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

With no annotations, the description carries the full burden and does well: it discloses that validation is local, that no processor is contacted, that the full number is never stored, and that the returned value is masked to first 6 and last 4 digits. It does not state what happens on an invalid number (error vs. a result with a validity flag), which is the main remaining gap.

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?

A single dense sentence that front-loads the action and checks, then adds the privacy and output-masking guarantees. No filler or redundancy.

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 the checks, the privacy model, and the shape of the returned value. It stops short of describing the full result structure (e.g., a validity boolean or detected network field), which would make it fully self-contained.

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?

Only one parameter exists and schema coverage is 100%, with the schema already documenting that spaces and dashes are allowed. The description adds no further meaning about the number parameter, 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?

States a specific verb (validate) and resource (payment card number) and enumerates the exact checks performed: Luhn, length, network detection, test-card detection. An agent can distinguish this from validate_email or validate_vat purely from the description.

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 phrase 'locally' and 'no network call is made to any payment processor' implies when this tool is appropriate (offline format validation rather than authorization), but there is no explicit when-to-use/when-not-to-use statement or named alternative. Usage is inferable rather than stated.

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