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

Annotations cover the safety profile (readOnly, idempotent, closed-world), and the description goes well beyond them by disclosing the payment/auth requirement: $0.05 per successful call settled over x402 with USDC on Base. That cost and payment-protocol detail materially affects whether and how an agent should invoke the tool, and it is not available anywhere in 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?

Front-loaded with the operation, then the returned fields, then the negative scope, then cost. The sentences are dense but each carries distinct information; the enumeration of return fields is long but not redundant with the schema. Minor trimming would be possible in the field list.

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 carries the return-value burden and does so well by naming each returned field plus the rounding increment use case. It also closes the exchange-rate expectation. The remaining gap is that all three parameters are optional, and the description never says what a no-argument call yields.

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%, so the schema already documents code, country, and include with their accepted formats, making 3 the baseline. The description adds only a light gloss on include ('historic codes on request') and does not clarify whether code and country are mutually exclusive or what happens when neither is supplied.

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 ('Currency lookup by ISO 4217 alphabetic or numeric code') and enumerates the exact facts returned (name, minor-unit exponent, fund flag, cash rounding, territories, historic codes). It also carves out negative scope ('returns currency facts only and never an exchange rate'), which prevents confusion with rate tools. It does not, however, differentiate itself from the adjacent sibling country_lookup, which a caller could easily reach for by mistake.

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?

It gives a purpose statement ('the correct rounding increment for cash amounts, which is what a caller needs to render or settle a price') that implies when the tool is useful. But there is no explicit when-to-use vs when-not, no alternative named, and no guidance on whether the code, country, or include path should be chosen for a given task. Usage is inferable rather than stated.

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