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

Annotations already declare readOnly/openWorld/idempotent/non-destructive, so the safety profile is covered. The description adds material context annotations cannot express: a paid call ($0.15 USDC via x402 on Base or Solana) and the upstream source (CoinGecko) — the payment requirement is the critical behavioral fact here.

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?

Two tight sentences plus a one-line price notice, with the core capability front-loaded before the cost caveat. No filler and nothing redundant with the schema.

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 2-param, fully-enumerated, no-output-schema tool this is nearly complete: what it returns, when to use it, and the payment gate are all stated. It only omits the shape of the returned series (e.g., list of date/close pairs), which matters somewhat since there is no output schema.

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% and both params carry enums, so the schema already defines valid values. The description adds only 'any supported cryptocurrency' and the 30/90/365 windows, both of which restate the schema; baseline 3 applies.

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 and resource ('Returns historical daily closing prices'), plus scope (30/90/365 days) and data source (CoinGecko). It implicitly contrasts with the sibling get_crypto_price (current price) via 'historical', but never names an alternative explicitly.

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 use cases ('trend analysis, drawdown calculation, or training data'), which tells the agent when this tool is appropriate. It stops short of exclusions or naming which sibling to use instead (e.g., get_crypto_price for a spot quote).

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