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

Annotations already declare readOnlyHint=true, destructiveHint=false and idempotentHint=false, so the safety profile is covered. The description adds genuinely non-structured behavior: per-call pricing ($0.01 via x402 USDC or card key, first 25 free, failed calls free) and the modelling assumptions (fixed rate, monthly payments). The 'each call is charged, so repeats are not free' line usefully reinforces, rather than contradicts, idempotentHint=false.

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 what is computed and what is returned, followed by usage and pricing; every clause is informative. The pricing sentence is somewhat dense (mixing x402, card keys, free tier and failure policy) and could be trimmed without losing meaning.

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 carries the return-shape burden and does so well, enumerating monthly payment, total paid, total interest, and schedule columns (principal, interest, balance). Minor gaps remain: assumed currency and behavior when term is omitted or when both years and months are given.

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 schema already documents units and defaults for rate, years, months and rows; baseline 3 applies. The description adds only a sample value pair (principal=300000, rate=4.5) and implies that only 'the first months' of the schedule are returned, without clarifying precedence when both years and months are supplied.

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 precise verb-and-resource: computes the monthly payment of an amortised loan plus total paid, total interest and the first rows of the repayment schedule. The qualifiers 'fixed rate, monthly payments' and 'amortised' scope it tightly enough that an agent can distinguish it from calc_compound_interest or calc_tip without opening the schema.

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?

'Use to compare loan offers or plan a mortgage' gives clear positive usage context, and the worked example (principal=300000, rate=4.5) shows a canonical invocation. However, no alternatives or when-not conditions are named, so the agent gets no routing guidance against the other calc_* siblings.

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