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

Annotations only give generic hints, but the description adds the substantive details an agent needs: it is format-only with no lookup, it is a pure read/compute, each call is billable so retries are not free, pricing is $0.01/call via x402 USDC or card key, and the first 25 are free with failed calls free. That explains the idempotentHint=false annotation rather than merely repeating it.

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 purpose is front-loaded in the first clause, and the later sentences carry distinct information (scope, example, billing). It is somewhat dense, with mild repetition of "Luhn" and "card numbers", but no sentence is pure filler.

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 one-parameter, non-destructive tool with no output schema, the description covers scope, proof-of-concept example, and billing behavior thoroughly. The one real gap is that it never says what the result looks like (valid/invalid plus check digit field), which the description implies but does not define.

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 schema already documents the accepted format (digits, spaces, hyphens), so the baseline is 3. The description contributes a worked example value (79927398713) but adds nothing about constraints or normalization beyond what the schema says.

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 names a specific check (Luhn mod-10 checksum) and the resource classes it applies to (card numbers, IMEI numbers, many IDs), and it also states the secondary computation (check digit to append). It is clearly distinguishable from siblings like validate_iban, validate_isbn, bic_validate and phone_validate, which cover other identifier formats.

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 gives an explicit use case ("Use to catch typos in card or ID numbers") and a clear scope exclusion ("Only the format is checked; nothing is looked up"). It never names a sibling alternative such as validate_iban or validate_isbn for other identifier types, so routing is implied rather than spelled out.

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