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

Annotations already declare readOnly=true, idempotent=true, openWorld=false, destructive=false, so the 'sin efectos secundarios / determinista' sentence largely restates structured data. What is genuinely new is the operational envelope (64 KB JSON input, 1 s per call, up to 1 MB result), which is useful context an agent cannot get from annotations or schema. Net: modest added value over a well-annotated baseline.

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?

Purpose is front-loaded, but the text is bloated by full ES/EN duplication plus a meta-note pointing at inputSchema.description, which itself repeats the same paragraph. Roughly half the tokens are redundant with each other, though no individual sentence is filler-free enough to score higher.

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?

Output schema exists, so return values need no explanation, and the description still covers scope, scale, exclusions, idempotence and payload/time limits. The one remaining gap is that same-family siblings (es-number-parse, number-to-words-en) are never disambiguated, but nothing needed to call the tool 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 description coverage is 100%, so the baseline is 3. The description clarifies gender agreement and apocope meaning ('veintiún, un' before a noun), which adds a little semantic color, but it never mentions the 'uppercase' parameter and adds no syntax detail beyond the schema.

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 ('Escribe un entero ... en palabras en español'), plus the numeric domain (up to 10^18), scale convention (escala larga) and the two output-shaping options. An agent can identify this as the Spanish cardinal-spelling tool without opening the schema, and the description names the sibling it is not (co-amount-in-words).

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?

Gives an explicit 'Cuándo' condition (Spanish text/numbers/dates must follow written language rules, not approximation) and an explicit 'No para' exclusion pointing to co-amount-in-words. It does not mention the inverse sibling es-number-parse or the English variant number-to-words-en, so routing among the three spelling/parsing tools is not fully closed.

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