I built a partner's finance app

I paid 150€ more for groceries than my girlfriend needed to fix this.

To solve this, I built an app that tracks both of our expenses and who owes who.

Then I asked myself: could this ever work for other couples, not just us? Turns out, no — there was no real separation between "her stuff" and "my stuff," just one shared bucket of data. Fine for us, a real problem for anyone else.

So I rebuilt it properly:

• Real accounts. Separate logins, secure passwords, actual sessions. Joining a household is just an 8-character code — no email system needed.

• Real data isolation. Every expense, goal, and setting now belongs to a household, and the app checks that before touching anything. Before this, a bug could let you delete someone else's data by guessing an ID.

• Private goals. I can save for Ellen's birthday without the app leaking that a mystery goal exists.

• No downtime. All of this went live without breaking the app we already use daily.

Along the way I also rebuilt the "auto-split our savings across goals" feature. First version used linked sliders — technically fine, but felt chaotic. Second version was one single bar you drag, which is much easier to trust at a glance.

Before shipping any of it, I did a proper security and bug review. Found and fixed a few real issues — including a subtle timing bug in the login check, and one feature that had been silently broken for a while without either of us noticing.

It now requires both of us to use it a bit more actively, but that's more of a us problem than the app problem. lol