A bill-splitting app ends up knowing a lot: who your friends are, where you go, what you spend, and who owes whom. That is a lot of trust to hand to code nobody outside the company can read.
You can check the claims
Every app says it is private. With open source you do not have to take that on faith. Anyone can read how Settl stores data, who can see what, and what leaves your phone. Settl's database rules, for example, mean people only ever see expenses they are part of, and you can read those rules yourself.
It cannot quietly get worse
Closed apps change their free plans when the business needs it. An open source app can still change, but the code that works today stays available. If a hosted version ever becomes something you do not like, you or anyone else can run the version you did like.
Your data can leave with you
Settl exports everything you are part of as a spreadsheet, in the same format it imports. No lock-in, no support ticket.
Bugs get found faster
Money maths has edge cases: rounding a ₹1,000 bill three ways, multiple payers, item-by-item splits. Settl does all of it in whole paise and the ledger code has its own tests. More eyes on that code means fewer wrong balances.
What open source does not mean
It does not mean your data is public. Your expenses live in a private database; only the code is open. And it does not mean unpaid forever for every feature: the core stays free, and if paid extras ever come they will be clearly marked.
If this matters to you, the next step is to run your own copy of Settl.
