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

With no annotations, the description carries the full burden. It discloses the paid nature, the need for x_payment, and the exact flow. It doesn't describe return format or error behavior, but the payment mechanism is the critical non-obvious behavior and is well explained.

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?

Two sentences with dense, relevant information. No filler. The sentence about the payment flow is necessary and well-structured, and the mention of `pricing` is directly useful.

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?

No output schema or annotations, so the description must be self-sufficient. It covers the tool's purpose and the unusual payment interaction. It could mention response format or error cases, but for a narrow FX rate lookup with a well-known data source (ECB), this is adequate and reasonably complete.

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 already describes base/quote and x_payment, but the description adds essential semantics for x_payment: it's an x402 payment payload, base64, omitting it triggers a terms request, and calling again with it settles and returns data. This goes beyond schema and is crucial for correct usage.

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?

Clearly describes a specific resource (ECB reference exchange rate for a currency pair) with an example (EUR/USD). Distinguishes itself from the sibling `pricing` tool by noting it's paid and lists a single pair, while pricing lists every price at once.

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?

Explicitly explains the two-step payment flow: call without x_payment to get terms, sign, then call again with x_payment. Also provides an alternative: use the free `pricing` tool to list all prices at once, giving clear when-to-use guidance.

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