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 real behavior: a per-call price of $0.002 USDC over x402 on eip155:8453, the ECB fixing as source, a peg-derived fallback for CVE, and the guarantee of no estimation. It stops short of stating freshness/lag, caching, or rate limits, and does not describe the response envelope beyond naming the 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?

Cost and protocol are bracketed and front-loaded, then the purpose and output fields follow in one dense but well-ordered sentence. Every clause (rate date, method, attribution, no estimate) adds information; only the phrase 'Deterministic, sourced, no estimate' is slightly redundant with the preceding source attribution.

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 covers source, fallback semantics, returned fields, and the payment requirement, which is enough for correct invocation. Missing only secondary operational details such as update frequency and error behavior.

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 the baseline is 4; nothing needs explaining. The description usefully spends its space on what the zero-arg call returns instead of inventing parameter guidance.

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 the exact resource ('Official AED/CVE reference rate') and the underlying source ('latest European Central Bank fixing', with peg fallback), so the agent knows precisely what value comes back. It does not explicitly contrast itself with fx_convert, fx_latest, or fx_timeseries, so the disambiguation relies on the tool name and the pair-specific description rather than an explicit statement.

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 that needs a spot AED/CVE rate can infer this is the right tool. There is no explicit guidance on when to prefer fx_convert (conversion math), fx_timeseries (historical series), or fx_latest (multi-pair snapshot), which matters given hundreds of near-identical siblings.

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