Harness Engineering: Anatomy, Architecture, and Evolution of Coding Agents -- A Source-Code Study of Eleven Systems

Abstract: 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 eleven production coding harnesses (Claude Code, Codex CLI, Gemini CLI, Mistral Vibe, OpenHands, Aider, Mini-SWE-Agent, Hermes, Pi, OpenCode, OpenClaw), plus Omnigent, the first meta-harness, analyzed as a contrast point. We define what a harness is, map its seven canonical subsystems with the minimal and maximal implementation of each, and dissect all eleven systems along those subsystems. The audit yields 13 cross-cutting observations and a catalog of 29 recurring design patterns. Two absences survive a threefold corpus expansion: across roughly four million lines of Python, TypeScript, and Rust, no agent runtime imports a general-purpose agentic framework, and none retrieves code with vector embeddings; the field runs on hand-rolled async loops and deterministic retrieval. SKILL.md skills lead MCP in adoption (9/11 vs. 8/11), and ACP ships in six systems with a new third role: harness hosting. Because the original eight systems were re-pinned rather than replaced, the study also contains a controlled longitudinal sample -- the same harnesses source-diffed across one quarter -- showing convergence becoming imitation and behavioral policy migrating from prompt prose to configuration. These threads converge on the paper's thesis: in the first half of 2026 the coding harness completed a turn from tool to platform. The paper closes with 18 design recommendations and a 90-line minimum-viable-harness scaffold.