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

Annotations already declare readOnlyHint, idempotentHint and a closed-world scope, so safety is covered. The description adds meaningful behavior beyond that: pricing at $0.05 per successful call via x402, a 10^15 value ceiling, and fallback minor-unit handling for non-listed currencies.

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?

The description is two sentences and front-loads the core capability before pricing. The first sentence is dense with language names and currency details, but every clause carries useful information for selection and invocation.

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?

Given the moderate complexity, full schema coverage and no output schema, the description covers what an agent needs: supported languages, all forms, value limits, currency behavior, and cost. The 'spell out' phrasing implicitly establishes a string-returning text transformation, leaving no important gap for this 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 semantic detail the schema does not: currency form agrees the number with the currency noun, supplies real minor-unit names for 19 currencies, and falls back to fractions like 56/100 for other ISO codes. It also restates the 10^15 range, reinforcing the schema.

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 opens with a specific verb and resource: 'spell a number out' in 12 named languages and in cardinal, ordinal or currency form. This distinguishes it clearly from sibling tools such as base_convert (base conversion) and currency_lookup (currency metadata).

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 clear contextual guidance by naming the supported forms (cardinal, ordinal, currency, year) and explaining that currency requires an ISO 4217 code. It does not explicitly name alternatives or state when not to use this tool, so it falls short of the top score.

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