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

Annotations already declare readOnly/idempotent/non-destructive, so the description adds beyond that: input normalization (spaces and hyphens ignored), payment/auth requirements ($0.001 USDC per successful call via x402 on Solana), and an alternate HTTPS transport. It doesn't say what a result looks like, but cost and auth disclosure are exactly the kind of context the annotations can't carry.

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 purpose, then algorithm, then input normalization, then pricing/endpoint — a sensible order with no filler. The pricing and mirror-URL clauses are necessary context; only the alternating-weight algorithm sentence is arguably redundant detail for a validator caller.

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 read-only validator this covers formats, normalization, scope caveat, cost, and transport. With no output schema, the one real gap is that the return value (boolean vs. computed check digit vs. error shape) is never described, which an agent needs to interpret results.

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% and the single `value` param has no description, so the description must compensate — it does, by stating which formats are accepted and that embedded spaces/hyphens are stripped before validation. It doesn't state whether prefixes like 'EAN-13:' or non-digit characters are rejected, leaving some ambiguity despite the 30-char cap.

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 a specific operation (GTIN check-digit validation) and enumerates the supported symbol formats: EAN-8, UPC-A (12), EAN-13, GTIN-14. It also states the scope limit ('does not check the GS1 prefix was assigned'), which separates it from full barcode/product lookups like food_barcode or barcode_generate.

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 exclusion ('does not check that the GS1 prefix was assigned') tells an agent not to use this for product/company resolution, and the listed formats make the intended inputs explicit. It stops short of naming a sibling alternative or stating conditions when another validator (isbn_validate, imei_validate) would be the right pick.

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