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

With zero annotations, the description carries the burden and does so well: it discloses that the call is metered ($0.002 USDC/call via x402 on eip155:8453), the upstream source (European Central Bank), the derivation method (direct, cross via EUR, or peg-derived), and that results are deterministic and not estimated. It does not cover freshness/caching behavior on non-publication days or error responses, which keeps it below a 5.

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?

A single dense sentence with the cost/payment terms bracketed up front so an agent sees the metering before anything else. Every clause adds information (source, method, deliverable fields, determinism) with no filler, though the bracketed prefix is slightly heavy for a first read.

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 zero-parameter lookup with no output schema and no annotations, the description is nearly self-sufficient: it tells the agent what it will receive (rate, rate date, method, attribution), where the data comes from, and that access is paid. Only the payment mechanics' practicalities and non-publication-day behavior are left unstated.

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?

The tool takes no parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The description correctly implies the pair is fixed by the tool itself rather than supplied as input.

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?

Names the exact resource (ILS/XAF) and states precisely what is returned: the latest ECB fixing, or the official peg when XAF is pegged, plus rate date, derivation method and attribution. That is far more specific than the bare pair name. It stops short of naming sibling alternatives (fx_convert for conversions, fx_latest, fx_timeseries), so differentiation is inferred from the 'reference rate / no estimate' wording 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?

Usage is implied by the resource description: call this to obtain a sourced reference rate for the ILS/XAF pair. There is no explicit when-to-use/when-not rule and no routing to fx_convert for converting an amount or fx_timeseries for history, which matters in a family of hundreds of near-identical fx_* tools.

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