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

With no annotations, the description carries the full burden and does notable work: it discloses the paid model ($0.002 USDC per call via x402 on eip155:8453), states results are deterministic and sourced with no estimation, and explains the peg fallback path. It stops short of rate limits, caching/freshness window, or failure behavior when no fixing exists.

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?

Essentially one sentence, front-loading the payment/x402 preamble before the substantive definition, and every clause (source, peg fallback, returned metadata, no-estimate guarantee) carries information. The bracketed billing tag is dense but it is the only place cost and protocol are disclosed.

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 annotations and no output schema, the description covers the important ground: what the number is, where it comes from, the derivation method, and that response metadata includes rate date and source attribution. Only the numeric precision/quote convention (OMR per CHF vs CHF per OMR) and the freshness window 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 zero parameters, which is the baseline-4 case; the pair is baked into the tool name and the description correctly explains that the currency pair and the derivation method (direct, cross via EUR, peg-derived) are determined server-side. Nothing about parameters needs clarifying.

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 the exact resource (OMR/CHF reference rate) and the specific source (ECB fixing, or the official peg when CHF is pegged), so an agent knows precisely what value it gets back. It does not, however, distinguish itself from the near-identical reversed sibling fx_chf_omr or from fx_latest/fx_convert, which is the one differentiator an agent most needs in this ~600-tool family.

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 rather than stated: 'official reference rate' and 'no estimate' signal this is for authoritative/compliance-grade rates rather than ad-hoc conversion. There is no explicit when-not guidance and no pointer to fx_convert (amount conversion), fx_latest (many pairs at once), or fx_timeseries (history), all of which sit in the sibling list.

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