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

With annotations already covering read-only/idempotent/open-world, the description layers on genuinely new behavioral facts: $0.002 per call via x402 with USDC on Base (MCP payments Base-only), errors not charged, data source with CoinGecko fallback via the source field, ~1 min update cadence, and an explicit disclaimer that momentum/sentiment labels are fixed rules, not advice.

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 purpose, then usage, then return shape and cost/caveats, so structure is sound. The return-value enumeration is a long comma-chained sentence, but nearly every detail (cost, source, cadence, disclaimer) is decision-relevant rather than 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?

With no output schema, the description carries the return-value burden and does so fully: nested asset fields (priceUsd, change24hPct, momentum), top-level momentum/sentiment/direction/avgChange24hPct/asOf, plus cost, source, and staleness. Nothing an agent needs to call or interpret it is missing.

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 are zero parameters, so the baseline is 4; the description usefully confirms 'Call GET with no parameters', removing any doubt about required input. Nothing further is needed.

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+resource ('Crypto prices API: BTC, ETH, SOL spot price') and pins the exact asset set and quote currency, which distinguishes it from siblings like market_quote and market_search. An agent can identify it without 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?

Gives an explicit trigger ('Use when you need current BTC, ETH and SOL spot prices in USD') plus a refresh cadence warning that calling more often returns identical values, which implicitly steers agents away from polling. No explicit when-not or named alternative sibling, so it stops short of a 5.

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