Understanding Harness Engineering: From the Agent Loop to Context, Sandboxing, Verification, and Long-Running Execution

This technical manual defines an agent harness as the runtime and control mechanisms that sit tightly around the model, and breaks harness engineering down into engineering problems such as interfaces, context strategy, state, execution environment, permissions, feedback, recovery, and verification. The article distinguishes between the model, the harness, the session, tools, and the sandbox, points out that MCP is only an interoperability layer rather than a complete agent architecture, notes that compression is lossy, and argues that completion status should be separated from self-reporting. Finally, it provides a production-grade self-check checklist covering context, where actions are executed, operations that can be safely retried, and stopping conditions.