I have a project where I asked the coding agent to write a design doc for each new feature. It now has 80 or so design docs, which aren’t kept up to date, so it’s a historical reference that we don’t go back to in practice. At some point I will probably delete them.

In a different project, I instead have it maintain a project overview and a couple other docs, and we delete plan.md once the work is done and the docs updated. I like that better.

The design document and plan are just there to ensure the agent has all the context and instructions to implement it correctly. After it's done, I ask the agent to salvage any useful information into proper documentation and delete the implementation files.