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

Annotations are absent, so the description carries the full burden. It discloses the cron refresh mechanism, the daily/reference nature, the response fields `snapshot.captured_at` and `snapshot.age_hours`, the per-call cost, the payment challenge on unpaid calls, and that no-data calls are never setted. That is comprehensive behavioral disclosure for a read-only data tool.

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 sentences, each earns its place: first defines the resource, second explains data nature and freshness fields, third explains cost and payment behavior plus the equivalent HTTP route. It is front-loaded with purpose and contains no filler.

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?

The description covers freshness, payment/settlement behavior, and the key response fields, which matters because there is no output schema. It omits the precise rate-mapping format, but the core semantics are complete enough for an agent to invoke it 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?

Schema description coverage is 100%, and both `base` and `as_of` already have clear schema-level descriptions. The tool description adds only marginal input-relevant context, such as 'chosen base currency' and 'ECB observation date', but does not meaningfully compensate or extend parameter semantics beyond what the schema provides.

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 identifies a specific resource — European Central Bank reference foreign-exchange rates for a chosen base currency — and explains that these are daily reference rates, not live tradable quotes. This makes the tool distinct from the crypto/defi/treasury siblings and leaves no doubt about what it does.

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 it: when official ECB daily reference rates are needed, with freshness information. It also explicitly excludes live tradable quotes, which is a key usage boundary. It does not name alternative sibling tools or provide when-not-to conditions beyond the 'not live' qualifier, so it stops short of full routing guidance.

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