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

With no annotations, the description carries the full behavioral burden. It usefully discloses cost ($0.05 paid), the checks performed (numeric format, Mod-10 checksum), and the implementation (Rust WASM engine), but never states what the tool returns, so the agent cannot know whether the result is a boolean, a status string, or a structured report.

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?

Four short sentences with no filler; the paid-cost note and the core purpose are front-loaded before the implementation detail. Slight redundancy between 'Checks numeric format and Mod-10 checksum' and the algorithm name keeps it from a 5.

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, low-complexity tool the description covers purpose, cost, and method adequately. However, with no output schema and no annotations, the absence of any statement about return values leaves a real gap for an agent deciding whether to call it.

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 'number' parameter is already documented with its 12-19 digit constraint and space tolerance. The description adds no further meaning about the input beyond what the schema states, so the baseline of 3 applies.

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 gives a specific verb ('Validate') plus a specific resource ('payment card or account number') and names the exact algorithm and standard (Luhn, ISO/IEC 7812-1). This clearly separates it from siblings like iban_check, lei_check, and mod97_check, which target different 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?

Usage is implied by the resource type (card/account numbers), but there is no explicit when-to-use or when-not-to-use guidance and no pointer to sibling tools for other identifier types such as IBANs or LEIs. An agent must infer the routing from the resource name alone.

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