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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered; the description adds real value beyond that with the per-call price ($0.001 USDC via x402 on Solana), the 36-digit input limit, and an HTTPS fallback endpoint. It does not describe latency or failure/refund behavior for paid calls, which would have pushed it higher.

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 capability and language/grammar detail, then pricing and the HTTP alternative at the end, so an agent gets the essentials first. It is dense and slightly run-on in the parenthetical grammar examples, but nearly every clause carries information.

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 and an input schema that covers the enums, the description supplies enough for correct invocation: supported languages, mode semantics, input bounds, and the paid-call caveat. Only the exact return shape (e.g., capitalization behavior in check mode) is left implicit.

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 only 50%, but the description compensates: it explains what each language does grammatically, clarifies what 'check' mode means (cheque wording) and that roman mode works in reverse, and bounds the number parameter (up to 36 digits, sign, decimals). The capitalize and style parameters are still explained only in 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?

States a specific verb (spell out) and resource (numbers), then enumerates the covered modes (cardinal, ordinal, currency, cheque, year, roman), languages and edge cases (negatives, decimals, 36 digits). It is clearly distinguishable from numeric siblings like base_convert or currency_format.

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 concrete use cases — invoices, cheques, contracts, accessibility, voice output — which map well to the check/currency/cardinal modes, and the language list guides lang selection. It stops short of saying when to prefer a sibling such as currency_format instead, so no explicit alternative routing.

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