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chrischall

zillow-mcp

by chrischall

Estimate rent-vs-buy break-even over a horizon

zillow_estimate_rent_vs_buy
Read-onlyIdempotent

Compare the cumulative costs of buying a home versus renting over N years, factoring in down payment, mortgage, taxes, maintenance, appreciation, and rent growth to determine the break-even year.

Instructions

Project the cumulative cost of buying a home versus renting a comparable place over N years. Accounts for down payment, closing costs, monthly PITI, maintenance (~1%/yr default), property appreciation (~3%/yr default), rent growth (~3%/yr default), and the opportunity cost of the down payment (renter invests it at the investment_return_rate, default 6%/yr). Returns the year-by-year cumulative costs, the break-even year, and the net difference at the horizon. No network — pure local math.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
home_priceYes
hoa_monthlyNo
down_paymentYes
monthly_rentYes
horizon_yearsNoDefault 7
interest_rateYes
loan_term_yearsNo
insurance_annualNo
maintenance_rateNoAnnual % of home value, default 1.0
rent_growth_rateNoAnnual %, default 3.0
appreciation_rateNoAnnual %, default 3.0
closing_cost_rateNo% of home price, default 2.5
property_tax_rateNo
selling_cost_rateNo% of sale price, default 6.0
investment_return_rateNoAnnual return on the renter's parallel-invested down payment, default 6.0
Behavior5/5

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

Annotations already indicate readOnlyHint=true and idempotentHint=true. The description adds significant transparency beyond annotations by stating 'No network — pure local math,' which confirms the tool is self-contained and safe. It also explains default values and the opportunity cost concept, giving agents deeper understanding of behavior.

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?

The description is well-structured, starting with the main purpose and then detailing components and outputs. It is slightly verbose (multiple sentences) but every sentence adds value. Could be trimmed slightly while retaining clarity.

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?

Given the tool's complexity (15 parameters, no output schema), the description covers the key model assumptions, outputs (year-by-year costs, break-even year, net difference), and the fact it is local math. However, it does not mention edge cases (e.g., negative interest rates) or validation of inputs.

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 47% (low), so the description should compensate. It explains the model and mentions defaults (e.g., maintenance ~1%/yr, investment_return_rate default 6%/yr) that are not in the schema for all parameters. However, it does not detail each parameter individually, and some parameters like loan_term_years or hoa_monthly are not described in either schema or description.

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 it projects cumulative cost of buying vs renting over N years, listing specific components (down payment, closing costs, PITI, maintenance, appreciation, rent growth, opportunity cost). It distinguishes from sibling tools like zillow_calculate_mortgage or zillow_calculate_affordability by focusing on the rent-vs-buy comparison and break-even analysis.

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

Usage Guidelines3/5

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

The description implies usage for rent-vs-buy decision making but does not explicitly state when to use it vs alternative tools (e.g., zillow_calculate_mortgage for mortgage-only calculations) or mention any prerequisites or exclusions.

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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