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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint. The description adds substantial context beyond that: ECB as the source, publication cadence around 16:00 CET on working days, ~30 currencies, rate-date freshness, and the paid access model with x402 USDC on Base at $0.001 per successful call, including the unpaid-call PaymentRequired behavior.

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 front-loaded with the primary use case and then layers on freshness, scope, intended uses, and payment details in a single tight paragraph. Some of the use-case enumeration ('for pricing, reporting, invoicing and conversions') is slightly redundant but still earns its place for routing.

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, read-only rate retrieval tool with full annotation coverage and 100% schema description coverage, the description supplies everything an agent needs: data source, update frequency, scope, filtering behavior, rate-date freshness, cost, and payment protocol. No output schema exists, but the description adequately conveys the nature of the returned rate information.

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%, so the schema already documents both parameters. The description adds only conceptual framing ('re-based to any supported currency', 'optionally filtered to the symbols you need') without new syntax or format details, which matches the baseline 3 for high-coverage schemas.

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 ('returns') and resource ('latest European Central Bank reference rates') with scope and freshness. It does not explicitly differentiate itself from sibling tools like fx_convert or fx_history by name, though the word 'latest' and the rate-focused framing provide implicit distinction.

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?

Provides a clear when-to-use condition ('when you need a current, citable exchange rate') and a when-not-to-use exclusion ('Not for trading'). It does not name alternative tools for conversion or historical rates, so sibling routing is left to inference.

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