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 price ($0.002 USDC), the payment protocol (x402) and network (eip155:8453), states the data is deterministic and sourced rather than estimated, and explains that the method (direct, cross via EUR, peg-derived) and rate date accompany the value. It stops short of describing failure modes, e.g. what happens outside ECB fixing days or when a peg changes.

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 compact sentences, with the payment tag front-loaded so an agent sees the cost before invoking, then the substantive description. The parenthetical 'direct, cross via EUR or peg-derived' is dense but informative; nothing is redundant.

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 what an agent needs: what value comes back, its source, its dating, its derivation method, and its price. The remaining gap is defensive behavior (rate staleness, market-holiday handling) and disambiguation from the sibling conversion/history tools.

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 no parameters, so the baseline is 4. The description compensates usefully by saying what the returned payload conveys (rate date, derivation method, source attribution), which is the only semantic context available given the empty schema and absent output schema.

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 a specific resource and scope: 'Official AED/CHF reference rate: latest European Central Bank fixing', plus the peg fallback. An agent can tell exactly which pair and which source it gets, but it never differentiates itself from sibling entry points like fx_convert, fx_latest, or fx_timeseries.

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 by the pair name and 'latest ECB fixing' – there is no explicit when-to-use statement and no routing guidance versus fx_convert (for arbitrary conversion) or fx_timeseries (for history). The cost note tells the agent it is a paid call but not when that call is the right one.

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