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

With no annotations, the description carries the full burden and does well: it discloses the ECB source and 'reference only' caveat, weekend/holiday fallback to the prior fixing, the output shape (per-target inverse and converted result plus fixing date used), and the paid x402 access with a concrete price. Remaining gaps are minor: no batch-limit or error behavior, and no statement on how failures for unsupported currencies are reported.

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 trigger condition is front-loaded, followed by input/output/source/price in a compact, well-ordered sequence. It is dense and packs a lot into a few sentences, but nearly every clause 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?

There is no output schema and no annotations, yet the description defines the return shape, the fixing date semantics, and the payment requirement, which are exactly the parts an agent cannot infer from the schema. Nothing material is missing for correct invocation.

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%, so the baseline is 3; the description adds marginal value by restating the 1–10 target limit, the amount default of 1, the date format constraint, the 'since 1999-01-04' floor, and an illustrative query string. It does not add much beyond the schema field descriptions, so not a 5.

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 verb and resource ('convert an amount between currencies') and pins the rate source to ECB reference rates, with date scope 'today or on a past date'. This clearly separates it from the sibling fx_rates (raw rates) and fx_history (time series), and gives an example call.

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 opening 'Use when you need to convert an amount between currencies' gives a clear trigger condition plus concrete examples (USD↔KRW, EUR→JPY). It does not explicitly name alternatives such as fx_rates or fx_history or state when not to use it, so it stops short of a 5.

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