This largely mirrors my experience building my custom agent

1. Start from the Claude Code extracted instructions, they have many things like this in there. Their knowledge share in docs and blog on this aspect are bar none

2. Use AGENTS.md as a table of contents and sparknotes, put them everywhere, load them automatically

3. Have topical markdown files / skills

4. Make great tools, this is still opaque in my mind to explain, lots of overlap with MCP and skills, conceptually they are the same to me

5. Iterate, experiment, do weird things, and have fun!

I changed read/write_file to put contents in the state and presented in the system prompt, same for the agents.md, now working on evals to show how much better this is, because anecdotally, it kicks ass