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

With annotations only declaring safety hints, the description carries the behavioral load: it discloses the price ($0.05 USDC on Base), the payment-wall behavior (returns payment_required with x402 v2 URL and header name, not a bare payment link), and credit-spend semantics. This is unusually rich context beyond what the annotations supply.

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 core purpose and methodology, then payment behavior. Mostly earns its place, though it repeats the title verbatim as its first sentence and includes a slightly defensive closing line ('does not return the rate as a bare payment link').

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?

No output schema, but the description explains the two possible return paths (data vs payment_required) and the payment mechanics, which mitigates the missing output schema. For a 2-param, 100%-coverage read tool, this is nearly complete.

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 already 100%, so the baseline is 3. The description adds meaning to pass_token (valid on eligible shelves, returns data and spends credits, not spent above the ceiling) and reasserts the required pair, giving context beyond the schema's own field descriptions.

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 (street/parallel-market FX rate) with explicit scope (USD-NGN or USD-GHS) and defines the methodology (Bybit P2P median VWAP / USDT-USD basis + Frankfurter reference + spread). It also distinguishes itself from siblings by stating 'Not a central-bank rate' (vs get_fx_official) and noting OKX is archived.

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 conditions under which data is returned (valid Shelf Pass on a shelf ≤ $0.25) versus when payment_required is returned, and clarifies it is not a central-bank or bureau-de-change quote. It does not explicitly name get_fx_official/get_fx_africa_brief as the alternatives to use instead, so routing vs siblings is only implied.

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