Build your own Microsoft Excel with Claude Code

Build my own take on Microsoft Excel: a spreadsheet app. Give it an original name, logo, colours and copy, and do not use Microsoft Excel's name, branding or assets.

Version one should have:
- Grid with cell editing
- Formulas such as SUM, AVERAGE and IF
- Multiple sheets
- Formatting and number formats
- Import and export CSV and XLSX

The hard part: A formula engine with dependencies. Use HyperFormula and a virtualised grid.

How to build it:
1. Read this folder first. If it is empty, set up Next.js (App Router, TypeScript, Tailwind) with Postgres through Prisma or Supabase, and Supabase Auth or Auth.js for sign-in. Write a short PLAN.md with the data model, screens, third-party services and what you are leaving out of version one.
2. Build the core loop end to end before anything else: grid with cell editing, working with real data saved to the database.
3. Add the rest of the features one at a time, and seed realistic demo data so every screen looks alive.
4. Keep API keys in environment variables, and put any paid service behind a small interface with a local stub, so the app runs on a fresh machine without accounts.
5. Run it, click through every screen on desktop and mobile widths, fix what breaks, and finish with a summary of what works, what is stubbed and how to deploy it.

Paste it into Claude Code in an empty folder. It plans first, so read PLAN.md and steer before it starts building.