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

With no annotations, the description carries the burden of behavioral disclosure. It clearly reveals the paid nature and the mandatory x402 payment flow, explaining what the first call returns (amount, asset, network) and how settlement works. It does not mention update frequency or result format, but the critical payment behavior is transparent.

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 exactly two sentences, each earning its place: the first states the tool's purpose, the second explains the payment workflow and points to the free alternative. There is no filler or redundancy.

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 simple currency-rate tool with a nontrivial payment workflow, the description adequately covers purpose, usage flow, and alternative tool. It does not specify the exact return schema, but no output schema is provided and the data type (a reference rate) is straightforward.

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 input schema already documents all three parameters at 100% coverage. The description adds significant value by explaining the x_payment parameter's dynamic role: omitting it returns terms, including it settles and returns data. This goes beyond the schema's static type description.

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 opens with a specific and identifiable resource: 'ECB reference exchange rate for a currency pair,' including a concrete example (EUR/USD). It distinguishes itself from the sibling `pricing` tool by noting the paid nature and pointing to the free alternative.

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?

The description explicitly explains the two-step paid workflow: call without x_payment to receive terms, sign, then call again with x_payment. It also gives a usage pointer to `pricing` for a broader listing, though it doesn't explicitly enumerate all cases where `pricing` should be preferred.

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