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

Annotations already declare readOnlyHint=true and destructiveHint=false, and the description reinforces this ('Reads or computes only and changes nothing'). Beyond that it adds information annotations cannot express: per-call billing ($0.002 via x402 USDC or card key), 25 free calls, failed calls free, and that repeated calls are charged — which explains the idempotentHint=false in practical terms.

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?

The sentence is dense but front-loaded: the resource and scale (0-100) come first, then return contents, then use cases, then commercial terms. Nothing is filler, though the billing clause lengthens a one-parameter read tool more than strictly necessary for invocation.

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 one optional parameter and no output schema, the description does the work of explaining what comes back (value, label, multi-horizon changes, window statistics, up to 90 days of history) and how it is priced. The only missing piece is disambiguation from the similarly named sibling tool.

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%, with 'Days of history, 1-90 (default 30)' already in the schema. The description restates the 90-day cap and gives a usage example, but adds no new semantics such as what a longer window costs or how partial windows behave. Baseline 3 is appropriate since the schema carries the specification.

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 a concrete resource and source ('Crypto Fear and Greed Index today ... from alternative.me') and enumerates the returned fields (value, label, 1/7/30-day change, window low/high/average, daily history). However, it never distinguishes itself from the near-identically named sibling 'crypto_fear_greed', so an agent faced with both must guess.

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 use cases ('a market-sentiment input for a trading agent, a dashboard or a daily brief') and a concrete invocation example ('days=30'), which is genuine context. It offers no when-not guidance and no routing rule against the sibling fear-and-greed tool, so selection is still implied rather than specified.

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