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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is covered. The description adds genuinely useful non-structured context: it is free, requires no wallet and no payment, and returns a sample rather than live data. It does not specify what the sample contains, but for a zero-arg preview that is a minor omission.

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 sentence that front-loads the [FREE] marker and moves directly through what it returns, why it exists, and what it costs. No filler or repetition.

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-argument preview tool with full annotation coverage and no output schema, the description supplies everything needed to call it correctly: cost model and the fact that it returns a sample shape. It could be slightly more complete by stating when to graduate to the paid fx_rates, but nothing essential is missing.

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 the baseline is 4. The description reinforces this with 'no arguments needed,' which is consistent with the empty schema and adds no misleading parameter claims.

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 the exact resource (FX Rates, ECB reference rates via Frankfurter) and states precisely what this variant returns: a sample response for the paid fx_rates tool. It is immediately distinguishable from the paid sibling fx_rates and from unrelated siblings like weather_preview by naming the underlying tool it previews.

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?

It gives clear context for use ('so you can see the shape before paying 0.004 USDC') and names fx_rates as the paid counterpart, implying the switch condition. It stops short of an explicit 'call fx_rates when you need real data' exclusion, so it is clear but not fully prescriptive.

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