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

Goes well beyond the annotations: it discloses per-call pricing ($0.003), the free tier (first 25 free), that failed calls are free, and explicitly reconciles the counterintuitive readOnlyHint=false by explaining the charge-based reason. This is exactly the kind of context annotations cannot convey.

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?

Purpose and formatting rules are front-loaded, followed by usage, example, and billing. Every sentence carries information, though the pricing clause is dense; overall efficient for the amount of needed disclosure.

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 no output schema, the description should carry the return burden; it implies a check result and ISBN-10→13 conversion but never specifies the response shape (boolean vs. converted string vs. error). Strong on input, billing, and safety context, but the return contract is left implicit.

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 coverage is 100% and the parameter is the only input, so the schema baseline is 3. The description adds real value by stating that hyphens and spaces are ignored and providing a concrete example (isbn=0-306-40615-2), clarifying accepted input formats beyond the schema's bare 'The ISBN'.

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 and resource: 'Check an ISBN-10 or ISBN-13 check digit and convert valid ISBN-10s to ISBN-13.' The ISBN resource name inherently separates it from validate_iban and validate_luhn, so an agent can route to it without opening any schema.

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?

'Use before cataloguing or ordering books' gives a concrete usage context, which is more than most validators offer. It does not name explicit alternatives or exclusions, but the resource-specific framing makes the boundary with sibling validators self-evident.

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