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

With no annotations supplied, the description carries the full burden and does well: it discloses the x402 payment requirement, price, network (eip155:8453), the data source, the direct/cross-via-EUR/peg-derivation method, and that the result is deterministic and sourced rather than estimated. It does not state fixing freshness (ECB publishes once per business day, so weekends/holidays return a stale date) or error behavior when no fixing exists.

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 clauses: a bracketed billing preamble followed by one front-loaded sentence that gives the resource, source, fallback, and returned fields. Every phrase carries information; nothing is redundant.

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, no-output-schema tool, the description covers the return contents (rate date, method, source attribution) and the payment terms an agent needs before calling. The remaining gap is operational: no indication of how often the fixing updates or what a caller sees when the market is closed.

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 no parameters, so there is nothing for the description to disambiguate beyond what it already does by naming the currency pair in the description itself. Baseline 4 applies for a zero-parameter tool.

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?

States a specific verb-plus-resource: the tool returns the official CNY/KRW reference rate, names the exact source (ECB fixing), and specifies the fallback (official peg when KRW is pegged). The pair is unambiguous and distinguishable from the hundreds of sibling fx_*_* tools by name and 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 only implied: 'reference rate' and 'no estimate' signal this is a spot reference lookup, not a forecast or conversion. There is no explicit routing guidance telling an agent to use fx_convert for amounts or other pairs, or to use fx_timeseries for history, even though those siblings exist.

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