Harness

Key facts

• It is the software that actually runs the model-to-tool-to-result cycle: handling API calls to the model, executing tool calls the model emits, feeding results back, managing the context window, and enforcing sandboxing and permissions.
• Well-known harnesses include Claude Code, Cursor, Cowork, Cline, Aider, Windsurf, and Devin.
• A freshly installed harness is unconfigured — a binary or IDE extension with no role, no mission, and no active task.
• It becomes useful only when someone points it at work with a specific prompt, system prompt, or AGENTS.md-style configuration.
• At that point the harness is operating as an agent, but the harness itself is the runtime, not the task.
• A note on industry usage.
• Many articles and vendors use "harness" and "agent" interchangeably, or skip "harness" entirely and call every agentic tool an "agent." On this site we reserve harness for the unconfigured runtime and agent for a harness that has been given a role, mission, and scope.
• The distinction matters because a single harness (e.g., Claude Code) can host dozens of different agents just by swapping AGENTS.md files — same binary, different configuration state.
• See the agents directory for a catalog of products in this category, and the agent concept for how harnesses get pointed at specific work.

References & related pages

• agents directory
• agent

Models Mentioning Harness(10)