How to Build a To-Do App

<context>
App: To-Do App
Difficulty: Beginner
Estimated build time: 1–3 days
Tech stack (intermediate): Next.js + Supabase + Clerk auth + React Query for optimistic updates

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

FIRST RUN REQUIREMENT:
Seed 3 example tasks across 2 lists (Work, Personal) so the app never opens to an empty screen. Seed data loads from a static config — no manual database entry needed.

DONE WHEN — verify each before marking V1 complete:
  □ User adds a task and it appears in the list instantly — no page reload
  □ Completed tasks are visually marked done and persist after a browser refresh
  □ Tasks within a list can be dragged to reorder, and the new order survives a refresh
  □ Filtering by list, priority, and due date work simultaneously
  □ New user sees 3 seeded example tasks on first load — no blank empty state

Recommended build order:
  1. Define API contract + schema + seed data (always first)
  2. Static UI — Build the task list UI with hardcoded data first — input field, task row, checkbox, delete button. No state yet, just HTML/CSS.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. End-to-end verification — walk every Done When condition above (always last)

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.
</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>

⚠️ Do NOT start planning or writing code until I answer. Present the questions, then stop and wait.
</task>

<task id="step-2-architect">
After I answer your questions, produce the following in order:

1. TECHNICAL SPECIFICATION
   - System architecture using → to show data and event flow
   - Core data model (refer to data model in <context>)
   - API contract — list ALL routes with request/response shapes BEFORE any implementation
     WHY: agreeing on contracts first prevents rewrites when frontend and backend shapes diverge
   - External APIs and integration points

2. MVP PLAN in three phases:

   SETUP   ✦ Step 1 (always): API contract + schema migration + seed data
     Done when: schema is migrated, seed script runs, app starts with demo data, zero manual setup.

   CORE FEATURES — build in this exact order:
   2. Static UI
   3. Add React state
   4. Persist to localStorage
   5. Add due dates and priority
   6. Add lists / projects
   7. Swap localStorage for Supabase
   8. Drag-and-drop reorder

   DONE WHEN — one observable condition per feature:
   □ User adds a task and it appears in the list instantly — no page reload
   □ Completed tasks are visually marked done and persist after a browser refresh
   □ Tasks within a list can be dragged to reorder, and the new order survives a refresh
   □ Filtering by list, priority, and due date work simultaneously
   □ New user sees 3 seeded example tasks on first load — no blank empty state

   STRETCH GOALS — post-launch additions that are explicitly out of V1 scope.

3. BLOCKER ANALYSIS
   Flag: API rate limits, auth complexity, scope risks, cold-start problems, top 2–3 failure modes.

   FIRST RUN REQUIREMENT: Seed 3 example tasks across 2 lists (Work, Personal) so the app never opens to an empty screen. Seed data loads from a static config — no manual database entry needed.
   ✓ Done when: app launches with seed data and the full user journey works without any manual setup.

<self_check>
Before presenting your output, verify:
□ Every answer I gave to clarifying questions is reflected in the spec
□ Step 1 is ALWAYS: API contract + schema + seed — never UI first
□ Done When criteria are observable user behaviors, not internal states
□ The plan is realistic for one person to ship within 1–3 days
□ 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, and all major AI coding tools at the start of every session.

Include:
- Project overview (2–3 sentences)
- Tech stack with exact version numbers
- Folder structure with one-line descriptions per directory
- Key architectural decisions and the WHY behind each
- Coding conventions: camelCase components, kebab-case files, max 200 lines per file, one concern per file
- Available commands: dev, build, test, lint, db:migrate, db:seed
- MVP scope boundaries — features explicitly out of V1

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

<task id="step-3-implement">
Implement the full project in the exact order from step 2. For each step:
- Write complete code (no placeholders or TODOs)
- Confirm the step works before moving to the next

<constraints>
- Step 1 is ALWAYS: define all API routes + TypeScript request/response interfaces + run schema migration
  WHY: agreeing on contracts before writing a single component prevents shape mismatches that require rewrites
- Final step is ALWAYS: start the app and walk every Done When condition from <context> end-to-end
  WHY: a spec that passes unit tests but breaks the core user journey is not done
- Max 200 lines per file. WHY: every file must fit in one AI context window for complete reasoning
- 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
- All database queries in server components or API routes only. WHY: keeps credentials server-side
- All environment variables documented in .env.example. WHY: next developer sets up in under 5 minutes
</constraints>
</task>