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

The annotations already declare readOnlyHint=true and idempotentHint=true, so the safety profile is covered. The description adds useful context about the temporal scope ('hoy'), granularity ('hora a hora'), and market ('mercado regulado español'). It does not disclose data source, update frequency, or exact return format, which keeps it at a mid-range score.

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 description is two sentences with no filler. The first sentence states exactly what the tool returns, and the second sentence gives practical usage guidance. It is front-loaded and every word earns its place.

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 no-parameter, read-only price lookup, the description is complete enough: it specifies the resource, time scope, granularity, market, and use case. No output schema exists, but the phrase 'hora a hora' makes the expected return shape clear. There are no missing details that would prevent an agent from invoking it correctly.

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?

The tool has zero parameters, so the description does not need to explain parameter semantics. The baseline for zero-parameter tools is 4, and the description adds relevant context about what the returned data represents. There is no schema information to compensate for, making this appropriate.

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?

The description clearly states the resource: 'Precio PVPC hora a hora de hoy (mercado regulado español).' It uses a specific verb and scope, and the use case helps an agent understand exactly what data this tool provides. There is no sibling with a similar purpose, so confusion is unlikely.

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 explicitly says when to use it: 'Úsalo para decidir cuándo conviene cargar o para fijar un margen sobre el precio real.' It provides clear practical context, though it does not mention alternatives or when not to use it. Given the sibling list contains no other price lookup tool, this is sufficient.

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