SmartFin

Compare renting and buying a home

smartfin_compare_rent_vs_buy

Read-onlyIdempotent

Compares buying a home with a fixed-rate mortgage against renting a similar home and investing the money not spent on the home, over the years someone plans to stay, the way SmartFin's rent vs buy calculator does. Both start with the same cash and spend the same each month; whichever costs less that month invests the difference. Returns the modeled money each choice leaves (home sold and loan paid, or kept as equity, against the investments), the difference, the first year-end at which buying leaves at least as much as renting, the starting rent at which both leave the same, the monthly payment, month-one costs and totals. Use when someone asks how renting and buying compare, how long they would need to stay for the two to cross, or what rent matches buying a home. Do not use for rental-property investing, for how much someone can borrow, for tax planning or for adjustable-rate loans. Rates are held constant; income-tax effects of owning and tax on the home sale are not modeled. It reports modeled money only and does not say whether to rent or buy. Amounts can be in any one currency; none is converted.

Input Schema

Name	Required	Description
`pmi_pct`	No	Mortgage insurance (PMI), % of loan a year, 0 to 3. Default 0
`currency`	No	Currency for the amounts in the summary and the link. Default USD; amounts are never converted
`keep_home`	No	True: the home is kept at the end of the stay and counted as equity, not cash; false: it is sold. Default false
`home_price`	Yes	Home price, in the currency chosen, 1000 to 100000000. Default 400000
`term_years`	No	Loan term, years, 1 to 40. Default 30
`hoa_monthly`	No	HOA or condo fees, a month, in the currency chosen, 0 to 10000. Default 0
`monthly_rent`	Yes	Monthly rent, a month, in the currency chosen, 1 to 1000000. Default 2000
`years_to_stay`	No	Years you plan to stay, whole years, 1 to 40. Default 10
`annual_rate_pct`	No	Mortgage rate, % a year, fixed, 0 to 25. Default 6.5
`cost_growth_pct`	No	Yearly increase in insurance and HOA, % a year, -5 to 15. Default 3
`maintenance_pct`	No	Maintenance, % of home value a year, 0 to 5. Default 1
`buying_costs_pct`	No	Closing costs when buying, % of price, 0 to 10. Default 3
`down_payment_pct`	No	Down payment, % of price, 0 to 100. Default 20
`property_tax_pct`	No	Property tax, % of home value a year, 0 to 5. Default 1
`rent_increase_pct`	No	Yearly rent increase, % a year, -10 to 15. Default 3
`selling_costs_pct`	No	Agent and closing costs when selling, % of sale price, 0 to 10. Default 6
`investment_tax_pct`	No	Tax on investment gains, % of gains, 0 to 50. Default 0
`home_insurance_yearly`	No	Home insurance, a year, in the currency chosen, 0 to 100000. Default 1500
`home_value_growth_pct`	No	Home value growth, % a year, -10 to 15. Default 3
`investment_return_pct`	No	Return on invested savings, % a year, -10 to 20. Default 6
`renters_insurance_monthly`	No	Renter's insurance, a month, in the currency chosen, 0 to 1000. Default 0

Output Schema

Name	Required	Description
`cite`	Yes	How to credit this result
`data`	Yes
`next`	Yes	Suggested follow-up calls
`links`	Yes
`how_to`	Yes	Steps for the user to see or redo this on smartfin.fyi
`source`	Yes
`summary`	Yes	Plain-language result the assistant can quote

Schema Changelog

Changes observed during successful MCP inspections.

AddedOct 13, 2026

TDQS

A4.4/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so credit is for added context: it explains the simulation mechanics (same cash, same monthly spend, difference invested), states key modeling limitations (rates held constant, income-tax effects and sale tax not modeled), and clarifies it reports modeled money only rather than a recommendation. The limitations are the valuable additions here.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loads the comparison method and behavior before usage guidance, which is well-ordered. However, the description is dense and lists many return fields (first-year crossover, break-even rent, monthly payment, month-one costs) in one long sentence that could be trimmed, especially since an output schema exists.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given a 21-parameter tool with an output schema and full annotations, the description covers the algorithm, limitations, exclusions, currency handling, and the distinction between modeled output and advice. Nothing material for correct invocation is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so every parameter is already documented with ranges and defaults. The description adds only high-level modeling context (currency never converted, rates fixed) rather than per-parameter meaning. Baseline 3 is correct when the schema does the heavy lifting.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource (compares buying vs renting) and immediately distinguishes itself from siblings like calculate_savings_vs_investing by naming the exact methodology: fixed-rate mortgage vs investing the difference. An agent can select this without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly gives three triggering questions ('how renting and buying compare', 'how long they would need to stay', 'what rent matches buying a home') and four exclusions (rental-property investing, borrowing capacity, tax planning, ARMs). This is model when-to-use/when-not-to-use guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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