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

With no annotations, the description carries the full burden and does a solid job: it discloses that the call is paid, the exact cost and accepted chains (Base or Solana), the absence of account/API-key requirements, and the refund policy for failed calls. It stops short of explaining the payment flow itself (e.g. how the 402 challenge is settled), which would be the last missing piece for a metered endpoint.

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 'Paid.', then the payload contents, price, auth model, billing policy and endpoint URL in telegraphic fragments with no filler. Every clause conveys a fact an agent or caller would otherwise have to discover by trial.

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?

For a zero-parameter tool with no output schema, the description covers both what is returned (daily values and a 7-day average) and the commercial/auth mechanics of the call. The only gap is the concrete payment protocol, which is a modest omission given how much else is disclosed.

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 schema has zero parameters, so there is nothing for the description to disambiguate; baseline is 4. The description does not add parameter meaning, but none is needed for a fixed-window, zero-argument call.

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?

The description names the resource (Crypto Fear & Greed Index) and its exact scope (last 7 days, daily values plus an average), which implicitly separates it from the sibling buy_fng_30d, buy_fng_90d and buy_fng_now. It never names those alternatives explicitly, so the differentiation is inferable rather than stated.

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 context is only implied: 'Paid', the $0.002 price, 'Failed calls not charged' and 'No account, no API key' are decision-relevant facts, but the description never says when to choose this window versus the 30d/90d/now siblings or when a paid index is preferable to a free price tool. An agent must infer scope selection from the name and the phrase 'last 7 days'.

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