Your CLAUDE.md is doing something. Whether it helps depends entirely on how you wrote it.

We've been operating on faith here. Everyone tells you to write an AGENTS.md or CLAUDE.md, you write one, and you assume it helps because it feels like it should. Now there's data, and it's more interesting than "yes, write the file."

A study of 15,549 agentic pull requests across 148 projects measured what instruction files actually do to merge rates. The result splits clean down the middle: 27.7% of projects saw merge rates rise by at least 20% after adding instructions, while 26.35% saw merge rates drop (arXiv). A bad instruction file doesn't do nothing. It actively makes your agent worse. The projects that improved had substantially longer, better-structured files with more sections. Presence isn't the variable. Quality and structure are.

Pair this with the companion finding from the same research wave, because together they tell the real story. Across the AIDev dataset, 46.41% of fixes proposed by Copilot, Devin, Cursor, and Claude get rejected (arXiv). Analyzing 306 non-merged PRs, the authors sorted the rejections into 14 reasons across four buckets: incorrect implementation, CI failures, the agent couldn't even execute the fix, and low priority. Nearly half of agentic PRs are noise. The instruction-file study tells you one of your strongest levers against landing in that 46% is a well-built guidance file.

So treat your CLAUDE.md like production config, not a README afterthought. The teams winning have structure: explicit sections for build commands, test invocation, code conventions, what "done" means, and crucially, what the agent should not touch. If you've got a three-line instruction file, the data says you might be in the cohort that's actively hurting your merge rate. Audit it. Add the sections that map to your actual rejection reasons. If your agent's PRs keep failing CI, your file needs an explicit "run the full suite before you open the PR" instruction and a self-validation step. The position paper crowd is out there arguing human code review is obsolete (arXiv). With a 46% rejection rate, I'd hold that thought. The gating isn't the bottleneck yet. The agent's hit rate is.