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

With no annotations provided, the description carries the burden of behavioral disclosure. It discloses that the operation costs 2 credits and requires an API key, which are important operational details. It also implies a read-only nature (ECB reference rates) and describes the two behaviors. It does not explicitly state side effects or error handling, but for a simple lookup tool, this is adequate.

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 three short sentences, front-loaded with the core purpose ('ECB reference rates'), followed by concise usage instructions and necessary caveats (credits, API key). Every clause is informative and there is no redundant wording.

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?

Given the tool's simplicity (3 optional parameters, no output schema, no annotations), the description covers purpose, usage, cost, and auth. It does not explicitly describe the return format, but the phrasing 'full latest table' and 'convert' implies the response structure. This is sufficient for an agent to select and invoke the tool, though a bit more detail on return shape would make it complete.

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 0%, so the description must explain parameters. It does so effectively: 'from' and 'to' are currency codes implied by context, 'amount' defaults to 1, and omitting both fetches the full table. This goes beyond the schema (which only provides patterns and types) by explaining the role and optionality of each parameter.

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 provides ECB reference rates and supports two modes: full table or conversion. It is distinct from sibling tools (holidays, timezone, VAT, IBAN) and uses specific language like 'Omit from/to for the full latest table, or pass from+to (+amount, default 1) to convert', making the purpose unmistakable.

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 explicit instructions on how to use the tool in its two modes ('Omit from/to...' and 'pass from+to...'), which serves as clear usage context. However, it does not mention when not to use it or name any alternative tools, so it lacks explicit exclusionary guidance, but the guidance provided is clear enough for a 4.

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