validate_nif

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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false, so safety and determinism are covered without prose. The description adds one genuinely new behavioral fact — 'Free', i.e. no credit consumption — but says nothing about how format problems (dashes, lowercase, wrong length) are handled versus checksum failures. A 3 fits because annotations carry most of the burden and the text adds only marginal context.

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 terse fragments with the core action and resource front-loaded and the cost note appended. Nothing is redundant and the whole definition is scannable in one pass.

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, deterministic validator with an output schema and full annotation coverage, the description supplies what an agent needs to select and call it: the identifier family and the fact that it is free. The main residual gap is input-format expectations, though the output schema relieves the need to describe return values.

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 0% for the single required parameter, which the schema only labels as a bare string titled 'Nif'. The description partially compensates by enumerating the accepted identifier types (NIF, NIE, CIF), which is real meaning beyond the schema. It still omits expected format details such as separators, case, or length, so compensation is incomplete.

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 concrete verb ('Check') and a precisely scoped resource ('Spanish NIF / NIE / CIF control character'), so an agent knows it validates tax-identifier checksums rather than performing a generic string check. It is clearly distinct by resource from sibling validators such as validate_facturae or validate_verifactu_xml, though it does not name those siblings explicitly.

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?

Usage is implied by the resource name: call it when a Spanish tax ID needs verification. There is no explicit statement of when not to use it, no mention of alternatives for non-Spanish identifiers, and no prerequisites, so guidance stays at the implied level. The 'Free' note does hint at a cost consideration versus paid siblings, which is a small added signal.

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