Agent Harness vs Agent Framework vs MCP: Which Layer Owns the Loop, State, Tools, Permissions, and Recovery

Harness, framework, and MCP get used interchangeably in agent architecture discussions. They are not the same thing. They sit at different layers, own different responsibilities, and increasingly overlap at the edges. This article separates the 3 with 1 question. Which layer owns the execution loop, state, tool transport, permissions, and recovery? Agent harness: The harness is the execution system that wraps a model and turns it into a working agent. OpenAI’s Codex as a platform post (August 19, 2026) defines it directly. The harness manages conversation state, streams execution, and uses tools. It also enforces sandbox and approval policies and carries work across turns. Anthropic’s Claude Code docs call the same thing an agentic harness. The Claude Agent SDK exposes ‘the same tools, agent loop, and context management that power Claude Code.’ A harness is opinionated. It ships a loop, a permission model, a sandbox, and a context strategy as one unit. Agent framework: A framework is a library of primitives for composing agents. It covers model clients, tool abstractions, graph orchestration, memory interfaces, and observability hooks. Examples include LangGraph, the OpenAI Agents SDK, and Microsoft Agent Framework, which reached 1.0 GA in April 2026. A framework gives you the…