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

Annotations already cover read-only, closed-world and idempotent, but the description goes further by stating no network access, no stored state, determinism, and concrete limits (64 KB input, 1 s per call, 1 MB result). It does not describe what the response reports for an invalid number, but the output schema covers that.

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 core purpose and followed by labeled When/Not-for/Side-effects/Limits blocks, which is easy to scan. The bilingual duplication and the trailing '[EN completo / full EN: inputSchema.description]' artifact add redundancy that earns little.

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 one-parameter validator with an output schema and full annotation coverage, everything needed to call it correctly is present (input semantics, limits, safety profile). Only the invalid-input reporting behavior is left implicit, and that is covered by the output schema.

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 baseline is 3, but the description adds real meaning beyond the schema by stating that spaces and dashes are ignored — a normalization rule that affects what input the agent may pass.

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 (validates) and a specific algorithm/resource (Luhn mod-10 checksum) plus the covered identifier types (cards, IMEI, national IDs). It also names the sibling card-brand-detect as the thing it is not, so an agent can route without opening schemas.

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?

Explicit 'When: verify format and check digit offline before use' plus an explicit 'Not for: detecting card brand from IIN prefix (card-brand-detect)'. Both the selection condition and the alternative are named.

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