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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so safety is covered. The description adds real value beyond them: it states the tool is syntax-only and does not verify the number is assigned or reachable, declares determinism, and gives operational limits (64 KB input, 1 s per call, 1 MB result). That is exactly the extra context annotations cannot carry.

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 core is front-loaded and tight, but the 'Cuándo: hay que leer, construir o revisar con exactitud una URL, una cabecera HTTP, un token, un rango de IP...' sentence is pure noise transplanted from an unrelated tool, and it is duplicated in the English half. A substantial share of the text does not earn its place and adds confusion.

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 an output schema present and rich annotations, the description need not explain return values, and it correctly covers syntax-only scope, determinism and resource limits. However, the misleading 'when to use' clause leaves the agent with an inaccurate usage signal, which is the one completeness failure that matters for tool selection.

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% and both parameters (phone, default_country_code) are documented in the schema itself, so the baseline is 3. The description contributes the E.164 output shape (leading '+' and 8-15 digits), which slightly clarifies what 'phone' should look like, but it never explains how default_country_code interacts with a '+' prefix — the one semantic gap the schema leaves open.

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?

The opening sentence gives a specific verb and resource — normalize a phone number to E.164 and identify the calling code — and the 'Solo sintaxis' clause sharply bounds what the tool does. It also names the sibling it is not (co-phone-validate), so it is distinguishable. The score is capped because the following 'Cuándo' sentence describes URLs, HTTP headers, tokens and IP ranges, which has nothing to do with this tool and muddies the purpose statement.

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?

There is a concrete exclusion, 'No para: verificar la SINTAXIS de un teléfono colombiano (co-phone-validate)', which routes the agent to the right sibling. But the positive 'when to use' condition is copy-pasted boilerplate about parsing URLs, tokens, IP ranges and Markdown files — actively misleading for a phone normalizer. Useful guidance exists, but only in the negative form.

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