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

Going well beyond the annotations (which only cover read-only, idempotent, open-world, non-destructive semantics), the description discloses pricing ($0.02/call over x402 on Base), the auth model (x-credit-token header for unlimited calls, no API key required), the absence of a free tier, and the 122-country scope. These are exactly the operational facts the structured fields do not carry.

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 leading question frames the purpose immediately and the operational facts are grouped at the end, so the structure is front-loaded and scannable. It runs slightly long with a marketing-tinged sentence ('most integrations discover only after a user fails'), but nearly every clause carries actionable information.

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 one-parameter, read-only lookup with an output schema already defined, the description supplies everything the agent needs: what it returns conceptually, when to call it, its cost, and its auth requirements. Return-value detail is correctly left to the 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?

There is a single parameter with 100% schema description coverage ('country', ISO country code), so the schema already does the heavy lifting. The description only restates 'Required input: country' without adding format, examples, or edge-case guidance, so this sits at the high-coverage baseline.

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 states a specific verb+resource: it reports which fiat-in and cash-out rails Coinbase runs in a country, plus currencies, limits, and supported asset/network counts. It is clearly a country-level feasibility lookup rather than a quote or transfer, but it never names its closest sibling onramp_quote, so the boundary is inferred rather than stated.

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 an explicit usage context — 'the feasibility check before you ask a user for money' — and explains the practical distinction between card-rail countries and transfer-only countries. What is missing is an explicit 'when not to use this / use onramp_quote for pricing instead' statement, so the agent must infer when to prefer a sibling.

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