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

With no annotations at all, the description carries the full behavioral burden and does so well: it discloses the paid access model ($0.002 USDC/call via x402 on eip155:8453), the authoritative source (ECB), the resolution method (direct, cross via EUR, or peg-derived), and the guarantee that values are deterministic and never estimated. It also enumerates what accompanies the rate (rate date, method, source attribution), which is exactly the provenance context an agent needs to trust or cite the number.

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?

Two sentences, roughly fifty words, with the cost/payment qualifier bracketed up front and the substantive definition immediately after. Every clause (source, fallback-to-peg, method, attribution, no estimate) adds distinct information; nothing is padding.

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-annotations, no-output-schema tool, the description covers source, fallback behavior, accompanying metadata, and payment terms — enough to call it confidently. The only gap is numeric format/precision of the returned rate, which is minor for a reference-rate lookup.

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 and the baseline is 4. It correctly avoids restating input fields and instead spends its words on the response 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?

Names the exact resource (AED/QAR) and the exact artifact returned (official ECB reference fixing, or the official peg when QAR is pegged). The pair is embedded in the name and restated in prose, so an agent can distinguish it from the hundreds of fx_<base>_<quote> siblings and from aggregate tools like fx_latest or fx_convert without opening a schema.

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: 'official ECB reference rate' and 'no estimate' signal this is for authoritative/reference-rate needs rather than spot conversion. However, it never names alternatives (fx_convert for amount conversion, fx_latest for a bundle, fx_timeseries for history) or says when NOT to use it, leaving routing to inference.

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