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

With no annotations at all, the description carries the full burden and does well: it discloses the paid x402 model with exact price and network (eip155:8453), the fact that results are deterministic and sourced rather than estimated, and that peg-derived rates are handled. It omits failure modes (stale fixing, non-trading days) and response shape.

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 dense sentences with the payment/network gating info front-loaded, then what the rate is and why it is trustworthy. No filler, though the bracketed pricing prefix is slightly awkwardly separated from the rest.

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 quote tool, the description covers cost, source authority, derivation methods and the fields returned. Only staleness/failure behavior is left unstated, which is a minor gap for a deterministic ECB-fixing 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; baseline 4 applies. The description usefully explains the returned attributes (rate date, derivation method, source attribution) even though it could not have documented parameters.

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 ('Official AED/EUR reference rate') and a specific provenance ('latest European Central Bank fixing'), which is far more than a restatement of the name. It does not, however, differentiate itself from the hundreds of sibling pair tools (fx_eur_aed, fx_convert, fx_latest) beyond the implied currency pair.

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 by 'official reference rate' and 'Deterministic, sourced, no estimate' — an agent can infer this is for authoritative, non-estimated AED→EUR rates. There is no explicit when-to-use versus fx_convert (conversion) or fx_latest (multi-pair snapshot), and no exclusions or freshness caveats.

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