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

With no annotations supplied the description carries the full burden, and it does substantial work: it discloses the payment model and price ($0.002 USDC per call, eip155:8453), the data provenance (ECB fixing, or peg-derived), the derivation method (direct, cross via EUR, or peg), and that values are deterministic with no estimation. Missing only operational details such as rate limits or publication timing of the daily fixing.

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?

A bracketed billing/payment tag followed by one dense sentence that front-loads the asset pair and source, then enumerates the returned metadata. No filler or repetition.

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?

There is no output schema, and the description compensates by enumerating the fields a caller receives (rate, rate date, method, source attribution) and by flagging peg fallback. It is nearly complete for a zero-parameter read tool, falling short only on freshness cadence and failure modes such as a missing fixing.

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; the baseline for a no-parameter tool applies. The description instead spends that space explaining what the fixed answer will contain (rate, date, method, attribution).

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 (the EUR/USD reference rate), the producing authority (European Central Bank fixing) and the fallback behavior (official peg when USD is pegged). An agent can identify this specific pair among the hundreds of fx_XXX_YYY siblings purely from the description.

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: the agent learns it is a paid x402 call costing $0.002 USDC, which is decision-relevant, but there is no explicit guidance on when to prefer this over fx_latest, fx_convert, fx_timeseries or the inverse pair fx_usd_eur. No when-not conditions or alternatives are named.

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