I think it also depends on how all in you are on vibecoding. Like if you just prompt it and then occasionally check if the token carnage produced anything usable, then having an agents.md is probably just best practices cargo culting. But in other cases, having a Here’s How Thing Are Done In This Repo, You Fucker markdown file should make going through the more slop-happy merge requests a little less soul crushing.

The +20% additional token churn is surprising, I would have though the markdown files get cached pretty early down the line, unless the agent file is really big compared to the chatbot output.