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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context: update frequency (daily at ~16:00 CET), the public-domain nature, and 'no auth required'. These go beyond the structured annotations and help the agent set expectations. No contradiction, so a 4 is appropriate.

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 sentences, front-loaded with the primary action and data source, then details on update frequency, options, and use cases. Every sentence contributes value; there is no redundancy or fluff. It is well-structured and appropriately sized.

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 simple read-only tool with an output schema and rich annotations, the description covers all essential aspects: data source, currency count, base default, update cadence, auth, and typical use cases. It does not mention error handling or rate limits, but those are not critical for this straightforward operation. Given the complexity, it is complete enough.

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% and each parameter already includes a detailed description (e.g., base lists supported codes, amount notes default 1.0, symbols explains comma-separated codes and omission behavior). The tool description only paraphrases these ('optional base currency override, target currency filter, amount scaling') without adding new information. Baseline 3 is correct when the schema carries the load.

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 verb ('Get') and the resource ('latest ECB reference exchange rates'), and specifies the scope (33 currencies, base EUR). It is specific and unambiguous. While it doesn't explicitly differentiate from sibling tools like historical or series, the 'latest' qualifier and the focus on current rates imply the distinction, so it earns a 4 rather than a 5.

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 provides clear use-case context ('FX dashboards, invoicing, multi-currency apps') and notes it's a read-only, no-auth tool. It does not explicitly state when to use this over the historical/series variants, nor does it mention alternatives, but the context is sufficient for an agent to understand typical usage. That matches a 4: clear context, no exclusions.

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