Definition
What is an agent harness?
An agent harness is the environment a single AI agent runs inside: the tool access (shell, files, browser, APIs), the permission model, the context management, and the guardrails. Anthropic’s agent SDK is one such harness. The model supplies the reasoning; the harness supplies everything that lets reasoning touch the world. Claude Code is a harness. A raw API call is not.
The harness matters to marketers because it sets what an agent can actually do. An agent with only a chat window can draft; an agent whose harness includes your CMS, your email platform, and your analytics can operate. Loops sit one level above the harness: the harness runs one agent for one session, and a loop schedules, dispatches, and verifies that work over time.
Stub · This definition is being expanded with verified examples from shipped systems. Updated .
VibeMarketers publishes one verified marketing agent system every week, with the installable files.