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

Annotations already declare readOnlyHint and destructiveHint, so the safety profile is covered. The description still adds real context beyond them: an explicit cost of 2 credits, that invalid input yields 'specific rejection reasons' rather than a bare boolean, and that successful calls return parsed parts plus both ISBN forms. It does not mention any limits on input length or batching, but for a single-parameter validator this is solid.

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 sentences, front-loaded with the core capability and the accepted input shape, and the cost note is a useful trailing detail. Minor waste from repeating the tool name at the start ('ISBN Validator: ISBN Validator checks...').

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?

With one required parameter, full schema coverage, an output schema, and annotations present, the description covers what the tool does, what input it takes, and what it returns. The only missing element is guidance on alternatives, which is not critical for a uniquely-scoped validator.

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 sole parameter is already documented with an example and a note about optional separators, so the schema does the heavy lifting. The description's 'raw book identifier with or without hyphens' largely restates that, and adds only the hint that unpadded/messy input is acceptable.

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 specific verbs (checks checksums, parses registration group/publisher/title, converts between ISBN-10 and ISBN-13) against a specific resource. Against siblings like CardValidator, IBANValidator and PhoneNumberValidator, the ISBN domain alone makes it unambiguous which tool handles book identifiers.

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?

It gives input-side guidance ('pass any raw book identifier with or without hyphens'), which tells the agent the tool tolerates unnormalized input. However, it never says when to choose this over a generic validator sibling or when the tool is inappropriate (e.g. non-ISBN identifiers), so usage is implied rather than directed.

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