Harness

Everything around the model that turns it into an agent: tools, system prompt, context-window management, permissions, hooks. Claude.ai and Claude Code run on the same model but behave differently because their harnesses differ.

Usage:

"Same model, why is Claude Code editing files and Claude.ai just answering questions?"

"Different harnesses — Claude Code has filesystem tools, a different system prompt, and a permission layer. The model isn't the variable here."