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

Annotations already cover read-only/idempotent/open-world, and the description adds substantial context beyond them: the ECB reference-rate source, the weekend/holiday fallback rule, the 1999-01-04 rate floor, the returned fields (amount, rate, rate date), and critically the paid x402 USDC pricing model plus the 'call without payment to receive PaymentRequired' behavior. That last detail is exactly the kind of non-obvious operational trait an agent needs.

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 primary sentence is front-loaded with purpose and use cases, and the payment caveat is separated into its own sentence where it belongs. The first sentence is long and clause-heavy, but nearly every clause carries real information, so little is wasted.

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 no output schema, the description compensates by naming the three returned values. Combined with rate-source, date-fallback, pricing, and error-path details, an agent has everything needed to invoke it correctly and handle the payment-required case.

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 a baseline of 3 is warranted. The description earns above baseline by adding semantics the schema lacks: that 'date' falls back to the previous working day on weekends/holidays and that rates only extend back to 1999-01-04, bounding valid input values.

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?

The description opens with a specific verb and resource ('convert money amounts') and immediately scopes it to concrete uses (quotes, invoices, expense reports, price comparisons). This clearly separates it from siblings like fx_latest and fx_history, which return rates rather than converted amounts.

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?

It states the situations the tool serves and clarifies the latest-vs-historical date behavior, which is the main selection decision here. It does not explicitly name fx_latest or fx_history as the alternatives for when you only want a rate, so routing guidance is implicit rather than complete.

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