MCP vs. CLI: Does an AI Agent’s Tool Interface Still Matter?

A controlled experiment compared AI agent performance using either a structured Model Context Protocol (MCP) or a command-line interface (CLI) for tool use. The study found that for less capable models, CLI can improve performance on retrieval-heavy tasks, but this advantage diminishes significantly with more advanced models like GPT-5.5 and Opus 4.8. This performance convergence occurs because newer models make far fewer of the tool-use errors that CLI helps to mitigate. While task success rates become similar for top models, the choice of interface still impacts operational costs, latency, and the number of steps required to complete a task.