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What an Agent Harness Is: The Ordinary Program Wrapped Around the Model
The horse-and-harness metaphor makes the concept clear: the layer of engineering code wrapped around the LLM — assembling context, executing tools, running the loop, managing permissions. It explains why the same model can perform worlds apart under a different harness, and why picking an AI tool is, to a large extent, picking a harness rather than a model.
2026/09/30 · Harness · Architecture · AI Agent
Evidence
YC Open-Sources QM: Source-Level Teardown of the 'Whole-Company Agent Operating System' That Hit 12K Stars in 7 Days, with More Security Code Than Model Loops
On July 31, 2026, Y Combinator open-sourced its internal Agent system QM (Quartermaster). Clone test: 241K lines of TypeScript, 379 test files, 4 interchangeable harnesses (Pi/OpenCode/Codex/Claude Code), egress proxy re-resolves DNS for each request, three-person gate 'a wall, not a hole', npm supply chain 7-day cooldown period: complete source-level dissection
2026/08/09 · Y Combinator · QM · Agent Harness