What is an Agent Harness?
A harness is the runtime environment that executes AI agents—the software that connects a language model to tools, files, and the outside world.
The Agent Runtime
Think of a harness as the "operating system" for an AI agent. It provides:
- Long-running execution
Agents can work for extended periods, handling multi-step tasks that span minutes or hours.
- Tool integration
Access to file systems, terminals, browsers, APIs, and other capabilities the agent needs.
- Skill loading
Skills get "attached" to the harness, extending what the agent can do.
How Harnesses Work
Startup
The harness initializes, scans for available skills, and establishes connections to tools (file system, terminal, etc.).
Request Handling
User sends a task. The harness prepares context (relevant files, skill descriptions) and sends it to the language model.
Agentic Loop
The model responds with actions (tool calls). The harness executes them, feeds results back, and continues until the task is complete.
Skill Invocation
When the model identifies a relevant skill, it loads the full skill content and follows those instructions for the task.
Skills Attach to Harnesses
Skills are not standalone applications—they need a harness to run. When you install a skill:
- •The harness discovers the skill in its configured skill directories
- •It reads the skill's name and description (lightweight metadata)
- •When a task matches, it loads the full skill content into context
- •The agent follows the skill's instructions using the harness's tools
Example Harnesses
Different vendors provide harnesses with varying capabilities:
Why Harness Choice Matters
Tool Availability
Some harnesses have browser automation, others don't. Some can execute arbitrary code, others are sandboxed.
Model Options
Each harness may support different language models—Claude, GPT-4, Gemini, or specialized coding models.
Execution Environment
CLI vs. IDE integration vs. web-based. Local execution vs. cloud-hosted. Interactive vs. batch mode.
Skill Compatibility
Not all skills work on all harnesses. A skill requiring shell access won't work on a sandboxed harness.