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

Annotations already declare readOnlyHint and destructiveHint=false, and the description reinforces this with 'Reads or computes only and changes nothing'. It goes beyond structured data by disclosing the billing model ($0.01/call via x402 or card key, first 25 free, failed calls free) and explains why repeats cost again, which clarifies the non-idempotent hint.

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 core behavior and example before the pricing/immutability notes, and every sentence carries information. The pricing clause is somewhat dense but is genuinely decision-relevant for an agent, so it earns its place.

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?

One required parameter, no output schema, and the description covers both return shapes (words, and the cheque form with cents), input limits, and the cost model. Nothing an agent needs to call this correctly is missing.

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% ('The number, decimals allowed'), so baseline is 3, but the description adds real constraints: the trillions ceiling, negatives permitted, and that a decimal input yields the 'and' cents cheque form. That is meaningfully more than the schema states.

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') and pins down scope with 'up to trillions, negatives allowed' plus the cheque-form variant for decimals. Combined with the sibling list (number_roman is the adjacent tool) the resource is unambiguous.

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' gives a clear trigger context plus a worked example (number=1234.5). It stops short of naming when-not-to-use or an alternative (e.g. number_roman), so it is clear context without exclusions.

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