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

With no annotations, the description carries the full burden and does disclose the paid x402 flow, cost, chain ID, data source, and non-estimated nature, plus the fact that rate date, method, and attribution are included in the response. It omits what happens on weekends/holidays when no ECB fixing publishes, which is a meaningful freshness gap for an FX rate.

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?

The paywall prefix is front-loaded so cost is visible immediately, followed by a single dense sentence covering source, method, returned fields, and determinism. The parenthetical method enumeration is slightly wordy but every clause adds information.

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 rate lookup, the description fully explains what comes back (rate, rate date, method, source attribution) and what it costs. Only edge-case behavior around unpublished fixings is left unaddressed.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?

There are zero input parameters, so the baseline is 4. The description usefully implies the parameterless semantics: the pair is fixed by the tool itself and 'latest' means no date, amount, or currency arguments are accepted.

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 ('CHF/KMF reference rate'), its provenance (latest European Central Bank fixing, or the official peg), and the computation method (direct, cross via EUR, peg-derived). 'Deterministic, sourced, no estimate' implicitly separates it from a generic conversion tool like fx_convert, though no sibling is named 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?

Payment terms ($0.002 USDC, x402, eip155:8453) give a clear precondition for calling, and 'latest' implies there is no date argument. However, there is no guidance on when to prefer this over fx_convert, fx_latest, or fx_timeseries for historical rates, so the agent must infer routing from the tool name alone.

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