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

No annotations are provided, so the description carries the full burden. It discloses a behavioral trait with '(test only)' and 'brand guess', implying heuristic or non-authoritative output. However, it does not explain what 'test only' actually means or whether real card numbers are rejected, processed, or echoed.

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 description is a single, tight sentence that front-loads the core purpose. There is no filler or redundancy, though it is minimal enough that some useful context (e.g., parameter expectation) is omitted rather than intentionally excluded.

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 simple one-parameter tool with an output schema, the description is nearly adequate, but it lacks usage guidance and leaves the parameter and 'test only' caveat under-specified. An agent would likely still call it correctly, but may not know how it differs from 'creditcard' or 'luhn'.

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?

The input schema has one parameter, 'q', with no description (0% schema description coverage), so the description must compensate. It does implicitly: 'Credit Card Validator' makes it likely that 'q' is the credit card number to validate. Yet it never explicitly states that 'q' expects a card number or any expected format, leaving room for inference.

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 states a clear verb and resource: 'Credit Card Validator' performing 'Luhn check + brand guess'. It tells an agent exactly what the tool does, but it does not explicitly distinguish itself from sibling tools like 'creditcard' or 'luhn', which likely overlap in function.

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?

There is no guidance on when to use this tool versus alternatives such as 'creditcard' or 'luhn'. The parenthetical '(test only)' hints at a restricted context, but it is ambiguous—it could mean test card numbers, testing purposes, or a temporary stub—and no constraints or exclusions are stated.

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