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

Annotations already declare a safe, idempotent read (readOnlyHint, idempotentHint, openWorldHint false). The description adds genuinely new behavior: it is structural validation only with no product database lookup, it enumerates what is returned, and it discloses a per-call cost and payment rail. Error behavior on malformed input is the only notable omission.

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?

The identifier list and return summary are front-loaded, and the closing note on database scope and pricing is compact. Three sentences carry distinct payloads with no filler, though the opening enumeration of formats is somewhat dense.

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 return-value burden and does so by naming the check-digit verdict, computed digit, prefix boundary, and GTIN-14 conversion. Input constraints (8/12/13/14/18 digits, spaces/hyphens) are covered by the schema, so the only residual gap is failure-mode behavior for invalid codes.

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%, so the enum values for op and to are already fully documented. The description adds incremental meaning by explaining that checkdigit derives a missing final digit from a partial code, but this only reframes what the schema already states. Baseline 3 is appropriate.

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 (validate/convert) and the exact resource class (GS1 identifiers) with an enumerated list of supported formats (GTIN-8/12/13/14, GLN, SSCC). This clearly separates it from siblings like id_national and id_security, which cover different identifier domains.

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 conveys when each mode applies (validate vs compute a missing check digit vs convert to GTIN-14), which gives clear context. However, it never explicitly routes the agent away from alternatives such as id_national or id_security, so selection among sibling identifier tools is left to inference.

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