US Mortgage Calculator

Work out how much house someone can afford

calculate_affordability

Read-onlyIdempotent

Maximum affordable home price from income, existing debt payments and available down payment, using the 28/36 qualifying ratios. Reports which of the two ratios is the binding constraint, which is the actionable part: a buyer limited by the back-end ratio can raise their budget by clearing debt, one limited by the front-end ratio cannot.

Input Schema

TableJSON Schema

Name	Required	Description
`state`	No	Full US state name, for example "Texas". Used to look up the average property tax rate. Either this or propertyTaxRatePercent is required.
`monthlyHoa`	No	Monthly HOA dues.
`loanTermYears`	No	Loan term in years. Most US mortgages are 15 or 30.
`annualInsurance`	No	Annual homeowners insurance premium.
`annualGrossIncome`	Yes	Household income before tax, in US dollars per year.
`downPaymentAmount`	Yes	Cash available for the down payment, in US dollars. A dollar amount, not a percentage.
`backEndRatioPercent`	No	Maximum total debt as a share of gross income. 36 is conservative; lenders often allow 45 and sometimes 50.
`monthlyDebtPayments`	No	Existing minimum monthly debt payments: car loans, student loans, credit card minimums. Excludes housing.
`frontEndRatioPercent`	No	Maximum housing cost as a share of gross income. 28 is the conservative standard.
`propertyTaxRatePercent`	No	Annual property tax as a percentage of home value. Omit and pass `state` instead to use that state's average.
`annualInterestRatePercent`	No	Annual nominal interest rate as a percentage, for example 6.5 for 6.5%.

TDQS

A4.2/5.0

Behavior4/5

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

Annotations already disclose read-only, idempotent, non-destructive behavior. The description adds valuable context by stating the use of 28/36 ratios and explaining how the binding constraint translates into actionable advice, such as clearing debt affecting only back-end-ratio-limited buyers.

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

Conciseness5/5

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

The description is two sentences long, with the first sentence stating purpose and method, and the second explaining the unique binding-ratio output. Every sentence earns its place, and the structure is front-loaded.

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

Completeness4/5

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

No output schema exists, so the description must convey return behavior. It explains the binding-ratio report and its practical interpretation, which is sufficient for an 11-parameter tool with required parameters clearly still covered by the schema. It could be more explicit about the output structure, but the core information is present.

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 coverage is 100% for all 11 parameters with detailed descriptions, so the tool description does not need to add parameter-level detail. It only groups three key inputs (income, debt, down payment) without enriching semantics beyond the schema, hence the baseline score of 3.

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?

The description clearly states the tool computes the maximum affordable home price from income, existing debt payments, and down payment, and uniquely highlights that it reports which ratio is binding, differentiating it from siblings like calculate_mortgage or calculate_rent_vs_buy.

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

Usage Guidelines4/5

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

The description provides clear context for when to use this tool (affordability calculation) and explains the actionable meaning of the binding ratio, but it does not explicitly name alternative tools or exclusion scenarios, lacking the 'when-not-to-use' guidance needed for a top score.

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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TDQS

A4.5/5.0

Disambiguation5/5

Each tool targets a distinct calculation or information need: affordability, amortization, full mortgage payment, PMI, property tax, refinancing, rent vs buy, loan comparison, closing costs, state rules, and state list. Even overlapping tools like calculate_mortgage and calculate_pmi are clearly separated by depth and purpose.

Naming Consistency5/5

All tool names follow a consistent verb_noun snake_case pattern, using verbs like calculate, compare, estimate, get, and list. The pattern is uniform and predictable across the entire set.

Tool Count5/5

With 11 tools, the server is well-scoped for a US mortgage calculator domain. Each tool covers a meaningful aspect of mortgage analysis without redundancy or bloat.

Completeness5/5

The tool surface covers the full lifecycle of mortgage decision-making: affordability, payment estimation, amortization, PMI, property tax, refinancing, rent vs buy, loan comparison, closing costs, and state-specific rules. No significant gaps are apparent for the stated purpose.

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