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

With no annotations supplied, the description carries the full behavioral burden and does well: it discloses that the call is paid ($0.005 USDC), the accepted payment chains, that no account or API key is required, and that failed calls are not charged. It omits rate limits, data freshness/latency beyond the 1d/7d/30d windows, and what a failure response looks like, which keeps it below a 5.

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 'Paid.' token is front-loaded so cost is the first thing an agent sees, followed by scope, price, prerequisites, and URL in a compact run of fragments. It is slightly telegraphic and mixes commercial terms into the same sentence cluster as the data description, but no sentence is wasted.

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?

There is no output schema, so the description must explain return values, and it does so by enumerating every field the caller gets. Combined with pricing, auth requirements, and the endpoint URL, nothing an agent needs to invoke this zero-parameter tool correctly 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?

The tool takes zero parameters, so there is no parameter surface for the description to explain and the baseline of 4 applies. The description correctly implies no filtering arguments are needed for the fixed apxUSD target.

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 specific resource (apxUSD stablecoin) and enumerates exactly what is returned: price vs peg, supply, 1d/7d/30d supply change, and supply per chain. An agent can distinguish this from the hundreds of sibling buy_stable_* and buy_coin_* tools 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?

Usage is implied by the data enumeration, and payment prerequisites (Base or Solana, USDC) are stated, but there is no explicit when-to-use versus alternatives guidance and no mention of when it would be preferable to buy_crypto_price or buy_stable_usdc. Alternatives are discoverable only via the sibling list, not the description.

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