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

Annotations only assert the generic flags (readOnly=false, openWorld=true, idempotent=false, destructive=false). The description adds real context beyond them: hyphens and spaces are silently ignored, conversion happens only for valid ISBN-10s, and each successful call costs $0.001 USDC on Base via x402 (implying failed calls are free). It doesn't describe what an invalid input yields, which is a remaining gap.

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?

Four short sentences, front-loaded with the action and followed by input normalization, usage trigger, and cost. No filler, though the pricing clause is dense enough that separating it or moving it later would read slightly better.

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 carries the burden of explaining return values, and it doesn't: it's unclear whether the result is a validity boolean, the converted ISBN-13, or an error on invalid input. Input semantics, usage, and cost are complete, but the response contract is unspecified.

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%, so the baseline is 3, but the thin schema text ("The ISBN") is surpassed by the description, which specifies accepted formats (ISBN-10 or ISBN-13) and that hyphens/spaces are normalized away. That is genuine input-format guidance the schema does not carry.

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+resource pair (check ISBN-10/ISBN-13 check digit) and adds the secondary transformation (convert valid ISBN-10s to ISBN-13). This clearly separates it from the other validator siblings (validate_iban, validate_luhn) by naming the exact identifier domain.

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 workflow trigger rather than leaving usage implied. It stops short of naming alternatives such as validate_luhn or validate_iban when the identifier type is uncertain, so it's clear context without explicit routing.

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