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

With no annotations, the description carries the full load and does disclose the payment model ($0.002 USDC/call via x402 on eip155:8453), determinism, non-estimated sourcing, and the alternate peg path — behavior an agent needs before calling. It omits freshness cadence, stale-rate behavior, and error modes, which keeps it below 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?

Two compact sentences with the cost/payment tag front-loaded and the substantive definition following; every clause contributes, though the bracketed payment prefix is dense.

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-param lookup with no annotations and no output schema, the description covers price, protocol, source, fallback logic, and the returned metadata fields. Only update frequency and failure behavior are missing.

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 the baseline applies; the description correctly leaves out argument discussion and instead describes what the response conveys (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 and direction ('Official SEK/CNY reference rate') plus its provenance ('latest European Central Bank fixing'), so an agent can distinguish it from the hundreds of other fx_* pair tools without opening a schema.

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 the pair and the ECB source, and the peg fallback condition is mentioned, but the description never says when to prefer this over fx_convert, fx_latest, or fx_timeseries, nor does it state exclusions or prerequisites beyond payment.

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