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

Beyond the readOnlyHint/openWorldHint annotations, the description discloses the upstream source (ECB fix), the computed cross-rate behavior, that the exact rate date is returned, that the source is named in every answer, that HTTP 400 occurs before payment, that unknown currencies or outages are not charged, and the exact price model (0.002 USDC per successful call via x402). This is unusually rich behavioral context that annotations cannot convey.

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 text is front-loaded with the usage trigger, then scope, then return behavior, then error and pricing terms — a sensible ordering with no filler sentences. It is dense and packs pricing plus error semantics into one paragraph, which slightly taxes scanning but each clause carries distinct 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?

With no output schema, the description compensates by stating that the exact rate date and the source are returned, and it covers errors and billing. The remaining gap is the response shape (field names, whether cross rates are echoed per symbol), which an agent must discover at call time, but nothing essential for invoking the tool correctly is missing.

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, but the description adds meaning the schema does not: base can be any ECB reference currency with cross rates computed from the fix, dates go back to 1999, and symbols defaults to all ~30 currencies. It does not elaborate on symbol validation or comma-format edge cases, keeping it just short of a 5.

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 verb and resource ('official exchange rate to convert prices, invoices or portfolios') and pins the data source to ECB euro reference rates covering ~30 currencies since 1999. It is immediately distinguishable from siblings like interest_rates, inflation, or treasury_yields, which cover different macro series.

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 when an agent needs an official exchange rate to convert prices, invoices or portfolios' gives a clear triggering condition, and the scope ('latest or for any day since 1999', 'any base currency') further narrows applicability. It does not explicitly state when not to use it or name a sibling alternative, but the domain is narrow enough that this is a minor gap.

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