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

Beyond the readOnlyHint annotation, the description discloses publication cadence, data freshness (daily reference, not live), the approximate size of the currency list (~30), and full pricing details ($0.001 USDC per successful call via x402, with failed calls not charged). These are material behavioral traits an agent needs before invoking the tool.

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 description is four sentences, front-loaded with what the tool provides and its cadence, followed by the nature of the data, the list size, and pricing. Every sentence adds necessary information with no repetition or filler.

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?

For a simple read-only data tool with three optional parameters and an output schema, the description covers the source, update frequency, data limitations, currency list size, and payment model. Nothing essential for correct invocation 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 description coverage is 100%, so the input schema already documents the 'to', 'from', and 'amount' parameters completely. The description adds limited meaning beyond the schema, mainly noting that conversion is between two listed currencies and that the list has about 30 currencies. Baseline 3 is appropriate when the schema handles parameter semantics.

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 states a specific verb and resource: it provides European Central Bank euro reference rates and an optional currency conversion. The scope (about 30 currencies, daily reference, not live market quote) is explicit, making it easy to distinguish from any live-market sibling.

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?

The description gives clear context for when to use it: rates are the ECB's daily reference published once per working day around 16:00 CET, and it explicitly says these are not live market quotes. It does not name an alternative tool, but the timing and nature of the data make the appropriate use case unambiguous.

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