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

With no annotations present, the description carries the burden well: it discloses the payment model and network ('PAID $0.002 USDC/call, x402, eip155:8453'), states the data is 'deterministic, sourced, no estimate', and explains that a peg may substitute for a fixing and that the returned method will say which. It does not cover failure behavior when no fixing exists (weekends/holidays), caching, or rate limits.

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 sentences, front-loaded with the price/access constraint followed by the purpose and the returned fields — nothing is buried. The parenthetical about methods is dense but each clause ('direct, cross via EUR or peg-derived') adds real information rather than restating the name.

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 no-parameter, no-output-schema lookup, the description covers the source, the derivation method, the peg fallback, and the fact that the rate date and attribution accompany the value, which is enough for an agent to call it and interpret the result. It stops short of stating freshness/latency or what the response looks like when the ECB has not published a fixing.

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 zero parameters, so there is nothing for the description to disambiguate beyond confirming the pair is fixed by the tool itself. Baseline 4 applies.

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?

The description names a specific resource and verb-equivalent ('Official MXN/XOF reference rate', 'latest European Central Bank fixing') and even states the derivation method, so an agent knows exactly which pair and which source this returns. It never names a sibling such as fx_convert or fx_latest, so the distinction from the generic/cross-pair tools is left implicit in the name 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 only implied: the fixed pair plus the $0.002 USDC/call price tag tells an agent this is a paid, single-pair lookup, which implicitly argues for using fx_convert when arbitrary pairs or conversion math are needed. There is no explicit when-to-use/when-not statement and no mention of an alternative for bulk or historical needs (fx_timeseries).

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