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

Annotations already declare readOnlyHint=true and destructiveHint=false, and the description reinforces that ('reads or computes only and changes nothing'). More valuable is the disclosure that each call is charged ($0.01/call via x402 USDC or card key, first 25 free, failed calls free), which explains why idempotentHint=false — repeats are not free. It stops short of describing output formatting details, but the cost/rate-limit disclosure is real added context.

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?

Purpose, scope, and the example are front-loaded, and the pricing sentence is dense but earns its place by answering cost and idempotency questions. The example value (number=1234.5) is slightly redundant with the preceding sentence but harmless.

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 single-parameter conversion tool with no output schema, the description covers input bounds, output shape (words plus cheque form), cost, and safety. Only the exact response structure is left implicit, which is a minor gap for a tool this simple.

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% and the single parameter is already documented as 'decimals allowed'. The description still adds meaning beyond the schema: the upper bound (trillions), that negatives are accepted, and that a decimal amount triggers a cheque-form output with 'and' cents.

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 immediately bounds the domain (up to trillions, negatives allowed, decimals get a cheque form). This clearly separates it from sibling number_roman, which converts to Roman numerals rather than English words.

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 the cheques/invoices framing gives a concrete usage context. It does not name an alternative tool or state when not to use it, so it falls short of explicit routing guidance.

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