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

Annotations already declare readOnlyHint, idempotentHint, and openWorldHint, lowering the burden on the description. The description adds useful context: the source (ECB), the default base currency (EUR), and the daily reference nature of the data, without contradicting any annotation.

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?

A single sentence, front-loaded with the action and result, with no filler. It efficiently includes the data source and default currency without unnecessary detail.

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 tool with one optional parameter, rich safety annotations, and no output schema, the description conveys the core return value and its context. It does not spell out the exact response shape, but 'exchange rates relative to a base currency' gives an agent enough to predict the output; explicit sibling differentiation is the main minor gap.

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 100%, so the base parameter is already fully documented. The description reinforces that rates are relative to the base currency and notes the EUR default, but this adds only modest meaning beyond the 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 uses a specific verb ('Return'), names a precise resource ('current daily reference exchange rates'), and adds the data source (ECB). This clearly distinguishes it from siblings like fx_historical, fx_pair, and fx_convert.

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 phrase 'current daily reference' and 'ECB' imply this is intended for daily reference rates rather than historical or intraday data, but the description never explicitly names alternatives or states when not to use it. With many fx_* siblings, explicit routing guidance would strengthen it.

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