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

With no annotations, the description carries the behavioral burden and discloses important traits: the tool is paid ($0.002 USDC/call), uses x402 on eip155:8453, and returns a full rate table in one call. It does not cover rate limits, error behavior, or response structure in detail, but the payment and output-scope context are substantial beyond the schema.

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?

The description is compact and front-loads the payment metadata before the core purpose. Every phrase contributes useful information: cost, protocol, chain, scope, supported pegged currencies, and the one-call full-table behavior. There is no redundant or filler text.

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 low-complexity, three-parameter rate retrieval tool with full schema coverage, the description is mostly complete. It covers payment requirements, base/date scope, pegged currencies, and the full-table output, though it does not describe the exact response format even though no output schema exists.

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 the baseline is 3. The description references the key concepts behind the parameters—'any base currency,' 'latest or a given fixing date,' and 'full rate table'—and adds that pegged currencies are included, but it does not add syntax or filtering details beyond what the schema already provides.

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 clearly states the resource and scope: all official ECB reference rates for a base currency on the latest or a given fixing date. It also notes that pegged currencies are included and that one call returns the full rate table. It does not explicitly name sibling alternatives like fx_convert or fx_timeseries, so it is clear but lacks sibling differentiation.

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 implied through phrases like 'latest or a given fixing date' and 'One call, full rate table,' which suggest retrieving a complete rate snapshot rather than performing a conversion. However, there is no explicit when-to-use guidance, no exclusions, and no named alternative tools for cases such as conversion or time-series requests.

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