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

With no annotations, the description does real work: it discloses the price ($0.0029), the execution engine (Rust WASM), the standard applied, and precisely what is verified (numeric format and Mod-10 checksum). It omits the return shape and the important caveat that passing Luhn does not mean the account exists or is chargeable, which keeps it from a 5.

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?

Three short sentences, with the cost front-loaded where an agent will see it before deciding to call. The engine note is marginally useful for trust but is the least load-bearing 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?

For a one-parameter, no-output-schema tool, the description covers purpose, standard, cost, and what is checked, which is nearly everything needed to call it correctly. The one gap is what the response looks like and how to interpret a pass versus a fail.

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 already documents format (12-19 digits, spaces allowed). The description adds no constraints or semantics beyond what the schema states, so the baseline 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?

States a specific verb (validate) plus the resource (payment card or account number) and names the exact algorithm and standard (Luhn, ISO/IEC 7812-1), which implicitly separates it from siblings like mod97_check and iban_check. It stops short of explicitly contrasting itself with those siblings, so a 4 rather than a 5.

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 only implied: an agent can infer this is the tool for pre-charge card-number sanity checks. There is no explicit when-to-use/when-not guidance and no referral to mod97_check or iban_check for other numbering schemes.

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