How to Build a Calendar App

<context>
App: Calendar App
Difficulty: Intermediate
Estimated build time: 1–2 weeks

Data model:
  User: id, email, timezone, google_calendar_token
  Event: id, user_id, title, description, start_at (UTC), end_at (UTC), all_day (bool), recurrence_rule (RRULE string), google_event_id, color, created_at
  Attendee: event_id, email, status (accepted/declined/pending)

Recommended build order:
  1. Month view grid — Build a 7×5 CSS grid for the month view. Populate each cell with the day number. Show today highlighted. Use date-fns to calculate the first day of the month and pad with days from the previous month.
  2. Create and show events — Add a click handler on each day cell that opens a modal to create an event. Store events with start_at and end_at as UTC ISO strings. Render event pills on the correct day cells.
  3. Week and day views — Build a week view: 7 columns with hourly time slots. Position events as absolute divs based on their start time offset and height based on duration. Day view is the same with one column.
  4. Timezone handling — All events are stored in UTC. Use date-fns-tz to convert to the user's timezone for display. When creating an event, convert the user's local time back to UTC before saving. Never store local times.
  5. Recurring events — Store recurring events as RRULE strings (RFC 5545 standard). Use the rrule.js library to expand them into occurrences for the current view range. Never materialise all occurrences in the DB — generate them on the fly.
  6. Google Calendar OAuth + sync — Register a Google OAuth app. Request the calendar.events scope. On connect: fetch all events from the Google Calendar API and import them. On new event creation in your app, create it via the Google API and store the google_event_id.
  7. Invites and RSVP — On event create, allow adding attendee emails. Send invite emails via Resend with Accept/Decline links. Each link hits /api/rsvp/[token]?status=accepted which updates the attendee row and sends a confirmation.

Known pitfalls to avoid:
  - Materialising all recurring event occurrences in the database — a "repeat forever" event would generate infinite rows. Store the RRULE string and use rrule.js to expand it for the current view window only.
  - Comparing dates without timezone awareness — new Date("2026-04-09") in JavaScript treats the string as UTC midnight, which renders as the previous day in western timezones. Always use date-fns-tz for timezone-aware comparisons.
  - Fetching all events for all time on load — query only events within the current view range (month start to month end + padding). A user with 5 years of events should not have them all loaded on the first render.

Tech stack (intermediate): Next.js + Supabase + FullCalendar + Google Calendar API + date-fns-tz
</context>

<role>
You are a Senior Full-Stack Engineer and product architect who has shipped production Calendar 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>