Calculate mortgage payment (local)

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

Annotations already cover the safety profile (readOnly, idempotent, closed-world), so the description's added value is the operational detail: no network call, fully deterministic, and the conditional PMI logic (auto-applies when LTV > 80% and pmi_rate is provided). It does not describe error behavior or loan-term edge cases, but it goes meaningfully beyond the annotations.

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?

Dense but front-loaded: what it is, what it returns, why it is safe, then the parameter rules. Every sentence carries information, though the parameter-rules block is a little packed and could be broken up for scannability.

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 10-parameter tool with no output schema and 40% schema coverage, the description covers the return structure (PITI components + total interest) and the key parameter ambiguities. Gaps are minor — permissions/edge-case behavior are irrelevant for a pure local computation.

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 coverage is only 40%, so the description has to compensate, and it does: it declares the down_payment/down_payment_percent mutual exclusivity, the down_payment 20% default, and the property_tax_annual vs property_tax_rate duality. It leaves hoa_monthly, insurance_annual, and loan_term_years semantics to the schema, but the ambiguous pairs are resolved.

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') and immediately qualifies it as local-only and deterministic, which separates it from the network-backed siblings like redfin_calculate_affordability and redfin_get_property. The return contents (PITI + total interest) are named, so an agent knows exactly what it gets.

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?

It gives a clear context for use ('safe to use for scenario comparison without burning a fetch'), which tells the agent when this tool is preferable to a network fetch. It stops short of explicitly naming the nearest alternative (redfin_calculate_affordability) or stating exclusions, so it is strong but not fully routing.

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