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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false, and destructiveHint=false. The description adds useful behavioral context beyond those: the result is the complete closed domain, contains rates and decimal places, and acts as an authoritative filter for other operations. This meaningfully complements the annotations.

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?

Two tight sentences with no filler. The first sentence states what the tool returns and the second immediately gives the operational implication. Every clause earns its place.

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 zero-parameter read-only list tool, the description is complete: it states the content, the scope, and the practical consequence for other tools. There is no output schema, but the description tells an agent enough about what to expect (currencies, rates, decimal places) to invoke and interpret the result correctly.

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 tool has zero parameters and the schema documents that fully, so there is nothing parameter-related to explain. The description correctly focuses on the returned content instead. The baseline of 4 applies because the schema already covers the complete parameter space.

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 the exact resource (the ECB daily currency set), the content (rate against the euro and decimal places), and the scope (the whole domain). It also differentiates from sibling conversion/history tools by stating that any code not on this list cannot be used elsewhere.

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 clearly implies when to use this tool: to enumerate the complete set of valid currencies or validate whether a code is supported. It does not explicitly name sibling alternatives like convert or rate_history, but the domain-boundary statement ('a code not on this list cannot be converted, quoted or historised here') gives strong contextual guidance.

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