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

With no annotations, the description carries the full burden and does well: it discloses the paid x402 cost and network (eip155:8453), the upstream source (ECB), and the derivation method variants (direct, cross via EUR, peg-derived), which tells the agent how much to trust the number. It stops short of stating freshness semantics (weekend/holiday staleness), error behavior on unsupported pairs, or response shape.

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 single dense sentence that front-loads the payment metadata, then the resource, then the return contract — every clause adds information (source, method, date, attribution, determinism). The bracketed x402/cost prefix occupies the lead position and slightly delays the tool's actual purpose, but it is legitimately actionable information.

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-output-schema lookup tool, the description covers what an agent needs: what the number is, where it comes from, how it was derived, that it is not an estimate, and what it costs to call. The only remaining ambiguity — staleness on non-fixing days — is a minor edge case for a reference-rate tool.

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 input schema is empty (0 parameters, 100% coverage), so there is nothing for the description to disambiguate at the parameter level. Baseline 4 applies; the currency pair is fully encoded in the tool name and description.

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 a precise resource (HUF/CNY) and states exactly what value is returned: the latest ECB fixing or the official peg, with rate date and derivation method. It is far from tautological. What keeps it from a 5 is that sibling differentiation is only implicit in the currency pair — it never says why an agent should pick this over fx_convert or fx_latest for the same 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 this is for authoritative reference lookups rather than conversions. There is no explicit when-to-use or when-not-to-use guidance and no pointer to fx_convert for arithmetic on the rate or fx_timeseries for historical series, both of which are plausible alternatives among the siblings.

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