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

Annotations already establish readOnlyHint=true, destructiveHint=false and openWorldHint=true, so safety is covered. The description adds genuinely new behavioral context beyond that: the response is tier-gated (paid plans add symbol, full name, ISO numeric code, precision, live rate), the data includes a live USD rate, and the call costs 1 credit — the credit cost and tier gating are exactly the kind of context annotations cannot convey.

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 short sentences, front-loaded with the core matching behavior and followed by tier/cost facts. Minor waste in the redundant 'Currency Symbols: Currency Symbols' opening that simply restates the name and 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?

An output schema exists, so return-field explanation is not required, and the credit cost is disclosed. What remains missing is behavior when neither optional parameter is provided, and how the response differs between free and paid tiers in terms of which fields an agent can rely on — a real gap for a tool whose output shape is plan-dependent.

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 both parameters are already documented in the schema with examples. The description corroborates the input domain ('ISO currency codes or country names') but adds no format rules, mutual-exclusivity behavior, or fallback semantics beyond what the schema states, so the baseline 3 applies.

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 states a concrete verb+resource: it matches ISO currency codes or country names to the countries that use them, and it enumerates the richer fields available on paid plans. It is clear about what the tool does, though it never names the nearby siblings (CurrencyConverter, CurrencyRate, ExchangeRate) that an agent could confuse it with.

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?

There is no explicit when-to-use or when-not-to-use guidance, and no mention of the obvious alternatives CurrencyConverter, CurrencyRate or ExchangeRate that sit in the same sibling list. With two optional parameters, the description also fails to say whether at least one of country/currency must be supplied.

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