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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and non-destructive behavior, so the bar is lower — yet the description adds substantial context the annotations cannot convey: cost (0.002 USDC per call via x402 on Base, Polygon, or Solana), the failure mode of an unpaid call (isError, structuredContent with x402Version 2 and accepts), and the exact retry protocol (sign an accepts entry, retry with PaymentPayload in params._meta['x402/payment']). It also says where the settlement appears (result._meta['x402/payment-response']), which is critical for a paid tool.

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 description is front-loaded with purpose and scope, then covers payment mechanics — all of which are necessary for a paid x402 tool. It is a single dense paragraph rather than structured sections, and the x402 retry instructions are verbose, but every sentence carries operational value. Slightly tighter formatting would earn a 5.

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?

Given the complexity of a paid x402 tool with no output schema, the description is complete: it explains the payment-required result, the retry handshake, the return format ('Returns JSON text'), and where settlement metadata lives. An agent has everything needed to call it correctly on the first attempt and handle the payment challenge.

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%, so the schema already documents 'to' (quote currency), 'from' (base currency, default USD), and 'amount' (base-currency amount, default 1). The description reinforces this with 'Send {from, to, amount}' and ISO examples (USD, EUR, ARS, BRL, JPY), but adds no syntax or format detail beyond what the schema provides. Baseline 3 applies when the schema does the heavy lifting.

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 gives a specific verb and resource: 'Daily mid-market exchange rate between two fiat currencies (ISO 4217...) with optional amount conversion.' It names the data source (blended central-bank reference rates via Frankfurter) and the exact input shape ({from, to, amount}), so an agent can tell this apart from the Argentina-specific siblings (ar_dolar, ar_cbu, etc.) without opening either 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?

The description implies when to use it (currency conversion between fiat pairs) and states one edge condition — 'Unsupported currency = no charge.' But it never explicitly says when to prefer this over a sibling such as ar_dolar, nor does it state exclusions or prerequisites beyond the payment flow. Usage is inferable but not routed.

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