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

The annotations already establish a safe, idempotent, closed-world read operation, but the description adds material context they cannot: a per-call price of $0.05 paid over x402 (USDC on Base) and a hard value ceiling of 10^15. Payment/auth requirement and input ceiling are genuinely useful behavioral disclosures beyond 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 capability, then scoping, then pricing in a compact two-sentence block. No filler, and the most decision-relevant facts (languages, forms, currency handling) come first.

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?

For a deterministic conversion tool with no output schema and fully documented parameters, the description covers capability, limits, currency edge cases and cost. The only gap is the unmentioned 'year' form value, which leaves the agent to discover it from the schema alone.

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 value, lang, case, form and currency. The description adds real semantic value on currency behavior (agreement plus minor-unit naming for 19 currencies, fraction fallback otherwise), but notably omits the schema's fourth form value, 'year', while claiming only cardinal/ordinal/currency exist.

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+resource ('spell a number out') and immediately scopes it by language set, and by cardinal/ordinal/currency form. This clearly distinguishes it from siblings like base_convert, money_split or currency_lookup, which handle different jobs.

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?

The description conveys form-specific rules (currency form agrees the number with the currency noun; other ISO 4217 codes fall back to a fraction) which implicitly tells the agent when each mode applies. However, it never states when to choose this tool over a sibling or names any alternative, so usage guidance is only implied.

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