Mortgage Prepay Decider — Pay Down or Invest?
Pay down the mortgage, or invest?
Most calculators only tell you how much interest you'd save. The real question is where this money does the most good. Money you prepay is hard to get back, investment returns carry risk, and if your emergency fund is thin, both choices are dangerous.
What you get
- Three options side by side: prepay, invest, or keep cash — ending balance, risk, and liquidity for each.
- Shorter term vs lower payment: how much interest each prepayment style saves, and how many years earlier you're debt-free.
- Small extra payments: what paying a little more every month adds up to.
- One clear recommendation: how much to top up your emergency fund first, and how to split the rest.
Who it's for
Anyone with a mortgage and a bonus, windfall, or savings they're not sure how to use. Works in any country and currency — you enter the rates and loan terms.
How to use
- Download from Capafy — you get a zip file (no need to unzip).
- In Claude, turn on code execution, then upload the zip under Settings → Skills. (Claude Code users: unzip into ~/.claude/skills/.)
- Describe your situation in plain English. Claude asks for the numbers it needs, runs the model, and walks you through the results.
Output and language
Claude explains the results in your language. The Excel workbook and written report are currently labeled in Traditional Chinese; English labels are planned for a future version.
Privacy
Runs locally in your own Claude environment. No external connections, no API keys, and your numbers never leave your machine.
Requirements
A Claude environment that can run Python (e.g. Claude Code, or Claude with code execution enabled). Uses openpyxl; Claude can install it on first run.
Disclaimer
Investment returns are assumptions, not predictions. Penalties and taxes depend on your loan contract and local rules. For personal planning only — not investment, tax, or legal advice.