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 model ($0.002 USDC per call via x402 on eip155:8453), the authoritative source (ECB), the peg fallback, the rate date and method metadata, and guarantees determinism with no estimation. It omits operational traits such as staleness/holiday behavior, caching, or what happens when no fixing is available.

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 preceded by the payment preamble, with the key facts (source, fallback, returned metadata, determinism) front-loaded. The nested parenthetical about pegging and cross-via-EUR method is compact but adds real meaning rather than 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?

No output schema exists, and the description compensates by enumerating the returned components: rate date, derivation method, and source attribution. Remaining gaps are minor for a zero-parameter lookup, though it never states the quote direction convention explicitly (HKD per CHF vs CHF per HKD) beyond the name ordering.

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 4 applies. The description correctly implies the pair is fixed by the tool identity rather than supplied as arguments.

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 resource and direction (HKD/CHF reference rate) and qualifies it precisely as the ECB fixing with peg fallback, which is far more than the bare pair name conveys. It does not, however, differentiate itself from adjacent siblings such as fx_convert (arbitrary amount conversion) or fx_latest, so routing among the hundreds of fx_* tools is left to the pair name.

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: an agent can infer this is the tool for the official HKD-to-CHF reference rate, and the 'no estimate' clause hints it should be preferred over approximations. There is no explicit when-to-use/when-not statement and no mention of fx_convert as the alternative for converting actual amounts, which is the most likely misuse.

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