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

Annotations only cover read-only and open-world hints; the description adds substantial context beyond them: ~30 currency coverage, ECB reference-rate source, returned fields (rate, result, as_of), and a full pricing model including that 4xx errors are not charged and that payment runs via x402 on Base or MPP on Tempo. That cost/auth disclosure is exactly what an agent needs before calling a paid endpoint.

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 the purpose and example before the pricing detail. The final parenthetical is dense (payment rails, charge semantics) but every clause is decision-relevant; slight tightening would make it a 5.

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?

No output schema exists, and the description compensates by naming the returned fields. For a 4-parameter read-only conversion tool, everything needed to call it correctly — currencies, formats, cost, result shape — is present.

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 ISO 4217 and YYYY-MM-DD formats are already documented; the description's added value is the worked example and the ~30-currency bound. No syntax or date-range limits beyond what the schema states, so baseline 3 applies.

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?

Specific verb and resource ('convert an amount between any two of ~30 currencies'), plus a concrete example (100 USD to JPY) and the rate source. It is distinguishable from the rates_* siblings, which surface rates rather than converting amounts.

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 is implied by the example and by 'at the latest ECB reference rate or as of a past date', which tells the agent the date param selects historical conversion. It never names rates_latest or rates_historical as alternatives, so the agent must infer when to convert versus when to fetch rates.

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