Developer Launches Coast, a Bookkeeping App Built Around FIRE After 10 Years of Tracking Expenses

The developer wrote that AI initially advised against building the app. Bookkeeping is a crowded category with similar features and high user migration costs, and most users will not abandon an app they have used for years. The developer still proceeded because several problems remained unsolved after a decade of tracking. The post included an AI usage note saying that some analysis used AI and that some screenshots and the header image were generated with Codex.

The developer began bookkeeping in college because the monthly allowance was insufficient and asking family for money was embarrassing. The allowance was not low compared with classmates, but spending was careless. The original goal was to stop living paycheck to paycheck and save something. Bookkeeping showed where money went, but it also created an illusion of control. Spending habits did not change, and after graduation the developer once borrowed money from a friend to repay a credit card.

After years of reflection, the developer summarized the method as recording as little as possible. The first step was reducing accounts. Five bank credit cards were merged into one account, WeChat and Alipay were merged into a single micro-pay account, and daily records use only a savings card, a credit card, and the micro-pay account. Consumption was also merged. Metro fares are recorded monthly through a transit card recharge. Coffee shops are recorded when recharged. Spring Festival red envelopes are not recorded individually and are checked against the balance after the holiday. Phone credit is bought as a large recharge card. Travel spending is merged into one entry, with shared costs calculated separately and foreign currency recorded after conversion. Another change was classifying by nature rather than type. Meal categories moved from breakfast, lunch, dinner, late-night snack, fruit, and snacks by time, to takeout, home cooking, gathering, and supermarket by scene, and finally to meals and gatherings by whether they can be cut. Transportation is now split only into transport and taxi, because transport is necessary and taxi is optional.

The developer said these methods addressed what to record but not how to use the records. A turning point came when the developer sent years of bills to AI for spending trend analysis. AI gave useful guidance and suggestions, but continuing this required exporting, downloading, and sending data to AI each time. The developer realized that years of accumulated data had produced little extra value beyond showing monthly spending. There were also two long-standing pain points: no goal and useless budgets. Budgets had been set in spreadsheets and abandoned within months.

The app is named after Coast FIRE, a FIRE variant in which enough principal has been accumulated that compound growth can reach financial independence at traditional retirement age even without additional saving. Coast's framework is built on FIRE, and its FIRE page puts the time remaining to financial independence in a prominent position. Once total assets and some consumption data are recorded, the app automatically calculates the time to retirement. A sensitivity slider adjusts monthly spending and income to show changes in retirement time in real time. A large-expense simulator lets users enter planned spending and see its impact, which the developer describes as a cooling-off period. The home page shows the FIRE date, and each recorded expense changes the date to give immediate feedback.

The category system uses fixed, necessary, flexible, and other expenses. Fixed and necessary expenses form the base for Lean FIRE, while flexible expenses are the main area for optimization. Custom categories must map to a FIRE role, though names and icons can be changed. The developer separately tracks personal improvement and travel as first-level categories because they are large and worth monitoring. Automatic budgeting uses median rather than average, prioritizes the same month of the previous year for annual budgets, and allows tightening by a percentage on top of the historical baseline. Other features include price tracking for similar goods. The developer once found that a large takeout milk tea from Chagee cost 7 yuan more on the day of the first cup of autumn milk tea promotion even after using a coupon. The app shows average price, lowest price, and price trends for an item. An ancillary spending feature covers tips, taxes, and service fees, which is important for users abroad or frequent travelers. AI integration is planned as an enhancement for spending trend analysis, unusual expense alerts, and personalized financial advice. The developer is watching Apple's on-device AI capabilities.

The developer used Vibe Coding to build the app, according to the post, and said it was not the first app. The post also refers to a previously released App Store app called EO, though the provided excerpt ends mid-sentence.

Editor's Summary

Coast is a new bookkeeping app built around FIRE by a developer with 10 years of expense-tracking experience. It turns spending records into a retirement timeline, automatic budgets, and immediate feedback on purchases, with features for price tracking, overseas tips, and planned AI analysis. The app enters a crowded personal finance market but focuses on using accumulated data rather than only recording it.