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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false, so safety and repeatability are covered by structured data. The description adds nothing behavioral beyond that: no statement about what happens to malformed lengths, non-numeric characters, or already-complete 8/12/13/14-digit inputs.

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?

Two short sentences, front-loaded with the operation and its input-length mapping, followed by the input-format note. Every clause carries information; nothing is padding.

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, read-only, idempotent utility with full schema coverage, the description covers the core contract well. Gaps are minor: behavior on invalid or already-valid inputs and whether output aligns one-to-one with the input list are unstated, and there is no output schema to compensate.

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 the schema says 'one per line', but the description adds a genuine alternative the schema omits: comma-separated input is also accepted. That is real meaning beyond the input schema, though it does not explain the output shape (order, error handling per line).

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: adds the GS1 check digit to GTIN numbers lacking one, and enumerates the accepted input lengths (7, 11, 12, 13) with their resulting formats (GTIN-8, UPC-A, EAN-13, GTIN-14). This implicitly separates it from validate_gtins and repair_gtin, but never names those siblings, so the distinction is inferential rather than explicit.

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?

Use when a number is 'missing' a check digit is implied clearly enough, but there is no instruction on what to do with numbers that already have one, nor any routing to validate_gtins or repair_gtin for related but different tasks. Usage is implied rather than stated.

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