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

Goes well beyond the readOnlyHint annotations: it discloses the exact price ($0.002/request), the charging model (one paid HTTP GET = one charge), the payment rails (x402 on Base, MPP on Tempo), and that 4xx responses are free. It also explains the data-availability edge case — non-publication days resolve to the prior publication date via as_of — which the agent cannot get from annotations or schema.

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?

Two sentences, front-loaded with the core behavior before the cost details. The pricing clause is long — multiple clauses on payment rails — but it is genuinely actionable for an agent about to spend money, so almost every element earns its place.

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?

An agent knows the cost, the fallback semantics, and the parameters before calling; the '(as_of)' mention hints at a response field. With no output schema, the missing piece is the actual response shape (rate fields, how symbols filters output), which leaves a modest gap for a data-returning tool.

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 baseline is 3, but the description adds real meaning: 'any base' clarifies the currency-inversion capability of base, and the fallback/as_of note explains what date actually resolves to on holidays. symbols is not mentioned in the description at all.

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?

States a specific verb+resource (ECB reference rates) and scopes it to 'a given date in any base', which implicitly separates it from rates_latest and rates_timeseries. It never names those siblings explicitly, so differentiation still requires the agent to infer from the sibling list.

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?

'On a given date' implies when this is the right choice versus rates_latest, but the description never says 'use rates_latest for today' or 'use rates_timeseries for ranges'. The non-publication fallback is a useful behavioral note but not a usage rule.

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