budget2.0/AGENTS.md

Budget 2.0 — agent guide

The instructions for changing this codebase, for any agent or any human. This is the only copy: CLAUDE.md and GEMINI.md are pointers to this file, so it cannot drift into three disagreeing versions.

Most of the rules below are here because breaking one cost real time. This file is the short, actionable form; the reasoning lives in docs/ARCHITECTURE.md and docs/specs/, and is not repeated here.

1. Invariants

• src/domain/ is pure. No React, no IndexedDB, no Date.now(), no new Date() without an explicit argument. It must be testable with no DOM (src/domain/smoke.test.ts imports it without one).
• Money is a float, rounded at every boundary. Use roundMoney from src/domain/money.ts after any division, after any FX conversion, and before persisting.
• roundMoney(amount, digits) takes a DIGIT COUNT, not a currency code. Decimal places live in Dataset.currencies, because the owner defines currencies; resolve them with digitsFor(dataset.currencies, code). The argument is required on purpose — a default of 2 is right for every currency the app shipped with, so a missed call site would pass every test and be silently wrong for the first zero-decimal currency added.
• When splitting an amount, never round the last part independently. Compute it as total - sum(others) so the parts sum exactly to the whole.
• Currency is an open string code; Dataset.currencies is the authority. There is no global list of valid currencies. A code is IDENTITY — it keys the FX table and every stored Money — so codes are normalised uppercase, are never editable, and a currency still referenced cannot be removed (currencyUsage names what refers to it). parseDatasetJson validates the table and every Money against it; that boundary check is what replaced the compile-time safety of the old closed union.
• The baked FX rates seed a new dataset only. They must NOT act as a fallback when a rate is missing: clearing a rate removes the row so MissingRateError surfaces, instead of silently converting money at a number the owner never chose.
• MonthId is the string "YYYY-MM". Stored data never contains Date objects.
• The envelope rollover fold has exactly one implementation, in src/domain/fold.ts. Year and summary views aggregate over it — they never reimplement the math.
• A post's allocation is a dated series, Post.rules: RuleVersion[], resolved by ruleAt(post, monthId). Post.standingRule no longer exists. A post with no applicable version allocates zero; a per-month override still wins outright.
• Posts are archived, never deleted, because purchases reference them. Archived posts still fold.
• An amount field parses its own currency: "30$" is thirty dollars. Use parseMoneyInput (src/ui/moneyInput.ts) and keep the input type="text" — a number input discards the "$" before the parser ever sees it.
• sw.js must stay unhashed and be served no-cache. A cached or hashed service worker can never be replaced, which pins users to an old build forever.
• A destructive whole-dataset write downloads a backup first. Both JSON import and "Reset everything" export the current dataset before replacing it, and abort if that export fails. This is the owner's only copy.

2. Guards that fail the build

Each of these shipped as a bug once. The test named beside it is why it cannot ship twice — re-introducing one is a regression, not a style choice.

• Never read event.target.value inside a mutate() callback. Capture it into a const in the handler. mutate defers behind the write queue, and React resets the input's DOM value first, so you commit the old value. → src/ui/eventCapture.test.ts, source-level because reproducing it needs real browser render timing that happy-dom does not have.
• Never define a bare --accent. shadcn owns it as a hover BACKGROUND paired with --accent-foreground; setting one without the other drops hover contrast below WCAG AA app-wide. Redefine shadcn tokens only in pairs, and put brand colour on --primary, never --accent. The app's own tokens are --budget-*. → src/cssPairs.test.ts
• A var() naming a token src/index.css does not define paints NOTHING, silently. An undefined custom property invalidates the whole declaration: no warning, no throw, no fallback. Renaming --rule this way killed the carry meter and no test noticed. → src/ui/cssTokens.test.ts, which matches source text — so it flags a token named in a comment just as readily.
• Every awaited store.* call in src/ui/ must be guarded by a try/catch or a .catch. IndexedDB fails for reasons no code can prevent (full disk, evicted storage, private-mode quota), and unguarded it is an unhandled rejection: the button appears to do nothing and the user is told nothing. useMutate is the guard for most call sites. → src/ui/storeWriteErrors.test.ts
• roundMoney call sites must resolve real digits, per §1. → src/domain/currencyDigits.test.ts is the only test that can tell a real implementation from one that assumes 2: it drives a JPY-style zero-decimal currency through rounding, both FX directions, both split modes, plan division, the fold and the month view.
• process.env.BUN_PUBLIC_* must be read via try/catch — see readBasePathEnv. Bun inlines the literal only when the variable is set; unset, the bare reference reaches the browser and throws before the app boots. A typeof process guard does not work: it survives inlining and discards the inlined value, silently breaking subpath deploys. → src/ui/basePath.test.ts covers that accessor. Nothing stops a new bare reference elsewhere, so this one still needs your attention.
• The pre-paint theme script in index.html and src/ui/theme.ts must agree. The script cannot import the module — it has to run before any module loads — so it repeats the storage key and the fallback logic. → src/ui/theme.test.ts
• Anything derived from the base path is asserted for both a root and a subpath deploy: the manifest (src/manifest.test.ts), the font URLs (src/fontCss.test.ts), the icons (src/icons.test.ts), and the service worker's precache list (src/swPrecache.test.ts) — which must contain no duplicate, because cache.addAll rejects on one, which is why the service worker had never once installed in any environment.

Two rules with no test behind them. Only review catches these:

• A React error boundary does not catch event-handler throws. Domain functions that throw must be guarded at the call site when called from one.
• Do not "fix" a domain throw by making it return a fallback. Silently wrong numbers are worse than a visible error in a budgeting app.

3. Deliberately absent validation

Adding a "missing" guard here is a regression:

• going over budget, anywhere;
• allocation percentages summing past 100%;
• splits that do not sum to the total (the remainder-absorbing split reconciles them by design);
• negative amounts — a refund is a normal line.

4. Environment

Bun only. bun <file> (never node or ts-node), bun test (never jest or vitest), bun install (never npm/yarn/pnpm), bun run <script>, bunx, bun build <file.html|file.ts|file.css> (never webpack, esbuild or vite). Bun loads .env by itself — do not add dotenv.

Serving and bundling. Bun.serve() with HTML imports — import index from "./index.html" in the routes map — plus WebSockets, HTTPS and development: { hmr: true, console: true } if needed. No express. An HTML file imports .tsx/.jsx/.js and <link>s stylesheets directly; Bun's bundler transpiles and bundles both, Tailwind included. Dev server: bun run dev.

If a dependency ever calls for one: bun:sqlite not better-sqlite3, Bun.redis not ioredis, Bun.sql not pg or postgres.js, the built-in WebSocket not ws, Bun.file over node:fs's readFile/writeFile, and Bun.$`ls` not execa. Full API docs are in node_modules/bun-types/docs/**.mdx.

Base path comes from BUN_PUBLIC_BASE_PATH. Never hardcode a subpath, and read the variable only through the try/catch accessor (§2).

Where new documents go. Design documents in docs/specs/, implementation plans in docs/plans/, both named YYYY-MM-DD-<topic>.md. Note for tooling: the brainstorming skill defaults to docs/superpowers/specs/ — override it. That directory was removed deliberately, because "superpowers" names the tool that wrote the documents, not a kind of document.

There IS a browser here. google-chrome is on PATH, and scripts/screenshot.ts drives it over CDP — it waits past the async IndexedDB read (which Chrome's own --screenshot does not, which is why the docs used to claim no browser existed), and it reports console errors and uncaught exceptions. Read the PNG it writes. Use it before claiming anything about layout.

The whole recipe, from a running bun run dev:

bun scripts/demo-data.ts /tmp/seed.js
bun scripts/screenshot.ts http://localhost:3000/month/2026-09 /tmp/shot.png \
  --eval-file=/tmp/seed.js --reload

Then read /tmp/shot.png. Other flags: --dark, --full, --w/--h, --click=<selector>, --profile=<dir> (keeps a service worker and IndexedDB across runs — required for anything stateful), --eval-after-file (observe a flow that ends in a navigation from the far side of it; it awaits a promise, so it can click through a confirm and read IndexedDB back).

Two things that would otherwise waste your time:

• --reload is not optional. Chrome starts from a fresh profile each run, so the app re-seeds itself empty; seeding IndexedDB only takes effect on the next boot. Without it every screenshot shows an app full of zeros.
• This container's fontconfig resolves every generic family to Fira Code, a monospace font. The UI then renders monospaced in a way no real user sees, and one agent nearly "fixed" a font bug that did not exist. scripts/screenshot-fonts.conf corrects it and the script applies it automatically — do not undo that, and do not trust a screenshot taken with FONTCONFIG_FILE set to something else.

Still not verifiable here, so do not claim it: real offline behaviour, the install and update prompts, whether a downloaded file actually landed on disk, opening the generated .ods in a spreadsheet, and anything about how the design feels. A screenshot proves a layout exists and is not broken; it does not prove it is good. Substitute a real check where one exists — structural validation of the .ods, curl against the dev server, pure-function tests — and state plainly what still needs a human.

5. Then, depending on what you are doing

• Changing anything structural → docs/ARCHITECTURE.md: layers, data flow, and why the non-obvious choices are what they are. This file holds the rules; that one holds the reasoning behind them.
• Picking up queued work → docs/TODO.md, ordered, each item carrying the reasoning already settled. It lists only what is still outstanding — finished work lives in the commit messages and in docs/specs/.
• Making a product decision → docs/PRODUCT.md: who this is for, what must never break, and what must not be invented.
• Wondering why something is the way it is → docs/DECISIONS.md, an index of every significant choice and what it was chosen over, each entry pointing into docs/specs/ for the argument in full. docs/plans/ has the task-by-task implementation plans.