to Route, What It Costs