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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered; the description adds real value beyond them by disclosing the billing model ($0.003/call, first 25 free, failed calls free) and clarifying why repeats cost money, which explains the idempotentHint=false annotation. The sentence 'reads or computes only and changes nothing' largely restates the annotations, keeping this from a top score.

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 core capability and example are front-loaded, and the pricing/behavioral caveats are packed into a dense second half. It is efficient overall, though 'Use when amounts must be written out in full' is mildly redundant with the preceding sentence.

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 usefully covers the return ('also returns the cheque form with and cents') and the input constraints, plus cost behavior. It stops short of describing the exact shape/format of the returned string, which is the one thing an agent cannot otherwise infer.

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% and the single parameter is documented as 'The number, decimals allowed', setting the baseline at 3. The description adds constraints the schema lacks — the supported range (up to trillions), that negatives are allowed, and a worked example — so it goes modestly beyond 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 and resource — 'Write a whole number ... out in English words' — with explicit scope (up to trillions, negatives allowed) and a concrete example (number=1234.5). This clearly distinguishes it from the closest sibling, number_roman, without either schema being opened.

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?

'Use when amounts must be written out in full' plus 'for cheques and invoices' gives a clear usage context, and the decimal case points to the cheque-form return. It does not name an alternative (e.g. number_roman) or state when not to use it, so it falls short of explicit routing.

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