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

Goes well beyond readOnlyHint/openWorldHint: discloses the upstream data source and license (Preciluz, CC-BY-4.0, from REE/ESIOS), a hard rate limit of 20 calls per client per UTC day shared across the Spain tools, and the exact failure behavior when the quota is exhausted, including the paid endpoint, price ($0.002 USDC per successful call, x402 on Base) and that no account/API key is needed. That is precisely the context an agent needs to decide whether to call and how to react to an error.

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-loaded with what the tool returns and why to use it, then quota/data-source caveats. Every sentence carries information, though the quota/paid-endpoint sentence is dense and could be split for scanability.

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 still specifies the return shape (hourly EUR/kWh plus cheapest hours and 2h/3h windows), the freshness constraint on tomorrow's data, provenance, and quota/error behavior. Nothing an agent needs in order to call it or interpret a failure is missing.

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%, so the baseline is 3, but the description adds real meaning: it establishes the output unit (EUR/kWh) and hour-by-hour granularity, and the publication timing for tomorrow materially clarifies what the 'tomorrow' enum value will actually return. The 'cheapest' parameter's default and role are left to the schema, which already documents them.

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 resource (Spain's regulated PVPC household price, peninsula), a specific granularity (hour by hour in EUR/kWh) and the derived outputs (cheapest hours and cheapest 2h/3h windows). An agent can immediately tell this apart from every sibling, none of which touch electricity pricing.

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?

Gives concrete use cases ('schedule appliances or EV charging', 'answer when is electricity cheapest in Spain today/tomorrow') and the operational caveat that tomorrow is only published around 20:15-20:30 Madrid time. It does not name an alternative tool or a when-not-to-use case, but no sibling overlaps this domain, so the practical guidance is close to complete.

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