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

Annotations declare readOnlyHint=false and non-idempotent, but do not reveal that the call costs money; the description fills that gap with the exact price ($0.01 USDC on Solana) and the two-step x402 handshake plus an HTTP fallback URL. That is real behavior context beyond structured fields. Minor gap: no mention of the forecast horizon or update cadence.

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 the purpose, then payment mechanics and the HTTP equivalent. Dense but each sentence carries information the agent needs to actually invoke a paid endpoint; the parenthetical x402 detail is slightly heavy but justified.

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?

No output schema exists, so the description should convey the shape of the returned curve (units, resolution, forecast horizon). It says only "average carbon intensity curve", leaving the agent unable to judge what it will receive, though the payment mechanics are fully covered.

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?

Only one parameter, and the schema already documents it at 100% coverage including the GB/IE/DK zone codes. The description adds nothing beyond "one zone", so baseline 3 is 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?

States a specific verb+resource+scope: "Forecast average carbon intensity curve for one zone." An agent can distinguish it from the sibling carbon_intensity_current (forecast vs current), though the description never names that sibling 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?

It gives clear procedural guidance for the payment flow (call without payment, read the PaymentRequired challenge, retry with _meta["x402/payment"]), which is genuinely actionable. However, it offers no explicit guidance on when to choose this over carbon_intensity_current or when-not to use it.

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