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

Goes meaningfully beyond the annotations: it discloses the data provenance (ECB reference publication), the returned fields (as_of date, source attribution), and critically that the call costs $0.001 USDC on Base via x402 — payment requirements are not derivable from readOnlyHint/openWorldHint. It does not cover rate limits, caching, or behavior on unsupported currency codes.

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?

Four dense sentences, front-loaded with what the tool returns, then filtering, return fields, and cost. No filler; each sentence contributes distinct information.

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 2-parameter, zero-required, read-only tool with no output schema, the description covers purpose, filtering, return fields, provenance, and cost — essentially everything needed to invoke it. Minor gaps remain around invalid currency codes and whether the ~30-currency list is discoverable.

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 coverage is 100% and already documents base (default EUR) and symbols (default all), so the schema carries most of the load. The description adds the 'any base currency' scoping and the USD→JPY example that clarifies the direction of conversion, which is a modest but real supplement.

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 ('Today's exchange rates, most recent ECB publication') with scope (~30 currencies, any base currency) and example pairs. The 'Today's/latest' framing implicitly separates it from rates_historical and rates_timeseries, though it never names those 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?

'Filter with symbols' hints at the filtering use case, and 'e.g. USD to JPY' shows a typical call, but there is no explicit when-to-use-this-vs-alternatives guidance despite three closely related sibling tools (convert, rates_historical, rates_timeseries). The agent must infer that this is the spot/latest path.

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