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

With no annotations, the description carries the full burden and does meaningful work: it discloses the paid model ($0.002 USDC/call via x402 on eip155:8453), the data provenance (ECB fixing, official peg fallback), and the fidelity guarantee (deterministic, sourced, no estimate). It does not disclose failure behavior when no fixing is available, rate direction/quoting convention, or precision — gaps that matter for an unpriced read.

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?

One sentence of substance plus a bracketed payment prefix; nothing is padded or repeated. The payment metadata is front-loaded ahead of the actual purpose, which slightly delays the core answer but is defensible since it warns of a billable call before the agent commits.

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, and the description partially compensates by naming the returned fields (rate date, method, source attribution, rate value). However, for a zero-parameter FX tool the quoting convention — whether the number means PLN per EUR or EUR per PLN — is never stated, and neither is precision or the behavior on a stale/absent fixing, which leaves real ambiguity for an agent that must use the value arithmetically.

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 for the description to disambiguate and the baseline is 4. The description correctly adds no parameter chatter and instead spends its words on source and output content.

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 (PLN/EUR reference rate) and a specific source (latest European Central Bank fixing, or the official peg when EUR is pegged), plus the exact fields returned: rate date, method, source attribution. Against a sibling list of hundreds of fx_* pairs, this reads unmistakably as the PLN/EUR rate rather than a conversion, timeseries, or generic lookup tool.

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?

Use is only implied: an agent can infer it should call this when it needs the official PLN/EUR rate. There is no explicit when-to-use framing and no mention of alternatives such as fx_convert, fx_latest, or fx_timeseries, nor any exclusion such as 'do not use for historical dates' or 'use fx_convert for amounts'.

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