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

Annotations already declare readOnly, non-destructive, open-world, and non-idempotent; the description reinforces the read-only nature ('reads or computes only and changes nothing') and adds genuinely new information about billing: $0.002/call via x402 or a card key, 25 free calls, failed calls free, repeats are not free. That billing detail explains and justifies the idempotentHint=false in a way annotations cannot. It does not mention rate freshness or whether ECB reference rates are daily rather than truly 'live', which the title implies.

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?

The purpose and scope are front-loaded in a single dense sentence, followed by usage, an abbreviated example, and pricing. The 'Example: from=USD.' fragment is slightly awkward, and the pricing sentences, while useful, add length, but every line carries information.

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?

There is no output schema, and the description does not say what the response looks like: whether it returns a rates map per currency, a converted amount, a timestamp, or both. For a conversion tool that an agent may parse programmatically, that omission is a real gap, though the pricing, safety, and currency coverage are complete.

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 from, to (comma-separated codes, default all), and amount (default 1). The description adds the list of supported currencies and an example (from=USD), but nothing about syntax or defaults beyond the schema. 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?

States a clear verb+resource: fetch exchange rates from a base currency to many targets and optionally convert an amount, sourced from ECB reference rates. It does not differentiate itself from the sibling tools fx, fx_all, and fx_symbols, which is the main clarity gap.

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?

Offers concrete usage contexts ('convert prices, settle an invoice... show a rate in an agent's answer') and an example call, so when-to-use is implied. It never says when to prefer a sibling such as fx_all or fx_symbols instead, so selection between the FX tools is left to inference.

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