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

Annotations already declare readOnly, non-destructive, idempotent and closed-world, so the 'no side effects / no network' sentence largely repeats structured data. What the description genuinely adds is quantitative operating limits: 64 KB JSON input, 1 s per call, 1 MB result, plus the determinism guarantee and the both-forms-when-convertible output behavior.

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?

Each language block is tight, front-loaded with the purpose, and structured into When / Not for / Side effects / Deterministic / Limits. The full bilingual duplication roughly doubles the token footprint, which is a real cost even though no single sentence is 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 one-parameter validation tool with an output schema (so return shape needs no prose), the definition covers purpose, routing, safety profile, determinism, input format nuances and operational limits. Nothing an agent needs in order to call it correctly is missing.

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% with a single documented 'isbn' field, so the baseline is 3. The description earns above baseline by specifying the accepted input semantics an agent cannot get from the schema: hyphenated forms, ISBN-10 with 'X' check character, and ISBN-13 restricted to the 978/979 prefix.

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 with technical detail: validates ISBN-10 (mod 11, X allowed) or ISBN-13 (EAN mod 10, 978/979 prefix) and returns both forms when convertible. It explicitly names the sibling it is not for (issn-validate), so an agent can separate the two identifier validators without opening a 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?

Has explicit 'When:' (verify format and check digit offline before use) and 'Not for:' clauses naming the alternative tool (issn-validate). Both the positive trigger and the exclusion are stated, leaving nothing to inference.

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