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

With no annotations, the description carries the full burden and does substantial work: it discloses the price ($0.002 USDC/call), the payment protocol and chain (x402, eip155:8453), the data source (ECB), peg handling, the rate date, the derivation method, and that values are deterministic rather than estimates. It stops short of explaining the actual payment handshake or what happens on non-fixing days (weekends/holidays), which would matter for a paid call.

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/x402 tag front-loaded so the agent sees the paid nature before anything else. Dense but every clause adds information; only the repeated 'Official… reference rate' phrasing is slightly redundant with the data-source clause.

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, but the description enumerates the meaningful return contents (rate date, method, source attribution), which is the key thing an agent needs to trust and cite the number. It is nearly complete for a single-value lookup; the remaining gap is response field naming and concrete payment mechanics.

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 the schema could document and nothing the description needs to compensate for. Baseline 4 applies; the description correctly adds no redundant parameter talk.

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 resource (the PLN/CNY reference rate) and pins down its provenance (latest ECB fixing, or the official peg when CNY is pegged). Combined with the fx_pln_cny naming convention, an agent can tell exactly which pair and which source this returns, versus the hundreds of other fx_*_* siblings.

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: the agent must infer it should call this when it needs a PLN-to-CNY rate rather than fx_convert, fx_latest or fx_timeseries. No explicit when-to-use, when-not-to-use, or routing to the generic conversion tool for amounts is given.

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