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

Annotations already cover read-only, closed-world, idempotent behavior, so the bar is lower. The description adds material context the annotations do not: a per-call price of $0.05 and the x402/USDC-on-Base payment requirement, plus the reassurance that no exchange-rate data is returned.

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-loads the lookup and return contents, then the rounding rationale, then scope and price. Each sentence carries information, though the price/payment sentence could be tighter. No filler or restating of the title.

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 by enumerating the returned fields, plus noting the cash-rounding use case and payment model. The one gap is that it never mentions the country-based lookup path offered by the 'country' parameter, which an agent would otherwise have to discover from the schema.

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 all three parameters, which sets the baseline at 3. The description adds nothing about the 'country' selector or the 'include' values (territories, historic) beyond what the schema states, so it does not exceed the baseline.

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 ('look up a currency by ISO 4217 code') and enumerates exactly what facts are returned (alphabetic/numeric codes, name, minor-unit exponent, cash denomination, territories). It also explicitly carves out what it is NOT ('never an exchange rate'), which cleanly separates it from any FX-style sibling.

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 clear usage context ('what a caller needs to render or settle a price') and an explicit exclusion ('never an exchange rate'), so an agent knows when this rather than a rate tool applies. It stops short of naming a concrete alternative tool or stating when to prefer the country-based lookup over the code-based one.

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