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

With no annotations, the description carries the full burden and does reasonably well: it discloses the paid x402 cost and network, the ECB as authoritative source, that a peg is used when NOK is pegged, and that results are deterministic rather than estimated. It stops short of disclosing freshness guarantees, caching, or error behavior for unsupported dates.

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 plus a bracketed payment/transport tag. The core claim (which rate, from which source, with which metadata) is front-loaded, and every clause — method, date, attribution, no estimate — carries information. Slightly dense but free of 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, no-output-schema tool, the description tells the agent what comes back in conceptual terms: the rate, its date, the derivation method and the source. The remaining gap is the unresolved quote direction and the absence of any return-shape hint, so not fully complete.

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 is 4, but the description adds nothing about how the single output pair is oriented. 'PLN/NOK' leaves the quotation direction (PLN per NOK vs NOK per PLN) genuinely ambiguous, which is exactly the kind of semantics this description could have resolved. Nudged down from baseline for that omission.

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 states a concrete verb and resource: it returns the official PLN/NOK reference rate, and adds that the value is the ECB fixing with date, derivation method and source. That is far more specific than the name alone. It does not, however, distinguish itself from related siblings such as fx_convert, fx_latest or fx_timeseries, which matters in a namespace of hundreds of pair-specific tools.

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: an agent can infer this is the tool for the PLN/NOK spot reference rate, and the pricing prefix signals a paid call. There is no explicit guidance on when to prefer it over fx_convert (arbitrary conversion) or fx_timeseries (historical series), and no stated prerequisites beyond the payment note.

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