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

Adds substantial context beyond annotations: cost ($0.005/call), payment rail (USDC on Solana via x402), and the exact two-step flow (call without payment, receive PaymentRequired challenge, retry with _meta["x402/payment"]). This is exactly the kind of auth/billing detail annotations cannot express. It does not state whether a failed calculation still incurs the charge, which is a meaningful gap for a paid tool.

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 sentences, front-loaded with the core purpose before the payment mechanics. The parenthetical payment instructions and HTTP endpoint are dense but each serves a real invocation need; nothing is filler.

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?

Covers the payment dimension thoroughly, but with no output schema the description should say what the answer contains (CO2e value, which factor vintage was applied, units). The schema's year field hints at vintage disclosure in the response, but the description never confirms the return shape.

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 description coverage is 100%, so factor_id, quantity, and the year/vintage behavior are already fully documented in the schema. The description only echoes 'from a published factor' and adds no format, unit, or vintage-selection detail beyond it — baseline 3 for schema-covered params.

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 and resource: 'Calculate CO2e for one activity from a published factor.' The phrase 'one activity' implicitly distinguishes it from emissions_calculate_batch, but the sibling is never named, so the differentiation is inferential rather than explicit.

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 frames the tool as pay-per-call and explains the x402 payment handshake, which tells the agent how to invoke it. However, it gives no explicit when-to-use guidance — nothing says to use emissions_calculate_batch for multiple activities or that a factor_id must first be retrieved from the factors endpoint (that hint lives only in the schema).

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