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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so the safety profile is covered; the description still adds real value by disclosing determinism and concrete limits (64 KB input, 1 s per call, 1 MB result) that no structured field carries. Some of the 'no network access, no stored state' phrasing merely restates the annotations.

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 operation, then labelled blocks (When / Not for / Side effects / Deterministic / Limits) that are easy to scan. The cost is that the entire text is duplicated verbatim in Spanish and English, roughly doubling length without adding information.

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 single-parameter pure function with an output schema, the description covers everything an agent needs: what is computed, what to send, what is not in scope, side effects, determinism, and hard limits. Nothing required for correct invocation is missing.

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 parameter is already documented, but the description adds meaning the schema does not: the body is 7-17 digits and excludes the check digit, and the tool returns the full code. That disambiguates the schema's 'body' wording and the example value.

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: computes the GS1 mod-10 check digit for a GTIN/EAN/UPC/SSCC body, and explicitly names the sibling that does the other job (gtin-validate). An agent can tell exactly what this returns (the full code) versus what it does not.

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 'When' clause is concrete (verify an identifier's format and check digit offline before it is used) and the 'Not for' clause routes to gtin-validate. However, the Spanish and English 'Not for' lists disagree — ES excludes EAN-8/UPC-A/EAN-13/GTIN-14 while EN excludes only GTIN-8 — which leaves a borderline case ambiguous for exactly the code types the tool advertises it supports.

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