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

With no annotations, the description carries the full burden and does substantial work: it discloses the price and payment protocol ($0.002 USDC/call, x402, eip155:8453), the upstream source (ECB fixing), determinism guarantees and the peg fallback path. It stops short of update cadence, rate limiting, or what happens when the ECB has no fixing for the day.

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 that front-loads the payment preamble and then the rate definition; every clause adds information (source, fallback, response fields, determinism). The bracketed pricing prefix slightly competes with the purpose statement, but no sentence is wasted.

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 tool the description is nearly sufficient: it enumerates the returned elements (rate date, method, source attribution) and pre-empts the obvious question about peg handling. The remaining gap is freshness/update cadence and expected failure modes, which an agent paying per call would want.

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 usefully notes that the pair, method and date are fixed server-side and reflected in the response rather than configurable by the caller.

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 (official KMF/JPY reference rate), the exact source (latest ECB fixing, or the official peg when JPY is pegged), and the discriminating property versus an estimation tool ("deterministic, sourced, no estimate"). The currency pair is unambiguous from the description alone, so an agent can distinguish it from the adjacent fx_kmf_eur / fx_kmf_usd siblings without opening a schema.

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 by the fixed pair: an agent infers it should call this when it needs a KMF/JPY reference rate rather than a converted amount. There is no explicit routing to fx_convert, fx_latest or fx_timeseries, and no statement of when this should not be used (e.g. when a live tradeable quote is required).

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