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

With no annotations at all, the description carries the full burden and does well: it discloses payment/authentication requirements (x402, USDC, chain id), the upstream source (ECB), the derivation method variants (direct, cross via EUR, peg-derived), and that output includes the rate date and source attribution. What it omits is failure and staleness behavior — e.g. what happens on non-fixing days or when payment is not settled.

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?

One sentence, front-loaded with the payment terms and then the core resource definition; every clause adds information (source, method, attribution, determinism). It is dense but not padded, though the bracketed pricing prefix slightly crowds out the core purpose.

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?

There is no output schema, and the description compensates by enumerating the returned fields (rate, rate date, method, source attribution) plus the guarantee that it is deterministic and not an estimate. For a zero-parameter read tool this is enough for an agent to call it and interpret the result.

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?

The tool takes zero parameters, so the baseline of 4 applies. The description correctly frames the call as parameterless by fixing both currencies in the name, so no additional parameter explanation is needed.

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 resource and scope: the official EUR/AUD reference rate, latest ECB fixing (or peg where applicable). The pair is named explicitly, which separates it from generic siblings like fx_convert, fx_latest, and fx_timeseries without needing to open any 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?

Usage is implied by the pair-specific name and the 'official reference rate' framing, and the cost line gives a useful decision input ($0.002 USDC/call over x402 on eip155:8453). However, it never states when to prefer this over fx_convert, fx_latest, or fx_timeseries, nor any exclusion such as 'not for arbitrary amount conversion'.

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