How to Build a To-Do App

<context>
App: To-Do App
Difficulty: Beginner
Estimated build time: 1–3 days

Data model:
  User: id, email, created_at
  List: id, user_id, name, color, created_at
  Task: id, list_id, title, completed, due_date, priority (low/med/high), position, created_at

Recommended build order:
  1. Static UI — Build the task list UI with hardcoded data first — input field, task row, checkbox, delete button. No state yet, just HTML/CSS.
  2. Add React state — Replace hardcoded data with useState. Wire up add, complete, and delete. Store tasks in an array of objects with id, title, completed.
  3. Persist to localStorage — Add a useEffect that writes tasks to localStorage on every change, and reads them back on mount. Now tasks survive a page refresh.
  4. Add due dates and priority — Extend the task object with dueDate and priority. Add a date picker input and a priority selector to the add-task form. Highlight overdue tasks in red.
  5. Add lists / projects — Create a List concept — tasks belong to a list. Add a sidebar to switch between lists. Store the active list ID in state.
  6. Swap localStorage for Supabase — Create a tasks and lists table in Supabase. Replace all localStorage reads/writes with Supabase queries. Add Clerk for auth so tasks are per-user.
  7. Drag-and-drop reorder — Add a position integer to each task. Use @dnd-kit/core to handle drag events. On drop, update the position of all affected tasks in a single batch upsert.

Known pitfalls to avoid:
  - Skipping optimistic updates — your UI should update instantly on click, not wait for the database round-trip. Use React Query's mutate + onSuccess pattern.
  - Storing order as an index integer — when you delete item 3 of 10, you have to renumber 4–10. Use a float-based ordering (1.0, 2.0, 1.5 for between) or the LexoRank algorithm instead.
  - Not debouncing title edits — if you save on every keystroke you'll hammer your database. Debounce the save by 500ms.

Tech stack (intermediate): Next.js + Supabase + Clerk auth + React Query for optimistic updates
</context>

<role>
You are a Senior Full-Stack Engineer and product architect who has shipped production To-Do Apps before. You know exactly where developers get stuck and how to structure the project to avoid rewrites.
</role>

<task id="step-1-clarify">
Before writing any code or spec, ask me 3–5 clarifying questions that will meaningfully change the architecture. Focus on: scale expectations, auth requirements, platform (web / mobile / both), must-have vs nice-to-have features for the MVP, and any hard constraints (budget, deadline, existing infrastructure).

<example>
BAD: "What tech stack do you want to use?" — too broad, doesn't change architecture decisions.
GOOD: "Do you need real-time sync across devices, or is single-device with periodic refresh acceptable? This decides whether we use WebSockets or simple REST polling and significantly affects infrastructure complexity."
</example>
</task>

<task id="step-2-architect">
After I answer your questions, generate a complete project specification including:

1. Final tech stack with version numbers
2. Full file and folder structure
3. Complete database schema — every table, column, type, index, and foreign key
4. All API routes with request/response shapes and auth requirements
5. Component tree with TypeScript props interfaces
6. Auth and authorisation flow (who can do what)
7. Step-by-step implementation plan in the exact build order from <context> above
8. All environment variables with descriptions

<self_check>
Before presenting the spec, verify:
□ Every answer I gave in step 1 is reflected in the spec
□ The database schema matches the data model in <context>
□ The implementation plan follows the build order from <context>
□ No circular dependencies in the component tree
□ Auth is wired up before any protected routes are built
□ All known pitfalls from <context> are addressed in the spec
</self_check>
</task>

<task id="step-2.5-agents-md">
Generate an AGENTS.md file at the project root. This file is read automatically by Claude Code, Cursor, Windsurf, Sourcegraph Cody, and all major AI coding tools at the start of every session.

Include:
- Project overview (2–3 sentences)
- Tech stack with version numbers
- Folder structure with one-line descriptions
- Key architectural decisions and why they were made
- Coding conventions: naming (camelCase components, kebab-case files), max 200 lines per file, one concern per file
- Available commands: dev, build, test, lint, db:migrate, db:seed

Note in the file: "Cursor users can symlink .cursorrules → AGENTS.md. Claude Code users can symlink CLAUDE.md → AGENTS.md."
</task>

<task id="step-3-implement">
Implement the full project following the spec from step 2, in the exact order defined. For each step:
- Write the complete code (no placeholders or TODOs)
- Confirm the step is working before moving to the next
- Note any deviations from the spec with an explanation

<constraints>
- Max 200 lines per file. WHY: every file must fit in one AI context window for easy review and editing.
- One concern per file. WHY: mixing auth logic into API routes makes it impossible to reuse — extract to lib/auth.ts.
- TypeScript strict mode, no `any` types. WHY: catches data shape mismatches at compile time, not in production at 2am.
- All database queries in server components or API routes only. WHY: keeps credentials server-side, prevents accidental client-side exposure.
- All environment variables documented in .env.example. WHY: the next developer (or your future self) should be able to set up the project in under 5 minutes.
- Comment every non-obvious decision. WHY: AI tools read your comments to understand intent — without them, the next edit will break the pattern you established.
</constraints>
</task>