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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, yet the description still adds genuinely new information: only a masked result is returned and 'nothing is stored'. That privacy/storage guarantee is exactly the kind of trait annotations cannot express. It stops short of saying what the result contains (valid flag, detected network), but the safety profile is well covered.

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?

Two short sentences, front-loaded with the mechanism and coverage list, then the privacy guarantee. No filler or repetition of the title; every clause carries information.

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, read-only tool with full schema coverage and strong annotations, the definition covers mechanism, supported networks, and data handling. The only gap is the shape of the response (validity plus detected network), which matters slightly more because no output schema exists to spell it out.

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 'number' parameter already carries a format hint and an example test number (4111111111111111). The description adds no further syntax or length guidance, so the baseline 3 for a fully documented schema 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 the concrete mechanism (Luhn check plus network detection) and enumerates the nine networks covered, so the resource is unmistakably a payment card rather than an IBAN, BIC, or GTIN. It doesn't explicitly name the sibling validators, but the card-network list makes the scope unambiguous.

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 by the domain: an agent can infer this is the tool for card numbers, but the description never says when to prefer it over validate_batch or the other validate_* siblings, nor any when-not condition. Adequate but leaves selection to inference.

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