Model Isn't the Decisive Factor: The Harness Battle for CLI Coding Agents in 2026

In mid-2026, the focus of the CLI coding Agent community shifted from foundation models to Harness—system prompts, retry logic, context compression, and sub-agent orchestration. On Terminal-Bench 2.1, Claude Code and Codex CLI were locked in a tight race with an accuracy rate of around 83%, and the same model could achieve scores differing by several percentage points under different Harness configurations. This paper sorts out the concept of Harness, compares four development routes including Claude Code, Codex CLI, OpenCode and Copilot CLI, and explains why context compression has become an invisible battlefield, to help developers select appropriate solutions according to their scenarios.