Calculate mortgage payment (local)

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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false; the description adds genuinely new behavior by stating there is no network call, that it is fully deterministic, and that PMI applies automatically only when LTV > 80% and pmi_rate is supplied. It does not cover error behavior for invalid inputs, but that is a minor gap given the annotation coverage.

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?

Front-loaded with purpose and return value before parameter rules; four sentences with no filler. Slightly dense in the parameter-rule sentence, but every clause carries information an agent needs.

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?

With no output schema, the description usefully enumerates the PITI return components and total interest, and the annotations cover the safety profile, so an agent has enough to call it correctly. Missing coverage of two parameters and no stated behavior on conflicting down-payment inputs keeps it from being fully complete.

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 description coverage across 10 parameters, the description does real work: it states the down_payment/down_payment_percent mutual exclusivity and 20% default, the property_tax_annual vs property_tax_rate alternatives, and the LTV condition for PMI. It leaves hoa_monthly and insurance_annual unmentioned, so the compensation is strong but not complete.

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?

States a specific verb+resource ('Local-only mortgage payment calculator') with a clear scope, and the 'local-only' qualifier implicitly separates it from the network-backed sibling tools. It stops short of explicitly naming compass_calculate_affordability, the nearest sibling, so the boundary between the two calculators must be inferred.

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?

Usage context is implied rather than stated: 'safe to use for scenario comparison without burning a fetch' suggests when it is preferable to a network tool, but there is no explicit when-to-use vs when-not rule, nor any pointer to the affordability sibling. The rest of the guidance is parameter mechanics rather than tool selection.

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