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

With no annotations present, 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 data source (ECB), the derivation method (direct, cross via EUR, or peg-derived), the inclusion of a rate date, and that values are sourced rather than estimated. It stops short of failure semantics — what happens on weekends/holidays when no fixing exists, or whether a failed call is still billed.

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, front-loaded with the price/access tag and then the substantive purpose. Every clause adds information (source, fallback, method, date, non-estimated guarantee), though the bracketed payment preamble competes with the purpose statement for the opening position.

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, the description usefully enumerates what comes back (rate, rate date, method, source attribution), which fills the gap left by the absent output schema. Remaining gaps are edge-case behavior — stale-rate handling and failure/charge-on-error semantics — rather than anything needed to invoke it correctly.

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 no parameter meaning to add and the baseline of 4 applies. The description does not waste space restating inputs and instead uses that budget on return-value semantics.

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 value: the official CHF/HKD reference rate, further specified as the latest ECB fixing with fallback to the official peg. Sibling differentiation is carried mostly by the tool name convention (fx_<base>_<quote>) rather than by the description, which never contrasts itself with fx_convert, fx_latest, or fx_timeseries.

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: an agent can infer this is the tool for a single spot CHF/HKD reference quote. There is no explicit when-to-use, no when-not-to-use, and no pointer to fx_convert for conversion arithmetic or fx_timeseries for historical series, even though those siblings are the obvious alternatives.

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