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

With no annotations, the description carries the full burden and does meaningful work: it discloses the paid x402/Base-USDC cost of $0.002 per call and the network (eip155:8453), which the agent must know before invoking. It also discloses behavior beyond the raw value — how the rate was derived (direct, cross via EUR, or peg-derived), the rate date, and source attribution, plus the guarantee that no estimation is involved. It stops short of describing failure modes or what happens outside ECB publication days.

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 payment/x402 tag is front-loaded in brackets, followed immediately by what the tool returns, which is the right order for an agent deciding whether to spend money. It is a single dense sentence with little waste, though the parenthetical about pegs and derivation methods makes it slightly clause-heavy.

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?

With no output schema and no parameters, the description is the only source of return-value semantics, and it covers the important parts (rate date, derivation method, attribution). The notable gap is the quote convention — nothing states whether the returned number is OMR per SEK or SEK per OMR — and no field names or response envelope are described, which matters for an agent wiring the result into a computation.

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 schema declares zero parameters, so the baseline of 4 applies; there is no parameter semantics for the description to elaborate. The description appropriately spends its words on the returned rate's properties instead of inventing inputs.

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 — the SEK/OMR reference rate — and specifies its provenance (ECB fixing, or official peg where applicable). Combined with the self-describing pair name, an agent can distinguish this from the hundreds of sibling fx_* pairs without opening a schema. It never names the more general siblings (fx_latest, fx_convert, fx_timeseries), so differentiation rests on the name rather than on described scope.

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 'deterministic, sourced, no estimate' signal this is the authoritative-fixing option, which tacitly distinguishes it from estimate-based or conversion tools. However, there is no explicit when-to-use or when-not-to-use guidance, and no pointer to fx_convert for cross-currency conversion or fx_timeseries for historical data.

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