Use the calculator from Claude
MCP server, free, no sign-in
The same model that powers this site is available as a Model Context Protocol (MCP) server. Connect it to Claude, or to any other MCP client, and ask questions in plain language: Claude runs the scenarios, comparisons and sensitivity sweeps for you and reports the numbers, instead of you moving sliders by hand.
Same numbers as the site. Same defaults, same units, same 2026 tax tables. Every answer includes the engine version so it can be checked against rentbuy.org.
Endpoint
https://rentbuy.org/api/mcp
Transport: Streamable HTTP. Authentication: none. Nothing you send is stored.
Connect
Claude on the web or Claude Desktop
- Open Settings, then Connectors, then Add custom connector.
- Name it RentBuy and paste the endpoint above.
- Choose no authentication and save.
Claude Code
claude mcp add --transport http rentbuy https://rentbuy.org/api/mcpOther MCP clients
Point the client at the endpoint as a remote Streamable HTTP server with no authentication. You can inspect it with the official tool:
npx @modelcontextprotocol/inspector --cli https://rentbuy.org/api/mcp --transport http --method tools/listWhat Claude can do with it
rentbuy_run_scenario
One 30-year comparison: the winner, both households' net worth, the break-even year, totals and year-1 monthly cash. Optional year-by-year rows.
rentbuy_compare_scenarios
Up to ten named variants side by side, with deltas against the first one. One call instead of ten.
rentbuy_sweep
Vary one or two inputs across a range and get the result at every point plus the crossover where the winner flips.
rentbuy://defaults
A reference of every input with its unit, default and allowed range, plus the model's conventions.
Try asking
- "At what monthly rent does buying beat renting for an $800,000 home at 6.5%?"
- "Compare a 15-year and a 30-year mortgage for a $650,000 home with 10% down."
- "Sweep the down payment from 5% to 30% and show the break-even year at each step."
- "I earn $150,000 and file single. Does buying win if rent is $4,200?"
How inputs work
- Every input is optional; anything you leave out uses the site defaults ($800k home, 20% down, 6.5% for 30 years, $3,500 rent, 7% return).
- Money is in US dollars and rates are percent points: a mortgage rate of 6.5 means 6.5%.
- Giving household income and filing status pre-fills the federal tax rate and the standard deduction from the 2026 IRS tables; you can still override them.
- Year-by-year net worth is before selling costs; the headline result sells the home at year 30 and subtracts them.
Good to know
- The server is read-only and deterministic: the same inputs always give the same answer.
- It is rate limited to 120 requests per minute per address, which is far more than a conversation needs.
- Results are estimates for comparison, not financial advice. The model does not include the SALT cap, the mortgage-interest limit, PMI or capital gains.
- Requests are computed and returned; we do not store them. See the privacy policy.