list_currencies

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

With no annotations, the description carries the burden and does disclose the salient behavior: it is a read-only enumeration limited to euro-tracked currencies, with a concrete return example. It stops short of noting that the list is static or whether any auth/rate constraints apply, but for a zero-argument lookup the disclosure is largely sufficient.

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?

Three short, front-loaded sentences: scope, return shape, and intended use, with no filler. The inline JSON example earns its place by pinning the exact field names.

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 lookup with an output schema that already defines return values, the description supplies everything else an agent needs: what the list contains, the shape of an entry, and when to call it.

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 takes zero parameters, so there is nothing for the description to disambiguate; the baseline for a parameterless tool applies. The description's mention of codes and names relates to output, not input, which is already covered by the output schema.

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 a specific resource with scope ('The ISO 4217 currencies Frankfurter tracks against the euro') and states exactly what is returned ('each currency's code and name'), so an agent can distinguish it from convert/latest_rates without opening a schema.

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?

It gives an explicit use case: 'check whether a currency code is valid before calling the other tools.' This routes the agent correctly relative to the sibling rate/convert tools, though it does not name which siblings consume the codes or state when the tool is unnecessary.

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