In my experience, a vague or outdated Agent.md causes more damage than not having one, because people assume it is accurate and stop asking questions. A simple, honest doc that is kept current is far more useful than a detailed one nobody maintains.
My approach is to keep them minimal. Make them a table of contents and spark notes, spread them across the repo, keep them up to date as the repo changes.
Something is better than nothing. It's pretty hard to have bad ones, also takes effort to craft good ones. The bad ones are the ones with incorrect information.
If paired this with system prompts derived from Claude Code. This makes gemini-flash nearly on par with the big boy models. Saves a ton of time and money
https://github.com/trailofbits/claude-code-config?tab=readme...
Start with nothing, and only add a single sentence to your Agent.md after the AI repeatedly makes the same specific mistake.
Something is better than nothing. It's pretty hard to have bad ones, also takes effort to craft good ones. The bad ones are the ones with incorrect information.