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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds meaningful behavioral context beyond that: the $0.003 USDC per-call cost via x402, the geographic scope, and the exact content and comparison basis of the response. It does not mention rate limits or response shape beyond the field list.

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?

One dense, front-loaded sentence covering resource, scope, output fields, and cost; nothing is wasted. The parenthetical fuel list is slightly verbose but useful for scoping expectations.

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 by enumerating the metrics returned (GWh, share by fuel, renewable/carbon-free share, five-year delta). Combined with the cost note and scope, an agent has enough to call it correctly; only pagination/format details are absent.

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 single parameter is self-documenting ('State name or code; omit for the US'). The description reinforces the default behavior by stating 'any US state or the whole US', adding a small amount of clarity 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 resource and scope: 'Electricity generation mix for any US state or the whole US', plus the exact units (gigawatt-hours) and breakdown dimensions (share by fuel, renewable and carbon-free share, five-year comparison). An agent can tell this apart from energy_electricity_prices or energy_forecast by content, though no sibling is named explicitly.

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?

Usage is implied rather than stated: an agent can infer this answers generation-mix questions, and the cost line hints at a paid call. There is no explicit when-to-use/when-not guidance or routing to alternatives such as energy, energy_electricity_prices, or energy_forecast.

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