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

Annotations declare it is idempotent and non-destructive, but the description adds genuinely useful behavioral facts the annotations cannot: a $0.005 per-call cost, deterministic output, primary-source citation, and a billing policy that errors/unverified rules are never charged. That pricing and billing behavior is material for a pay-per-call service; it stops short of describing result format or rate limits.

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?

Two sentences, front-loaded with the computed outputs, with the cost and billing guarantee packed economically into the second. No filler.

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?

Annotations and an output schema exist, so return values need not be explained. The description covers purpose, cost, and billing policy, but is silent on what distinguishes the two payment methods or how prepayment interacts with them, which is the kind of nuance an agent would benefit from.

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 100%, so the baseline is 3. The description echoes the two enumeration values (level payment/principal) and mentions prepayment effect, but adds no format or semantic detail beyond what the schema already documents.

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 names a specific resource (mortgage) and its outputs: schedules under level payment/principal methods, total interest, and prepayment effects. This clearly separates it from the adjacent Japanese-finance siblings (jp_payroll_net, jp_consumption_tax), though it never explicitly names an alternative tool.

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?

There is no statement of when to use this tool versus siblings, nor any prerequisite or exclusion. The agent can infer the domain from 'mortgage schedules', but no routing guidance is provided.

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