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

With no annotations, the description usefully discloses the exact per-call cost, x402 payment requirement and network, data source, peg fallback, calculation methods, determinism, and source attribution. It does not cover failure behavior, publication delays, or rate limits, but it provides substantial behavioral context for a simple paid read operation.

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 but efficient sentences front-load the payment requirement and then cover purpose, source, fallback behavior, returned metadata, and determinism. Each clause contributes actionable information without repetition or filler.

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 without an output schema, the description supplies nearly all essential selection and invocation context, including cost and the accompanying rate metadata. It could be more complete by stating the quotation convention explicitly, such as CVE per 1 SAR, and by clarifying freshness or routing to generic FX alternatives.

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 has no input parameters, so there is no parameter-documentation burden and the baseline score of 4 applies. The pair, rate date, method, and attribution details still clarify what the invocation concerns and what information accompanies the rate.

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 clearly identifies the resource as the official SAR/CVE reference rate and scopes it to the latest ECB fixing or an official peg-derived value. The exact pair distinguishes it from other pair-specific tools, but it does not explicitly differentiate itself from generic siblings such as fx_latest or fx_convert.

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?

Phrases such as "Official," "latest," and "no estimate" establish a clear use case: obtaining a current, authoritative reference rate rather than an estimated value. It does not state when another tool should be preferred for conversions, historical series, or other rate types.

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