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

With no annotations, the description carries the full load and does well: it discloses the metered cost model ($0.001 USDC per call via x402) and identifies the upstream data source, both of which affect whether an agent calls it. It omits rate limits, failure modes, and any caching/refresh cadence, which keeps it short of a 5.

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?

Three tight sentences with purpose, mode, and pricing front-loaded. The example output is slightly long but is doing real work in the absence of an output schema, so it earns its place.

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 no-annotation, no-output-schema tool, this covers purpose, cost, source, and return shape via the example payload. The remaining gaps are minor: no error/empty-result behavior and no explanation of the symbols filtering.

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?

Schema description coverage is 100%, so base/date/symbols are already documented, giving a baseline of 3. The description adds only the implied semantic that omitting date yields the latest rate and that the result is base-relative; it says nothing about the symbols parameter's effect.

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?

Names a specific resource (European Central Bank currency exchange rates) with concrete use cases (price conversion, invoicing, travel budgets) and states the two modes (latest or historical by date). None of the unrelated sibling utilities (ip_lookup, pdf_to_text, etc.) could be confused with it.

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?

'Latest or historical by date' tells the agent how to select the two operational modes, and the use cases give context for when it's appropriate. It stops short of explicit when-not-to-use guidance, but no sibling tool overlaps in function so there is little ambiguity to resolve.

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