Learn to build a custom harness, folks. Mods in Claude Code, DeepSeek Harness, and now Pi Durable. What do these all have in common? Increased extensibility. Builders need coding agents and harnesses to be more malleable. Say you are building a simple agentic app for your users, a personalized headless experience, a software factory, or a domain-specific harness; you will quickly find that the generic harness won't cut it. Sometimes you need a different behavior in the UI. Sometimes you…