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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety bar is largely met. The description adds genuine value beyond that: 'Returns a local structural assessment; never contacts a bank' clarifies the operational behavior and confirms no external calls, plus the privacy warning about real card details.

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?

Five short, front-loaded sentences with no filler; the scope constraint and safety warning come first, followed by use/non-use cases and return behavior. Every sentence earns its place.

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 single-parameter, read-only check with full annotation coverage, the description covers purpose, misuse exclusions, input constraint, and return nature. No output schema exists, but the description adequately characterizes the response as a local structural assessment.

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, and schema description coverage is 100%, so the schema already documents the 'number' field and its synthetic-only constraint. The description reinforces the same idea but adds no new format or syntax detail, 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+resource: screening a synthetic card number for network/length plausibility and Luhn checksum. This is clearly distinguishable from siblings like is-iban-safe, is-us-tin-safe, or is-phone-e164, and the scope ('Luhn/brand format only') is precise.

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?

Explicitly states when to use it ('screen synthetic card fixtures for plausible network/length and Luhn checksum') and when not to ('Do not use for real payment-card authorization or fraud detection'). It also warns against submitting real card details, covering the main misuse path.

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