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

Annotations already declare readOnly/idempotent/openWorld/no-destructive, but the description goes well beyond them: the weekend/holiday fallback (answers with the previous business day and says so), exactly what is returned (amount, per-unit rate, effective rate date), and the payment/auth model ($0.002/call via x402 on Base, x-credit-token header, 1 free call/day, no API key). This is unusually rich behavioral disclosure.

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?

Front-loaded with the core action in the first clause, then layered with caveats and cost. Every sentence carries information, though the pricing/auth sentence is dense and could be trimmed without losing meaning.

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 paid, single-purpose conversion tool this is complete: inputs, output meaning, effective-date behavior, cost, and authentication are all covered. An output schema exists, yet the description usefully pre-empts the weekend-date surprise, so nothing needed to call it correctly is missing.

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 schema already documents all four parameters; baseline is 3. The description adds only marginal semantics beyond the schema (date may be today or any past date, "Required inputs: from, to"), and it omits the schema's note that `to` accepts comma-separated currencies.

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 30+ currencies at the ECB daily reference rate — with scope details (today or any past date). It is clearly distinguishable from siblings like unit_convert or token_price by anchoring to ECB reference rates for accounting/reporting.

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?

Gives a clear use case (accounting and reporting) and an explicit when-not ("not tradable quotes"), plus pricing/free-tier conditions that affect whether to call it. It does not, however, name a sibling alternative such as unit_convert for non-currency conversions.

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