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 payment requirement and price, the protocol and chain, the authoritative source (ECB), the fallback rule for pegged HKD, and the derivation method returned with each result. It also asserts determinism and 'no estimate', which sets expectations about accuracy. What it does not cover is failure behavior when no fixing is available or how stale a cached rate might be.

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 segments: a bracketed pricing/access banner followed by a single dense sentence covering source, fallback, return fields, and determinism. Every clause contributes information and the core purpose appears immediately after the cost tag, with no filler.

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-value lookup with no output schema, the description covers what an agent needs: what is returned, where it comes from, how the rate is derived, and what it costs to call. The remaining gap is the exact response shape and error/empty-result behavior, which a short return-format note would have closed.

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 there is nothing for the description to disambiguate; baseline is 4. It adds useful context by explaining what the fixed output contains (rate date, method, source attribution), which is helpful given no output schema exists.

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?

States a specific resource and scope: the official SEK/HKD reference rate sourced from the ECB fixing, with fallback to the peg where applicable. The currency pair plus derivation methods (direct, cross via EUR, peg-derived) make it unambiguous which pair it returns, though it never names a sibling (fx_convert, fx_latest, fx_timeseries) as an alternative, so it stops short of full sibling 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?

Usage is implied rather than prescribed: an agent can infer this is the tool for a single SEK/HKD reference quote, and the paid-call banner ($0.002 USDC, x402, eip155:8453) gives useful cost/access context for deciding whether to call. There is no explicit when-to-use or when-to-avoid guidance, and no pointer to fx_convert for actual conversion or fx_latest for a broader snapshot.

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