Calculate mortgage payment (local)

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

Annotations already declare readOnlyHint, openWorldHint=false and idempotentHint, so the safety profile is covered. The description goes beyond them by disclosing determinism, the no-network guarantee, the automatic PMI trigger when LTV > 80%, and the 20% down-payment default — real behavioral context the annotations do not carry.

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-loads purpose and output, then moves systematically through the input rules with no filler sentences. Dense but every clause carries information; slightly over-packed into a single block.

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 correctly takes on the burden of describing the return value (PITI line items plus total interest) and covers the key input rules for a 10-parameter tool. Adequate for an agent to invoke correctly, with only minor incidental parameters left undocumented.

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?

Schema description coverage is only 40%, so the description has to compensate, and it does: it clarifies the down_payment vs down_payment_percent mutual exclusivity and default, and the property_tax_annual vs property_tax_rate alternatives. It leaves hoa_monthly and insurance_annual to their self-explanatory names, which is a minor gap.

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 and resource ("mortgage payment calculator") plus the exact output (full PITI breakdown and total interest). It also flags the local-only scope, which sets it apart from fetch-based siblings, but it never names an alternative like homes_calculate_affordability or homes_estimate_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.

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

Provides clear usage context: "safe to use for scenario comparison without burning a fetch," which tells the agent when this local calculator is preferable to a network call. It stops short of explicit exclusions or naming a sibling alternative, so it lands at clear-context rather than full routing guidance.

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