# Agent Harness

> The runtime around a model that supplies tools, state, prompts, permissions, logs, and execution loops.

- Category: Agents & Autonomy
- Stage: Rising
- Also called: Harness, Agent runtime
- Canonical: https://www.joelcsaji.com/ai-dictionary/agent-harness

## Plain English

The model is the brain, but the harness is the workbench, clipboard, tool belt, and supervisor.

## Example

A coding harness gives an agent shell access, file editing, browser QA, progress logs, and rules for ending each session cleanly.

## Why it matters

Many agent improvements now come from better harness design rather than only stronger models.

## Related

- [Agent Loop](https://www.joelcsaji.com/ai-dictionary/agent-loop)
- [Long-Running Agent](https://www.joelcsaji.com/ai-dictionary/long-running-agent)
- [Compaction](https://www.joelcsaji.com/ai-dictionary/compaction)

## Mentioned in

- [The Rise of AI Agents: How Autonomous AI Transformed Work in 2025-2026](https://www.joelcsaji.com/blog/rise-of-ai-agents-2025-2026)

## Sources

- [Anthropic: Effective Harnesses](https://www.anthropic.com/engineering/effective-harnesses-for-long-running-agents)
- [Anthropic: Building Effective Agents](https://www.anthropic.com/engineering/building-effective-agents)