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

With no annotations, the description carries the full burden, and it does disclose a lot: cost of $0.002 USDC per call, the x402/eip155:8453 payment protocol, ECB as source, the rate date, the derivation method (direct, cross via EUR, or peg-derived), and that it is deterministic with no estimation. What it omits is arguably the most operationally important fact: the quote direction (AED per KRW vs KRW per AED), plus any error behavior or response shape detail beyond listing fields.

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 with the payment cost front-loaded in brackets, then the data source, the fallback rule, the returned metadata, and a determinism guarantee. No filler and no repetition of the tool name beyond identifying the pair.

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-annotation, no-output-schema tool, the description covers cost, provenance, method, and returned fields reasonably well. The critical missing piece is quote direction, which an agent cannot infer and which matters for any downstream arithmetic; the meaning of 'peg-derived' for KRW is also left unexplained.

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?

There are zero parameters, so there is no parameter meaning for the description to add; the baseline for a parameterless tool applies. The schema coverage is 100% by definition, and the description correctly adds no redundant parameter chatter.

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 gives a specific verb and resource: the official AED/KRW reference rate, sourced from the ECB fixing, with fallbacks for the pegged case. An agent knows exactly which pair and which data source it is getting. It does not, however, distinguish itself from siblings such as fx_convert, fx_latest, or fx_timeseries in the massive fx_* family, leaving routing between them to the name alone.

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: 'Deterministic, sourced, no estimate' hints that this is for reference/official-rate needs rather than ad-hoc conversion, but there is no explicit when-to-use or when-not statement. Nothing tells the agent to prefer fx_convert for conversions 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.