What is a harness?

The harness is everything wrapped around the model that turns it from a text generator into something that can act.

Navid Moazzezby Navid MoazzezUpdated Sept. 12, 20261 min read
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Claude Code is a harness. So is Cursor, so is Codex. They supply the tools, the instructions, the file access and the permission prompts.

Swapping harness changes what you can do far more than swapping model does.