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

The annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so safety is covered. The description adds genuinely new behavioral information beyond those annotations: that the tool performs check-digit validation and also converts between ISBN-10 and ISBN-13, which an agent could not infer from the annotations or schema alone.

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?

A single front-loaded sentence covering both the validation and conversion capabilities with no filler. Nothing redundant and nothing wasted.

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 single-parameter tool with full annotation coverage this is nearly sufficient, but with no output schema the description leaves the return value undefined — it never says whether the result is a boolean, the normalized/converted ISBN, or both. Since 'convert between the two' implies an output the agent cannot predict, this is a meaningful gap.

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 with 100% schema description coverage, so the baseline is already 3-4. The description adds meaning beyond the schema by naming the two accepted formats (ISBN-10 and ISBN-13), which the bare schema example of a 13-digit number does not fully convey.

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 (validate) and resource (ISBN-10/ISBN-13 check digit) and adds the secondary capability of converting between the two formats. The resource is distinct enough from siblings (BIC, IBAN, GTIN, VAT, card) that an agent can route correctly, though the description never explicitly contrasts with those validators.

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 name and by the resource being an ISBN, and the 'ISBN-10 or ISBN-13' phrasing signals accepted input formats. However, there is no explicit when-to-use guidance, no statement about what happens with malformed or non-ISBN input, and no mention of when to prefer this over validate_gtin (which also handles 13-digit numbers).

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