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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is partly covered. The description adds meaningful context beyond that: 'Pure computation, free' tells the agent there are no side effects or costs, and 'Do not send real card numbers' is a useful privacy warning. It does not explain return format, but with the annotations carrying the safety profile this is a strong addition.

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 three short sentences and front-loads the core purpose. The safety warning and the 'pure computation, free' note are both useful and earn their place. Nothing is redundant or delayed.

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 pure computation, the description covers purpose, privacy constraints, and operational behavior. The annotations cover read-only and closed-world semantics, and the schema fully documents the parameter. It could mention that the result is a validity boolean, but that is a minor omission for a tool this simple.

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%: the single parameter already explains that it takes digits and that spaces and dashes are ignored. The description does not add any parameter-level detail beyond the schema. This is the expected baseline when the schema does the heavy lifting.

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 specific verb and resource: 'Validate a number with the Luhn checksum (cards, IMEI).' That is clear enough for an agent to know exactly what the tool does. However, it does not explicitly distinguish this tool from nearby validators such as bursia_iban_validate or bursia_email_syntax, so it stops short of a 5.

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 use case is implied by the examples ('cards, IMEI'), and the privacy warning gives a clear constraint on when not to send real data. But there is no explicit comparison to alternative tools or statement of when another validator should be chosen instead. This is minimum-viable guidance rather than routing guidance.

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