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

Goes well past the readOnly/idempotent annotations by disclosing cost ($0.001 USDC per successful call via x402 on Solana), the pay-per-success billing model, and the equivalent HTTPS endpoint. It also discloses the derived return behavior (valid ISBN-10 also returned as ISBN-13 and vice versa) and the honest limitation that publisher-prefix assignment is not verified.

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?

Front-loaded with the core operation, then normalization rules, then return/conversion behavior, then pricing and transport. Every sentence adds a distinct fact — no filler, no restatement of the title.

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 define the response shape, and it only partially does: it implies a validity verdict plus cross-converted ISBNs, but does not state the field names or, critically, what happens on invalid input (a false flag vs. an error, and whether an invalid ISBN still incurs the $0.001 charge). That is a material gap for a paid tool.

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 0%, so the description carries the full burden for the single 'value' parameter, and it does: it defines input normalization (spaces/hyphens ignored), the 9-digit shortcut, and the 978/979 prefix requirement. It never explicitly maps these rules to the parameter name, so an agent must infer that 'value' is the ISBN string, but the accepted-format semantics are otherwise well covered.

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?

Names the specific resource (ISBN-10 and ISBN-13 check digits) and the specific operation (check-digit validation), so an agent can distinguish it from neighboring validators such as ean_validate or issn_validate by the ISBN-10/13 framing alone. It also scopes the tool down with 'This does not check that the publisher prefix was ever assigned,' which sharpens the purpose rather than restating the name.

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 description gives concrete acceptance conditions — spaces and hyphens ignored, 9-digit input read as ISBN-10 with a leading zero, ISBN-13 must use the 978 or 979 prefix — which function as rules for when this tool will succeed. What it never does is name an alternative or an exclusion (e.g., 'use ean_validate for generic EAN-13'), so guidance is clear but not comparative.

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