Calculate mortgage PITI

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

Annotations already indicate readOnly, idempotent, and not openWorld. The description adds valuable behavioral context beyond annotations: it is local-only (no network call), PMI applies automatically when LTV > 80% and pmi_rate is provided, and down_payment_percent defaults to 20%. This enriches the agent's understanding of the tool's side effects and default behavior.

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

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

The description is concise and well-structured. It front-loads the primary purpose, then details the output and key parameter relationships. Every sentence adds value without redundancy or fluff.

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

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

For a calculator with 10 parameters and no output schema, the description covers the essential output (PITI breakdown and total interest), the key parameter alternatives, and the PMI rule. It does not explicitly state loan_term_years default (though the schema does) or error behavior, but the coverage is adequate for an agent to call the tool correctly.

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

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

With only 40% schema coverage, the description must compensate for undocumented parameters. It explains the down_payment/down_payment_percent alternatives, property_tax_annual/property_tax_rate options, and the default for down_payment_percent. However, it does not explicitly describe hoa_monthly or insurance_annual inputs (though they are implied by the PITI breakdown), leaving some parameters under-explained.

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

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

The description clearly states the tool's purpose: it is a local-only mortgage payment calculator that returns a full PITI breakdown and total interest. It distinguishes itself as local-only with no network call, but it does not explicitly differentiate from the sibling onehome_calculate_affordability, so it stops short of a full 5.

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

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

The description provides parameter usage rules (down_payment OR down_payment_percent, property tax alternatives, PMI condition) but does not explicitly state when to use this tool versus the affordability calculator or any other alternative. The 'local-only' and 'no network call' hints imply use for quick offline calculations, but that guidance is implicit rather than explicit.

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