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

With no annotations, the description carries the full burden and does meaningful work: it discloses the paid x402 model, price ($0.002 USDC/call), network (eip155:8453), and the shape of the response. It does not say what happens for weekends/holidays or unsupported currency pairs, which for a rate lookup is the main remaining behavioral gap.

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?

Two compact clauses with the payment/auth requirement front-loaded, then purpose, scope and return values. No filler; every clause carries distinct information.

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?

No output schema exists, and the description compensates by naming the returned fields (rate, amount, rate date). Combined with full schema coverage and the pricing disclosure, an agent has enough to call it correctly; only edge-case behavior (missing fixing dates, unsupported pairs) is unstated.

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 from/to/date/amount including defaults. The description adds only the outer date bound (1999) and mentions CFA franc and Gulf pegs, which hints at currency coverage beyond the ISO examples but does not define parameter behavior beyond the schema.

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 two currencies at the official ECB reference rate.' The scope ('today or any date since 1999') and the return payload ('rate, amount, rate date') let an agent distinguish it from fx_latest (single latest rate) and fx_timeseries without opening a schema.

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 date-range scope implies when this tool applies versus fx_latest and fx_timeseries, but no alternative is named and there is no explicit when-not guidance. An agent must infer the routing from the sibling names alone.

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