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

Goes well beyond the readOnly/idempotent annotations by disclosing cost ('Gasta 1 consulta'), degraded no-API-key behavior with a daily limit, and Pro-tier gating of address/revenue/group fields plus the include_directors requirement (Pro y Business). It doesn't say what happens on an invalid CIF or a non-existent company, which is the remaining behavioral gap.

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 precondition and core purpose come first, followed by return fields, then alternatives and cost — a sensible front-load. The trailing English restatement 'Spanish company lookup by tax ID' is redundant against the opening sentence and the title.

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 no output schema, the description correctly enumerates the returned fields (razón social, forma jurídica, LEI, CNAE, provincia, fecha de constitución) and what conditional extras unlock. An agent has enough to call it and interpret results; only failure/nonexistence semantics are unstated.

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 already 100%, so the baseline is 3, but the description adds real meaning: include_directors is tied to a Pro/Business entitlement and to specific extra payload fields (administradores, tramo de empleados, domicilio masivo). The cif parameter itself adds nothing 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 ('Ficha de UNA empresa española') and pins the precondition ('cuando ya tienes su CIF/NIF'), which immediately separates it from name-based lookups. It also enumerates the returned fields, so an agent knows exactly what retrieval this is versus search_companies or verify_company.

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 routing: 'Si no tienes el CIF, usa search_companies' and 'para decidir un alta (KYB), verify_company'. Both alternatives from the sibling set are named together with the condition that selects them, leaving nothing to inference.

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