Claude tends to default to and do best with first making ASCII diagrams in markdown files, which you can then ask it to translate into Mermaid if appropriate.

Prompts like, "Please write a comprehensive report on _____ to work with _____. Include a holistic report on architecture and meaning and purpose of all involved systems. Describe the why and how of the changes in depth and include a full glossary of terms and systems. Write as a new .md in docs when you are sure there are no major gaps in your understanding. Include a report on the plan to _____."

Will be the rough shape you want to get it to dig through all the relevant code and make relevant architectural diagrams. Guide more or less towards specifics as appropriate. This has worked well since Opus 4.5.

Gotcha. So this is more for diagrams of new systems than existing ones. Our maybe a blend of new and old.