That AGENTS.md You Auto-Generated? ETH Zurich Says It's Making Your Agents 3% Worse and 20% More Expensive.

ETH Zurich researchers ran the first serious study on context files for AI coding agents. 5,694 pull requests across 138 repositories, tested with three frontier models: Sonnet 4.5, GPT-5.2, and Qwen3-30B. The finding that caught me off guard: LLM-generated context files reduced task success by 3% and added 2-4 extra reasoning steps per task, increasing inference costs by more than 20%.

Human-curated files did slightly better. A 4% improvement. But with the same token overhead, and that overhead compounds across hundreds of agent invocations per day.

The recommendation is specific enough to act on today: keep context files under 60 lines. Limit content to details the model genuinely can't infer from the codebase itself. Custom build commands, non-standard test runners, project-specific naming conventions. And never auto-generate them with an LLM.

This validates something I've noticed in my own setup. My CLAUDE.md is tightly scoped: architecture overview, code conventions, file locations, testing commands. No AI-generated prose. No "detailed guidelines." The temptation to dump everything the agent might need into one file is strong, but the ETH study shows it creates noise that degrades performance.

The connection to harness engineering is direct. Context files are part of your harness. A bloated, auto-generated context file is like giving a contractor a 50-page specification when they needed a one-page brief. The model spends tokens processing irrelevant context instead of solving the actual problem.

An r/ClaudeAI post (51 upvotes) from the same day made the same argument from a practitioner angle: "context anxiety," agents losing track of what they're doing, is better solved by a well-structured CLAUDE.md than by adding more coordination layers. The academic data and the community wisdom converged.

If you have an AGENTS.md or CLAUDE.md over 60 lines, today's the day to cut it. Strip it to what the model can't figure out on its own. Your agents will work better and cost less.