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

Annotations only cover the read-only/idempotent safety profile; the description adds substantial behavior beyond that: the exact return contents (check-digit verdict, computed correct check digit, GTIN-14 form with indicator digit, conversions), the absence of company-prefix data and why, and the $0.05 x402 payment model. This is exactly the extra context annotations cannot 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?

Information is front-loaded with the capability statement first, followed by returns, exclusions, and pricing. It is dense with colon-separated lists rather than flowing prose, but every clause carries real information, so overhead is low.

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 compensates by enumerating the returned fields, and it discloses scope limits and the payment requirement. For a 3-parameter read-only validator, an agent has everything needed to call it correctly.

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 schema already documents op, to and value including their enums and the checkdigit omission rule. The description only echoes these (partial code for check-digit computation, conversion between formats) without adding format or syntax detail beyond the schema, so the baseline 3 applies.

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 and resource ('validate and convert GS1 identifiers') and enumerates exactly which identifier families it handles (GTIN-8/-12/-13/-14, GLN, SSCC). An agent can distinguish it from siblings like iban_validate, vat_tax_id_validate or isin_cusip_sedol_validate 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 maps the three operations to intent ('validate', 'checkdigit' to compute a missing final digit, 'convert') and states a clear non-scope ('No company-prefix split is returned', 'Structure only, no product database'). It does not explicitly name an alternative tool, but no sibling overlaps enough to require one, so the guidance is clear rather than exclusionary.

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