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

Annotations already mark it read-only and non-open-world, so the bar is lower. The description adds material behavior not in annotations: non-publication-day fallback and a $0.002 USDC x402 payment cost. It does not describe response format or error behavior, but for a read-only rates call this is substantial.

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 one compact sentence with three useful clauses: date/base scope, non-publication fallback, and payment cost. It is front-loaded and contains no 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 rates lookup with full schema coverage and no output schema, the description covers scope, fallback behavior, and payment. It stops short of describing the returned structure or error behavior, but the essentials are 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 base, date, and symbols parameters are already documented. The description reinforces 'any base' and date semantics but does not add syntax, defaults, or formatting details beyond the schema, making the baseline 3 appropriate.

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 clearly identifies the resource as ECB reference rates for a specific date and any base, which implicitly separates it from rates_latest and rates_timeseries. However, it does not explicitly name or contrast those siblings, so it falls short of a 5.

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 date-specific framing implies this tool is for single historical dates, and the fallback note gives useful context. But it never states when to use rates_latest or rates_timeseries instead, nor does it give explicit exclusions.

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