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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds genuinely useful context the annotations don't: the data is ECB-published, each rate carries a freshness/staleness flag, and it is free. It says nothing about update cadence, caching, or what a 'stale' result means operationally, so it goes only modestly beyond the structured fields.

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?

Short and front-loaded: the resource and its freshness metadata lead, and the disclaimers follow as a guardrail. The staccato fragments ('Information only.' 'Free.') are efficient, though 'Free.' is marginal information that could have carried a more useful caveat.

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?

With no output schema, the description does its job of sketching the return shape (rate, date, fresh/stale) and warns against misreading the values as tradable. It is complete enough for a one-optional-param lookup, with the only real gap being the default behavior when `currency` is omitted.

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 description coverage is 100%, with the single optional `currency` field documented as an ISO code such as USD, so the baseline is 3. The description adds nothing about the parameter — notably it never states what omitting it returns (all currencies?) or how the euro base interacts with a non-EUR currency filter.

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 specific resource (ECB euro reference rates) and what accompanies it (date, fresh or stale), and explicitly fences it off from rate/quote tools with 'Not a Keeta rate, not a tradable rate, and not a quote.' The verb is only implied by the tool name, but the resource and scope are unambiguous.

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?

'Information only' plus the triple negation gives clear when-not-to-use guidance: don't reach for this when you need something tradable or quotable. It stops short of naming the sibling to use instead (e.g. quote_stablecoin_fx), so the routing half is left to inference.

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