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

Annotations already declare readOnly, idempotent and closed-world, so the safety profile is covered. The description adds real value beyond that: the exact return payload (verdict, computed check digit, GTIN-14 form with indicator digit, cross-format conversion), a negative scope guarantee ('no product database', 'no company-prefix split'), and a per-call price via x402. Only the failure/error behavior is left unstated.

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 capability and identifier list, then returns, then explicit exclusions, then pricing — a sensible order. It is dense but every clause carries information; the only slight drag is the repeated enumeration of formats in two places.

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 takes on the return-value burden and does so well by naming the four returned artifacts. The gap is behavioral edge cases (what an invalid length or non-digit input yields, SSCC/18-digit return shape) which an agent calling it would want.

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% and both enums are documented in-schema, so the schema carries the parameter burden. The description's 'omit the final digit' and conversion notes merely restate what op=checkdigit and to=convert already say in the schema, adding little beyond the baseline.

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-plus-resource ('validate and convert GS1 identifiers') and enumerates the exact identifier types handled (GTIN-8/12/13/14, GLN, SSCC), which no sibling validator (iban_validate, isin_cusip_sedol_validate, crypto_address_validate) covers. Scope is unambiguous 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?

The description implies the three modes ('validate and convert', 'computes a missing check digit from a partial code') and explicitly rules out product lookup and prefix splitting, which tells an agent when this tool is the wrong choice. It stops short of naming an alternative or stating prerequisites, so not a 5.

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