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

Annotations declare readOnlyHint and openWorldHint, so the safety profile is already covered, yet the description adds genuinely new behavior: the per-call price ($0.002 USDC), the payment rail (x402 on Base, Solana or Arc), and the fact that failed calls are not charged. The billing semantics are exactly the kind of thing an agent cannot infer from annotations and must know before calling.

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 resource and source, then the disambiguation, then the return shape and cost. Every sentence carries practical information, though the aside about the route name being retained for the owner is a small detour an agent does not need in order to act.

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?

An output schema exists, so return formatting need not be explained; annotations cover read-only and open-world traits; the single parameter is documented. The description fills the remaining gaps an agent actually needs: data source, exact return window, and the payment model including non-charging on failure.

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?

There is one optional parameter and the schema describes it fully (1 to 24 months, default 3, 100% coverage). The description only restates the 24-month cap in the return summary ('plus up to 24 months of history'), adding no syntax or format detail beyond the schema. Baseline 3 is correct when the schema carries the parameter semantics.

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 (monthly European natural gas reference price), the exact source series (World Bank PNGASEUUSDM via FRED), and explicitly rules out what an agent might otherwise assume it to be (Dutch TTF hub settlement, day-ahead, storage level). That negative scoping separates it from energy_natural_gas_henry_hub and TTF-style siblings without the agent opening any schema.

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 clearly says what this is not, which steers an agent away from misfiring on hub-settlement or day-ahead use cases, and notes the route name is a legacy label for the owner. It stops short of naming the sibling tool to use instead for TTF settlement or storage data, so the agent has an exclusion but not a redirect.

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