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

Annotations already declare the read-only, idempotent, closed-world profile, so the description correctly focuses elsewhere. It adds genuine behavioral context the annotations cannot convey: each country carries a 'currency' field, some countries are returned in local currency unconverted (TRY, BRL, VND), and cross-currency comparisons must be converted first. Only return-shape/pagination detail is absent.

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?

Front-loads the purpose, then arg semantics, then currency caveats as a separate closing paragraph, so nothing important is buried. The sentence 'Datos de solo lectura, sin autenticación' is mild redundancy against the readOnlyHint annotation and openWorldHint=false, but the rest earns its space.

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?

There is no output schema, so the description carries the return-value burden and does so by specifying units (price per litre) and the per-country 'currency' field with the unconverted-currency warning. Combined with the annotated safety profile, an agent has everything needed to call and interpret this tool.

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 schema already documents the ISO 3166-1 alpha-2 code and the omit-to-return-all behavior. The description repeats this and adds an example ('ES'), which is a marginal gain over the structured field, so the baseline 3 applies.

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 ('Devuelve el precio medio nacional de gasolina G95, G98 y diésel por país' plus Brent) and names the aggregation source. The scope is clearly distinct from the sibling tools (alerts, city stations, country coverage), so an agent can route correctly without opening any schema.

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?

Explains the trigger condition for the single parameter ('Si se omite, devuelve todos los países disponibles') and notes it is read-only with no authentication. It gives clear usage context but does not explicitly reference alternatives among the sibling tools for related price/coverage questions.

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