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

Annotations already declare this a safe, read-only, closed-world lookup, so the bar is lower. The description adds genuinely useful behavior: what the free tier returns (total rate + application note), the autónomo rate of 1.30%, and that IT/IMS breakdown plus source_anchor are gated behind a paid plan.

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?

Three tight sentences with the core purpose front-loaded, followed by free-tier contents and the paywall boundary. No filler, though the parenthetical autónomo note could be trimmed.

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 carries the return-value burden and does so partially: it names the free outputs and the paid ones. For a single-parameter lookup this is close to complete, missing only error behavior for invalid or unmapped CNAE codes.

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% and the code parameter already documents CNAE-2025 format and accepted dot variants. The description only reaffirms 'actividad CNAE-2025' and adds no syntax, validation, or defaulting 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+resource: it returns the AT/EP contribution rate for a CNAE-2025 activity. An agent can distinguish it from detalle_cnae or detalle_iae without opening the schema, though no sibling is named explicitly to sharpen the boundary.

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 description implies the use case (look up the AT/EP rate for a CNAE activity) but never states when to use this versus detalle_cnae or other CNAE-related siblings. The free-vs-paid framing reads as product tiering rather than invocation guidance.

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