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

Annotations declare readOnly/idempotent hints, and the description adds context the annotations cannot: daily update cadence, cost ($0.01/call via x402 USDC or card key, first 25 free, failed calls free), and the explicit warning that repeated calls are billed — which explains why idempotentHint=false matters in practice. It also confirms 'reads or computes only and changes nothing', reinforcing the safety profile with a stated consequence.

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 definition and scale, then return contents, then usage and economics — a logical order with no filler sentences. It runs long for a single-parameter tool because pricing/licensing details are packed into the same block, which is useful but makes the final third dense.

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 no output schema, the description carries the return-shape burden and does so fully (value, label, deltas, window stats, history length), plus freshness, cost, and side-effect posture. An agent has everything needed to decide and call correctly; only pagination/truncation of the 90-day history is unstated, a minor gap.

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 coverage is 100%, so the baseline is 3, but the description adds meaning beyond the schema by explaining that `days` defines the window over which low/high/average are computed and that it drives the returned daily history (default 30, up to 90). That aggregation semantics is not visible in the bare parameter description.

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?

Names the specific resource (Crypto Fear & Greed Index from alternative.me), its scale (0 = extreme fear, 100 = extreme greed), and enumerates exactly what is returned: today's value and label, 1/7/30-day deltas, window low/high/average, and daily history. It also routes a closely-related need elsewhere ('for live prices use price_token'), so the agent can distinguish it from siblings without opening the 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?

Gives concrete usage contexts ('market-sentiment input for a trading agent, a dashboard or a daily brief') and names the alternative for live prices. It does not state explicit exclusions against the other market tools (btc_price, trade_insight, pack_crypto_market), so the boundary is clear for the price case but left implicit for the rest.

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