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

With no annotations, the description carries the full burden and delivers key facts: it is a paid call ($0.005 USDC, x402, eip155:8453) and returns derived statistics plus % change. It omits auth mechanics beyond the payment hint and pagination/error behavior, but the pricing disclosure is substantive context an agent would otherwise lack.

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?

Two compact sentences, with pricing front-loaded and the resource/scope statement immediately following. Every clause contributes; only the parenthetical pricing block is dense but justified.

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 exists, so the description compensates by naming the returned aggregates (min, max, mean, % change). Combined with the pricing and date-window disclosure, an agent has enough to call it correctly; only error/empty-range behavior is unaddressed.

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 all four parameters are documented in the schema (from/to ISO 4217, start/end dates with defaults). The description only adds the 366-day cap and the pair concept, which maps to the schema's start/end semantics but adds no syntax beyond it. Baseline 3 is appropriate.

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 (daily official ECB exchange rates for a currency pair) with scope (up to 366 days) and output content (min, max, mean, % change). Clearly distinguishable from fx_latest/fx_convert by the timeseries framing, but does not name those siblings explicitly.

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?

Offers use-case hints (invoicing, hedging checks, reporting) that imply when this tool fits, but never states when to prefer it over fx_latest or fx_convert, nor names alternatives. Usage is implied rather than explicit.

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