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

Annotations already declare readOnly, idempotent, non-destructive behavior, so the safety profile is covered. The description adds meaningful non-schema context: the pricing model (free within a daily allowance, then $0.002 USDC per call via x402), which is exactly the kind of cost/rate information annotations cannot express.

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?

Two short sentences, zero waste. The functional description is front-loaded ahead of the pricing detail, so an agent gets the 'what' first.

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 an output schema present and annotations covering safety, the description needn't explain return values. But 'recent history' is vague (how far back, what window), and the lack of any routing versus generation-now/generation-today leaves a real gap for a tool in a dense sibling cluster.

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 takes zero parameters, so there is nothing for the description to disambiguate. Baseline for a parameterless tool is 4.

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 and granularity: 'half-hourly generation by fuel type, recent history.' This clearly distinguishes a historical mix series from a snapshot. However, it never names the closely related siblings (generation-now, generation-today), leaving the agent to infer where this one fits.

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 phrase 'recent history' implies when this is appropriate versus a real-time or daily view, but there is no explicit guidance, no prerequisites, and no reference to the alternative generation tools. Usage is only weakly implied.

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