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

Annotations only declare readOnlyHint and openWorldHint; the description adds genuinely new behavioral context: this is pay-per-call via x402 USDC on Base, and a missing payment yields a payment challenge in the response rather than a normal result. That failure-mode disclosure is valuable. It omits pricing amount and any rate limits, keeping 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?

Two dense sentences, zero waste: the data scope is front-loaded and the payment caveat follows immediately. Every clause (source, latest + 30-day history, payment mechanism, challenge-on-missing-payment) carries information an agent needs.

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?

With an output schema present, return-value detail is not required, and the description still summarizes the payload (latest + 30-day history). Payment mechanics and the failure path are covered, which is the main risk for this tool. Missing only edge-case behavior such as unsupported currency codes or rate limits.

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% and both parameters ('from'/'to' ISO codes with defaults) are already fully documented in the schema, so the baseline is 3. The description adds only the notion of 'any pair' and the ECB source, not per-parameter semantics such as accepted code formats or behavior when a pair is unsupported.

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 and scope: 'FX rates for any pair (ECB): latest + 30-day history.' An agent immediately knows this returns currency exchange rates plus history, which is clearly distinct from the crypto/macro siblings. It stops short of naming any sibling or boundary, so it is clear but not differentiating-by-comparison.

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 only implied by the domain statement — an agent infers 'call this for FX conversion/rates,' and the ECB source hints at official reference rates rather than market quotes. There is no explicit when-to-use, when-not-to-use, or alternative (e.g. yields for interest rates), so guidance is implicit rather than stated.

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