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

With no annotations, the description carries the full burden and does well: it discloses the paid cost ($0.002 USDC/call via x402 on eip155:8453), the authoritative source (ECB), the possible derivation paths (direct, cross via EUR, peg-derived), and that the result is deterministic rather than estimated. It omits failure behavior when no fixing exists, caching/freshness policy, and 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 compact sentences with zero redundancy; the payment/x402 tag is bracketed up front so cost is visible before invocation, and the substantive description follows immediately. Every clause carries information the agent cannot get from structured fields.

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, single-purpose rate lookup with no output schema, the description covers source, method, date, and attribution fields plus the peg nuance, which is sufficient to call it correctly. What is missing is the concrete output shape and behavior when a fixing is unavailable or stale.

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 correctly frames the call as parameter-free by fixing both currencies in the tool name, and adds useful meaning about what the single implicit pair yields (rate, date, method, attribution).

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 verb-equivalent and resource: "Official SEK/KMF reference rate: latest European Central Bank fixing," and clarifies the peg fallback and derivation method. It is unambiguous about what the tool returns. It does not, however, distinguish itself from the hundreds of sibling fx_* pairs or from fx_convert/fx_latest, so an agent gets no explicit differentiation.

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?

There is no statement of when to use this tool versus fx_convert (for actual conversion) or fx_latest/fx_timeseries (for other views). The line "Deterministic, sourced, no estimate" implies it should be preferred for authoritative rates, but that is inference, not guidance. No prerequisites or exclusions are given.

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