Writing diagram prompts

The difference between a useful draft and a useless one is almost always the prompt, not the model.

#The pattern that works

Include	Example
The notation	"A Mermaid sequence diagram"
The actors or components	"between the mobile app, the API gateway and the auth service"
The flow, in order	"login request, token issued, token validated on each later call"
What to leave out	"do not show error handling"

Weak prompt	Strong prompt
A diagram of our login	A Mermaid sequence diagram of login: the mobile app posts credentials to the API gateway, which calls the auth service; the auth service returns a JWT; later calls carry the JWT and the gateway validates it locally. No error paths.
Show the architecture	A draw.io style component diagram of three services (web, api, worker) with a shared Postgres and a Redis queue, showing which service talks to which store.

Name your own components. The model does not know your service names unless you supply them, and it will happily invent some.

Longer prompts genuinely work better here. This is not a search query.
Asking for a notation you have not enabled produces a fallback, not an error.
If two attempts both miss, describe the diagram structurally rather than narratively.

Using the prompt.

Refining rather than re-prompting.

Knowing which notation to name.

Name the notation, the parts, and the order.