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

Goes well beyond the readOnly/idempotent/openWorld annotations by disclosing the payment model (x402 USDC on Base, $0.001 per successful call), the behavior when unpaid (returns a PaymentRequired object), the weekend/holiday fallback, and the return fields. This is exactly the behavioral context annotations cannot carry.

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 action ('Use to convert money amounts...') and densely packed but every clause carries information: rate source, date semantics, fallback, return values, payment terms. Slightly long as a single run-on, but no filler sentences.

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 returned values (converted amount, rate used, rate date) and fully covers the paid-call flow. An agent has everything needed to invoke it correctly and interpret the response.

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 already 100%, so the baseline is 3, but the description adds real meaning: the date parameter's ECB-rate semantics, historical range since 1999-01-04, and the previous-working-day fallback. The amount default is left to the schema, so it is not fully additive.

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 (convert) and resource (money amounts between currencies) plus the rate source (ECB reference rate) and scope (rates since 1999-01-04). This clearly separates it from fx_latest and fx_history siblings, which the description implicitly contrasts by defining its own conversion behavior.

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 concrete use cases (quotes, invoices, expense reports, price comparisons) and the fallback rule for weekends/holidays, so an agent knows when this applies. It stops short of explicitly naming when to prefer fx_latest or fx_history instead, leaving some routing to inference.

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