I wanted to know one thing before my first real Supabase invoice: if usage jumps, what does the bill do, and does the spend cap really keep it safe? I read the Billing and Usage pages of a Pro organization, the docs, and built a simulator that shows the bill with the cap on and off. All from my run on 2026-10-27. Prices change, so every number is dated.

How this was made: I wrote this with Claude's help: it drafted, and I ran every step, edited the text, and checked the results. The mistakes are mine. This is a personal learning project, not an official guide, and I don't work for Supabase.

The non-technical version (three boxes, and why "stop" and "bigger bill" are different): https://medium.com/p/fd38d7d84bb2

The code: https://github.com/tomozilla/cost-simulator-en-demo

What I did and did not do

I changed no billing setting. I did not toggle the spend cap, add an add-on or change a plan. I opened the cap dialog and cancelled it.

I did not trigger the cap.…