budget2.0/CLAUDE.md at de128e3c878b059df62de5fe75591e403be53676

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Lucas WintherandClaude Opus 5 fe9f6c6dec docs: add TODO, ARCHITECTURE and AGENTS guides for handover

TODO.md carries the remaining work with the reasoning already settled, so the
next agent does not re-derive decisions: tasks 3-6 of the versioned-rules plan,
the designed-but-unbuilt baked FX rates, the queued visual redesign, the small
residuals, and an explicit list of what only a human with a browser can check.

ARCHITECTURE.md explains the layer rules, the single-implementation fold, the
mutation pipeline's three deliberate properties, and the gotchas that each cost
a real bug — reading a deferred event.target.value, an unset BUN_PUBLIC_* var
reaching the browser, and overriding shadcn's --accent.

CLAUDE.md gains those gotchas, the dated-rule model that replaced standingRule,
the validation this project deliberately omits, and an explicit statement that
visual and offline behaviour cannot be verified in the agent environment.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2026-09-02 02:26:25 +02:00

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Default to using Bun instead of Node.js.

Use bun <file> instead of node <file> or ts-node <file>
Use bun test instead of jest or vitest
Use bun build <file.html|file.ts|file.css> instead of webpack or esbuild
Use bun install instead of npm install or yarn install or pnpm install
Use bun run <script> instead of npm run <script> or yarn run <script> or pnpm run <script>
Use bunx <package> <command> instead of npx <package> <command>
Bun automatically loads .env, so don't use dotenv.

APIs

Bun.serve() supports WebSockets, HTTPS, and routes. Don't use express.
bun:sqlite for SQLite. Don't use better-sqlite3.
Bun.redis for Redis. Don't use ioredis.
Bun.sql for Postgres. Don't use pg or postgres.js.
WebSocket is built-in. Don't use ws.
Prefer Bun.file over node:fs's readFile/writeFile
Bun.$ls instead of execa.

Testing

Use bun test to run tests.

import { test, expect } from "bun:test";

test("hello world", () => {
  expect(1).toBe(1);
});

Frontend

Use HTML imports with Bun.serve(). Don't use vite. HTML imports fully support React, CSS, Tailwind.

Server:

import index from "./index.html"

Bun.serve({
  routes: {
    "/": index,
    "/api/users/:id": {
      GET: (req) => {
        return new Response(JSON.stringify({ id: req.params.id }));
      },
    },
  },
  // optional websocket support
  websocket: {
    open: (ws) => {
      ws.send("Hello, world!");
    },
    message: (ws, message) => {
      ws.send(message);
    },
    close: (ws) => {
      // handle close
    }
  },
  development: {
    hmr: true,
    console: true,
  }
})

HTML files can import .tsx, .jsx or .js files directly and Bun's bundler will transpile & bundle automatically. <link> tags can point to stylesheets and Bun's CSS bundler will bundle.

<html>
  <body>
    <h1>Hello, world!</h1>
    <script type="module" src="./frontend.tsx"></script>
  </body>
</html>

With the following frontend.tsx:

import React from "react";
import { createRoot } from "react-dom/client";

// import .css files directly and it works
import './index.css';

const root = createRoot(document.body);

export default function Frontend() {
  return <h1>Hello, world!</h1>;
}

root.render(<Frontend />);

Then, run index.ts

bun --hot ./index.ts

For more information, read the Bun API docs in node_modules/bun-types/docs/**.mdx.

Project invariants (Budget 2.0)

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.
Money is a float. Always round with roundMoney from src/domain/money.ts after any division, after any FX conversion, and before persisting.
When splitting an amount, never round the last part independently. Compute it as total - sum(others) so the parts sum exactly to the whole.
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.
Over-budget is always allowed. Never add validation that blocks it.
Base path comes from BUN_PUBLIC_BASE_PATH; never hardcode a subpath, and read it through a try/catch accessor — see the gotcha below.
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.

Start here

ARCHITECTURE.md — layers, data flow, and why the non-obvious choices are what they are. Read it before changing anything structural.
TODO.md — queued work, each item carrying the reasoning already settled.
PRODUCT.md — who this is for and what must never break.
docs/superpowers/specs/ — design decisions, including alternatives rejected.

Gotchas that cost real bugs

Each of these shipped once. Re-introducing one is a regression, not a style choice.

Never read event.target.value inside a mutate() callback. mutate defers behind the write queue; React resets the input's DOM value first, so you commit the old value. Capture into a const in the handler. src/ui/eventCapture.test.ts fails the build otherwise.
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.
Never define a bare --accent CSS variable. shadcn owns it; overriding it drops hover contrast below WCAG AA app-wide. Project tokens are --budget-*.
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.

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.

Verification limits

There is no browser in this environment. Visual appearance, offline behaviour, install and update prompts, keyboard focus traversal, and opening the generated .ods in a spreadsheet cannot be verified here. Do not claim you checked them. 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.

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