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

With no annotations, the description carries the full behavioral burden and does well: it discloses the paid model ($0.002 USDC per call via x402 on eip155:8453), the underlying data pipeline (ECB fixing, with a documented peg fallback), and the guarantees (deterministic, sourced, no estimate). It does not cover failure modes, staleness limits, or whether a rate can be missing for a given date, which keeps it off a 5.

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?

It is a single dense sentence with the cost/payment metadata front-loaded in brackets, followed immediately by the substantive purpose. Every clause carries information (source, fallback behavior, output fields, determinism), though the bracketed payment prefix competes for attention with the purpose statement.

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 adequately enumerates the returned fields (rate, rate date, method, source attribution) and the payment prerequisite. What remains thin is operational detail such as error behavior when no fixing exists for a date or how the peg-derived case is signaled in the response.

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 no parameter semantics to convey and the baseline of 4 applies. The description correctly adds no spurious input discussion, though it also does not clarify whether optional date selection exists elsewhere (e.g., fx_timeseries).

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 (AED/SEK), the authoritative source (European Central Bank fixing, or the official peg when applicable), and what is returned (rate, rate date, derivation method, source attribution). An agent can tell instantly this is the EUR-cross-referenced AED→SEK rate tool rather than a converter, time series, or other 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 rather than stated: 'Deterministic, sourced, no estimate' signals when to prefer it over an estimate, but the description never names alternatives (e.g., fx_convert, fx_latest, fx_timeseries) or states conditions such as 'use for a single official fixing, not for conversion math.' An agent must infer the use case from the pair name alone.

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