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

Annotations cover the safety profile (readOnlyHint, destructiveHint=false, openWorldHint, idempotentHint=false), and the description adds substantial context beyond them: pricing ($0.003/call via x402 USDC or a card key at /credits), the 25-free tier, free failures, and the fact that repeat calls are charged. That last point directly explains the non-idempotent annotation, which is unusually helpful.

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 resource and currency coverage, then usage, then example, then pricing. Slightly dense in the final pricing sentence, but every clause carries operational weight and nothing is padding.

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 compensates by naming the return contents (rates, converted amounts, the date the rates apply to). Combined with the cost/auth model and the read-only guarantee, an agent has everything needed to call it correctly without opening anything else.

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 100%, so the baseline is 3; the description goes further by explaining the from/to/amount relationship in prose and giving a concrete invocation ('from=USD, to=EUR,GBP,SEK, amount=100'), clarifying that 'to' is a multi-value list and that rates, conversions, and the applicable date all come back together.

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?

Names a specific resource (ECB foreign exchange rates for ~30 currencies) and the concrete operation (rate lookup plus conversion), and the currency-list detail plus the example tell an agent exactly what it retrieves. It is clearly distinguishable from siblings like btc_price, price_token, or token_report, which are crypto-asset scoped.

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?

States concrete use cases ('currency conversion, invoice and price calculations, and reporting in another currency') and supplies a worked example, so the agent knows the calling shape. It does not name an alternative sibling to prefer or exclude against, so it falls short of explicit when-not guidance.

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