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

Annotations mark it readOnlyHint=false, and the description justifies that by disclosing the $0.05 USDC on Base cost, the pass_token credit-spend behavior, the ceiling rule (token not spent above the ceiling), and the exact non-success return shape (payment_required with x402 v2 URL and header name). This is far more than the annotations convey and materially useful.

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 the resource and methodology, and every clause carries information. Slightly dense, and the trailing 'It does not return the rate as a bare payment link' is marginally redundant, costing it a point.

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 still covers both return paths (the data, or payment_required with the x402 v2 URL), the price, and the pass eligibility rule. An agent has everything needed to call and interpret it.

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 coverage is 100%, so the baseline is 3, but the description adds real meaning about pass_token: it is spent only on an eligible shelf and not spent above the ceiling, which clarifies the parameter's side effect beyond the schema text.

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 resource and scope: the street/parallel-market rate for USD-NGN or USD-GHS, with the exact methodology (median VWAP of Bybit's USDT/fiat P2P book ÷ USDT/USD spot basis, plus Frankfurter official reference). It also carves out what it is not (central-bank rate, bureau de change quote), which lets an agent distinguish it from the get_fx_official sibling.

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 clear context for when this applies versus an official rate, and spells out the payment/eligibility conditions (valid Shelf Pass on a shelf priced ≤ $0.25 versus falling back to payment_required). It does not explicitly name get_fx_official as the alternative, so the routing is implied rather than stated.

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