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

With no annotations, the description carries the load and does so well on the operationally critical dimension: it discloses the paid cost, the x402 payment protocol, the chain identifier, and that data is ECB-sourced and deterministic rather than estimated. It also explains the peg fallback and method derivation (direct, cross via EUR, or peg). It stops short of describing error behavior, caching/freshness beyond 'latest', or the response shape.

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 dense sentences with no filler, and the payment/cost bracket is front-loaded so an agent sees the metering before deciding to call. The single long sentence after the bracket packs several distinct facts (rate source, peg fallback, method, attribution) that would read more clearly if split, but nothing 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 tool with no output schema and no annotations, the description covers what an agent needs: what value comes back, its date, how it was derived, and its attribution. The remaining gap is the payment execution flow (how the x402 charge is settled) and response structure, which is acceptable given the tool's simplicity.

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 the baseline is 4. The description makes clear the pair is fixed by the tool itself and requires no input, which is the only semantic an agent needs here.

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 precise resource — the official SEK/CVE reference rate — and specifies its provenance (ECB fixing, or the peg when CVE is pegged). The currency pair is unambiguous from name plus text. It does not, however, distinguish itself from siblings like fx_convert or fx_latest, which an agent must infer from naming convention alone.

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: 'deterministic, sourced, no estimate' suggests this is the authoritative-reference choice over a computed conversion, and the paid-call pricing signals a metered endpoint. There is no explicit when-to-use/when-not guidance and no mention of fx_convert, fx_latest, or fx_timeseries as alternatives for conversion, history, or bulk retrieval.

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