SMF·LUNCH-MONEY
Can a prompt replace Lunch Money?
Personal finance — budgeting, invoicing and small-business finance
Exhibit tracking slip
Verdict
A personal finance ledger with real multi-currency support — converting and displaying balances in a home currency while preserving the original transaction currency — is a genuine weekend build using a currency-conversion API. What doesn't survive: Lunch Money's public API ecosystem, third-party tools built against it, and live bank-feed connections.
Exhibit A — The prompt
Received on31.07.2026Build a personal finance ledger with genuine multi-currency support, Lunch Money's actual differentiator. Use Next.js/TypeScript with SQLite. Support CSV import, the same duplicate-detection and column-mapping approach as this catalogue's Tiller entry, with a per-transaction currency field, not assuming everything is in one currency. Use a free-tier currency-exchange-rate API to convert each transaction to a chosen home currency for display and totals, while always preserving and showing the original currency and amount too — never lose the source data to a conversion. Cache exchange rates by date, since a transaction from six months ago should convert at that date's rate, not today's, for accurate historical totals. Build budget and net-worth views computed in the home currency from the converted amounts. Expose a simple read-only REST API, with a personal API token, over your own data, so you can build small scripts or integrations against it yourself, echoing Lunch Money's API-first design in miniature. Do not build live bank-account connections or a public third-party integration ecosystem — those are out of scope; the API here is for your own personal use, not other developers. Requires hosting and a database; a free currency-exchange-rate API key.Opening prefills the prompt — press enter to run it.
Exhibit B — What you lose
- B.1 a public API ecosystem with third-party integrations
- B.2 live automatic bank-feed connections
- B.3 years of currency-conversion edge cases like historical rates and rounding
- B.4 budget rollover and category-group polish
Prior art
Exhibit C — Why people still pay: financial integrations, compliance, and trust
Converting one currency is an API call; building an ecosystem other developers build on top of, plus getting historical exchange-rate edge cases right for years of transaction history, is the ongoing work.
Questions
Does it convert old transactions at today's exchange rate or the rate at the time?
At the rate for that transaction's date, cached so historical totals stay accurate rather than drifting every time exchange rates move.
Can I still see the original currency a transaction was in?
Yes — the original currency and amount are always preserved alongside the converted home-currency value, never overwritten.
Does it have a public API other people can build integrations against, like Lunch Money's?
It has a simple personal API for your own scripts, but there's no public ecosystem — that's real, ongoing developer-relations work this build doesn't attempt.
What does it cost to run?
Hosting, a database, and a currency-exchange API's free tier — typically nothing beyond minimal hosting costs.
Related tools
Receipt