Why Better Models Can Break Your Agents: The Case for Harness Maintenance

Key Takeaways

• Vercel studied a top sales rep’s real workflow, built an agent around it, then improved it by removing tools — "the beginner instinct is to add; the maintenance instinct is to ask what to remove."
• Agents break when the model gets better, not just worse: a harness built to constrain a weak model can trap a stronger one, or hand a now-capable model too much reach.
• Agents inherit the crud of the systems around them — a stale wiki or a drifted CRM field that’s merely annoying to a human becomes dangerous when a proactive agent acts on it.
• Both OpenAI (Codex) and Anthropic (Claude Code) are winning by maintaining the harness — terminal, browser, computer use, memory, approvals, sandboxing, logs — not just shipping a smarter model.
• Everyone has a "harness" whether they call it that or not: your project folders, memory, source docs, approval habits and verification loop. Ask what yours is, how you ship it, and what you’ll need to delete later.
• A five-point checkup for any serious agent: what is it reading, what can it touch, what is its job now, what proof does it bring back, and is it still delivering value?