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

With no annotations supplied, the description carries the full load and does so unusually well: it discloses the x402 payment requirement with exact price and chain (eip155:8453), the upstream source (ECB), the resolution method variants (direct, cross via EUR, peg-derived), and that results are deterministic rather than estimated. What it does not cover is failure behavior (e.g. weekends/holidays when no ECB fixing exists) or the response shape, which matters since there is no output schema.

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 sentences, front-loaded with the payment metadata and then the substantive definition; every clause (source, date, method, attribution) maps to information an agent or downstream consumer needs. Slightly dense in the second sentence, but no padding.

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 zero-parameter, single-value tool this covers what an agent needs to call it and to trust the result: source, method, date stamp, and payment terms. The remaining gap is what happens when no fixing is available and what the payload looks like, since no output schema exists to answer that.

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 no parameters, so the baseline is 4 and there is nothing for the description to disambiguate. The currency pair is fully encoded in the tool name and restated in prose, leaving no ambiguity about direction (AUD base, MXN quote).

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?

Names the exact resource (AUD/MXN) and states it is the official ECB reference fixing, plus the fallback case (official peg). An agent can immediately tell this apart from other fx_* pair tools and from fx_convert/fx_timeseries, which serve different shapes of the same domain.

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 pricing prefix and by 'deterministic, sourced, no estimate', which tells the agent this is a single-pair spot lookup rather than a historical or conversion tool. However, no sibling alternative (fx_convert, fx_latest, fx_timeseries) is named, and no when-not-to-use condition is given.

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