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

Annotations already declare the tool to be read-only, idempotent, and non-destructive, so the safety profile is covered. The description adds important behavioral details: cross rates are computed through the euro, and the response includes the ECB rate date and cache age. This provides context beyond the annotations that helps an agent interpret results correctly.

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 brief and well-structured: the first sentence states the purpose and key interpretation, the second conveys additional behavioral details. Every sentence earns its place, with no redundant or filler content. The rate example is compact and immediately useful.

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 tool with two optional parameters, the description covers the essential context: when to use it, how rates are expressed, and what the response includes. There is no output schema, but the description partially compensates by mentioning the returned fields. It could mention edge cases like invalid base currencies, but this is minor given the tool's low complexity.

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?

The schema already provides full descriptions for both parameters, including the base/quote interpretation and the 'omit quotes for all' behavior. The description largely repeats this information, adding no new semantic detail beyond what is in the schema. With schema coverage at 100%, a baseline score of 3 is 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 states the tool's purpose: fetching the latest ECB daily reference rates against any base currency. It uses a specific verb ('call this tool for'), names the resource ('ECB daily reference rates'), and clarifies the rate interpretation with an example. This distinguishes it from sibling tools focused on historical rates or conversions.

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 usage context: use this tool when you need the latest ECB reference rates. It does not explicitly mention alternatives or exclusions, but the phrase 'latest ... rates' implies a contrast with historical rate tools like rate_history or rate_on. No conflicting guidance is present.

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