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

Annotations already declare readOnly/idempotent/closed-world, yet the description adds real value beyond them: it says mailboxes are not verified, that the domain part is lowercased, that scanning is linear-time, that no network access or stored state is used, and it publishes explicit limits (64 KB input, 1 s per call, 1 MB result). That is unusually complete behavioral disclosure.

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?

Within each language the text is well structured and front-loaded (purpose, when, not-for, side effects, limits). However, the Spanish and English blocks duplicate essentially the same content, roughly doubling token cost with no added information — a real conciseness penalty for a two-parameter extraction tool.

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?

An output schema exists, so return values need not be explained, and the description covers everything else an agent needs: non-verification semantics, determinism, side-effect profile, and hard limits. Nothing required to invoke it correctly 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 both parameters are already documented in the schema, and the description adds no explanation of the 'unique' flag or of the 'text' format. The only semantic addition is the domain-lowercasing behavior, which is a result property rather than a parameter one. 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 and resource ('extrae direcciones de correo de un texto') and immediately bounds it ('solo sintaxis; no verifica que existan'). It names the sibling it is not (email-syntax-validate), so an agent can distinguish it without opening another 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 'No para' clause is explicit and routes the agent to email-syntax-validate for pure syntax checking. The positive 'Cuándo' clause ('transformar, comparar o medir un texto con exactitud') is somewhat generic and describes a family of text tools rather than this one specifically, but the exclusion is precise enough to be actionable.

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