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

With no annotations at all, the description carries the full burden and does unusually well: it discloses the paid x402 cost and chain (eip155:8453, $0.002 USDC/call), the upstream source (ECB fixing), the peg fallback path, and the returned metadata (rate date, method, source). It does not cover error behavior, stale-rate handling, or rate limiting.

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 after the payment tag; every clause (source, peg fallback, method, date, attribution, determinism) adds information. The payment prefix is front-loaded but is arguably the most decision-relevant fact for an agent calling a paid endpoint.

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, no-output-schema lookup tool this is close to complete: the agent knows what value it gets, where it comes from, that it costs $0.002 per call, and what metadata accompanies it. Only the response shape and failure modes remain unspecified, which is minor at this complexity.

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?

Zero parameters, so there is nothing for the description to disambiguate and the baseline of 4 applies. The description does not waste space re-explaining an empty schema.

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 a specific resource (XAF/USD) and a precise source (latest ECB fixing, or official peg when USD is pegged), which is far more than a restatement of the name. It does not explicitly distinguish itself from fx_latest, fx_convert, or fx_timeseries, so the agent must infer from the pair-naming convention that this is the single-pair spot rate.

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?

"Deterministic, sourced, no estimate" implies the appropriate use case (authoritative reference rate rather than a computed or estimated value), but there is no explicit when-to-use or when-not-to-use statement and no routing to fx_convert for actual conversion or fx_timeseries for history.

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