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

Annotations already declare readOnlyHint, destructiveHint=false and openWorldHint, so the safety profile is covered. The description adds genuine context beyond that: it lists the exact checks performed, promises PCI-safe output (BIN + last 4, not full PAN), flags processor test cards, and discloses a 2-credit cost — an important operational trait not present in any structured field.

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?

Information is front-loaded and the enumeration is efficient, but the opening 'Card Validator: Card Validator is a...' redundantly restates the title twice before reaching substance. The cost note is useful, but the single run-on sentence packs six capabilities without hierarchy.

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?

An output schema exists, so return-value documentation is not required, yet the description still sketches the shape of the response (validity, brand, BIN, last 4, test-card flag, risk score). Combined with the credit cost, an agent has what it needs to decide to call it; only input-format expectations are left hanging.

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 itself stating 'The card number to validate'. The description adds no further meaning such as accepted formatting (spaces, dashes) or whether non-digit input is rejected, so it neither compensates nor detracts — baseline 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 opens with a specific verb+resource ('credit card number validator API') and enumerates what it does: Luhn checksum, scheme detection, BIN/last-4 extraction, test-card flagging, risk scoring. That is concrete enough to separate it from neighbors such as BINLookup or TestCardGenerator, though it never names those alternatives explicitly.

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 enumeration of checks rather than stated. There is no explicit 'use this when...' guidance, no mention of when to prefer BINLookup for pure BIN data or IBANValidator for bank accounts, and no note on what input formats are acceptable.

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