Developers keep mixing up agent harnesses, frameworks, and the Model Context Protocol (MCP). They shouldn’t. These three components sit at distinct layers of the AI stack, and blurring the lines leads to fragile architectures. The quickest way to separate them is asking one question: which layer owns the execution loop, state, tool transport, permissions, and recovery? If you think MCP manages your agent’s memory or decision cycle, you are building on a misunderstanding.
An agent harness is the execution system that wraps a model into a working agent. It is opinionated and complete. OpenAI’s Codex platform post from August 19, 2026, defines this clearly. The harness manages conversation state, streams execution, enforces sandbox policies, and carries work across turns. Anthropic’s Claude Code docs refer to this same concept as an “agentic harness.” The Claude Agent SDK exposes the exact tools, agent loop, and context management that power Claude Code. You do not rewrite the loop here. You accept the five-step cycle: receive prompt, evaluate, execute tools, repeat, and return. The harness decides when the turn ends.
Frameworks like LangGraph or the OpenAI Agents SDK offer primitives, not policies. They provide model clients, tool abstractions, and graph orchestration hooks. Microsoft Agent Framework reached 1.0 GA in April 2026, cementing this approach. In LangGraph, you draw the control flow. Nodes and edges define the path. You choose the durability mode for state persistence—“exit,” “async,” or “sync.” Pick wrong, and a crash mid-run wipes your progress. The framework gives you the skeleton; you must supply the muscle and the brain. It exposes state primitives but does not enforce how you save them.
MCP is strictly a wire protocol. Governed by the Linux Foundation’s Agentic AI Foundation since December 2025, it standardizes how hosts discover and call tools via JSON-RPC 2.0. It owns no loop. It holds no state. The July 28, 2026 specification update even retired the handshake process to keep requests stateless. If a server needs memory, it must mint a handle and pass it back as an argument. MCP defines the contract between the agent and its tools, nothing more. Treating it as a runtime environment is a fundamental architectural error that will limit scalability.
