Agent Harness in Python: Give LLMs Shell and File Access

Every popular coding agent you have heard of in 2026 — Claude Code, GitHub Copilot CLI, Codex, Cursor's background agent, the new Grok Build CLI — is built on the same hidden layer: an agent harness. The model does the thinking, but the harness is what lets it actually read your files, run a command, see the output, and decide what to do next, all without burning down your machine.

On April 3, 2026 Microsoft Agent Framework shipped 1.0 with a first-class harness component, and at Build 2026 this week Microsoft positioned it as the recommended way to put an LLM in front of a real OS. The discussion blew up in r/LocalLLaMA, r/ClaudeAI, and across the Hacker News front page because it finally puts a name on a pattern most developers had been re-inventing badly.