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

A detailed technical breakdown examines the architectural distinctions between agent harnesses, agent frameworks, and the Model Context Protocol (MCP), clarifying which layer is responsible for execution loop control, state management, tool invocation, permission enforcement, and failure recovery. As multi-agent systems become more common in production, confusion about layer responsibilities leads to brittle architectures and hard-to-debug failures. The article provides a structured mental model for developers deciding where to implement specific agentic behaviors — for example, whether tool permissions belong in the harness or the protocol layer. This kind of architectural clarity is increasingly valuable as MCP adoption grows and teams build more complex agent pipelines. Developers designing agent systems should use this framework to audit their current layer assignments and identify ownership gaps.