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

Beyond the readOnlyHint, the description discloses that validation is offline (no network call), that batching returns a results array, the batch cap of 50, and the pricing model ($0.002 USDC per successful call via x402 on Base/Solana/Arc, failures uncharged) – operational facts annotations cannot convey. Minor tension with openWorldHint=true, though that hint refers to external data scope rather than network I/O.

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?

One dense paragraph that leads with what is validated and the supported identifier families, then adds batching and pricing. Every clause adds new information – the type list, the derived fields (GS1 prefix country, VIN manufacturer/region/model year, ISIN country), and payment terms are all useful.

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?

An output schema exists so return values needn't be explained, and pricing/payment/rate-limit behaviour is covered. Only the absence of explicit sibling routing (e.g., when to prefer vin or iban) keeps it short of complete.

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 description coverage is 100%, and the description largely restates it: 'type=auto detects the type' mirrors the schema's 'auto (default)', and 'up to 50 comma-separated' mirrors the value description. Baseline 3 is appropriate since the schema already carries the parameter detail.

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 ('checks check digit and structure') plus a fully enumerated set of resources (ISBN-10/13, EAN-8, UPC-A, EAN-13, GTIN-14, VIN, IMEI, ISIN, Luhn), which lets an agent distinguish it from siblings like vin, iban, lei or uuid without opening the 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?

Gives clear operating context: no network call (offline/local checksum validation), type=auto detects the type, and up to 50 comma-separated values can be batched. It never explicitly routes the agent away from overlapping siblings such as vin or isin, so the when-not guidance is implied rather than stated.

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