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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely non-obvious behavior: holiday/weekend date resolution and a paid x402 micropayment model with a concrete price and chain. It does not describe rate source timing or what happens on unsupported currency pairs.

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, front-loaded with the operation, then the usage caveat, then the payment terms. No filler and each sentence contributes distinct information.

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 output schema exists, so return-value detail is unnecessary, and the description covers the mutation-free nature, date-resolution caveat and payment requirement. The remaining gap is base/quote orientation, which an agent needs in order to interpret the result 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 0%, so the description must carry parameter meaning and only partly does: 'three-letter ISO currency pair' clarifies the two currency params and 'specified historical date' clarifies the date. It never explains base-versus-quote direction, i.e. which currency the returned rate is expressed in, which is the key ambiguity for this parameter pair.

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 and resource: return the ECB reference rate for a three-letter ISO currency pair on a specified historical date. The word 'historical' plus 'ECB reference rate' implicitly separates it from get_fx_rate and convert_currency, but no sibling is named explicitly, so differentiation is inferred rather than stated.

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?

Gives a clear use context ('retrospective reconciliation or analysis') and warns that non-business dates may resolve to provider handling rather than an intraday quote. It stops short of naming get_fx_rate as the alternative for current rates or stating when not to use this tool.

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