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

With no annotations, the description carries the full behavioral burden. It does valuable work by disclosing the paid x402 cost ($0.002 USDC/call on eip155:8453) and by explaining the rate method (direct, cross via EUR, or peg-derived) plus source attribution. However, it says nothing about freshness/lag of the fixing, what happens for non-trading days, or error behavior — gaps that matter for a paid call with zero annotation coverage.

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/cost marker. The price and protocol come first, then the resource, then the qualifiers — front-loaded and 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, no-output-schema tool, the description covers what is returned (rate, date, method, source) and the payment requirement. It stops short of stating update cadence or the response shape, which an agent calling a paid endpoint would reasonably want.

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; the baseline for a parameterless tool applies.

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 states a specific resource (PHP/XOF reference rate) and source (latest ECB fixing, or the official peg when XOF is pegged), which is far beyond restating the name. It doesn't explicitly contrast with siblings like fx_convert or fx_latest, but the currency-pair naming makes the target unambiguous.

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: the ECB-fixing and 'no estimate' framing signals this is for official reference rates, not ad-hoc conversion. There is no explicit when-to-use/when-not guidance and no pointer to alternatives such as fx_convert for arithmetic conversion or fx_timeseries for history.

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