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

With no annotations, the description carries the full burden and does well: it discloses the paid access model ($0.002 USDC/call via x402 on eip155:8453), the authoritative data source, the derivation method variants (direct, cross via EUR, peg-derived), and that values are deterministic rather than estimated. It omits failure behaviour when no fixing exists and any staleness/caching policy, which keeps it from a 5.

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 dense sentence prefixed by a compact payment tag; the purpose clause is front-loaded and every phrase (source, peg fallback, rate date, method, attribution, no estimate) adds information. The x402/eip155 tag is cryptic to an unfamiliar agent, but it is a required cost disclosure.

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, so the description must describe the return payload, and it does: rate value, rate date, derivation method, and source attribution. What remains unexplained is the numeric precision/quote convention and error behaviour for missing fixings, which are minor for a parameterless single-pair lookup.

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 there is nothing for the description to disambiguate and the baseline is 4. The description correctly does not waste space re-explaining inputs and instead spends its words on output semantics.

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 names a specific resource (official AUD/THB reference rate), its source (latest ECB fixing) and the fallback case (official peg when THB is pegged). That is far more precise than a name restatement, but it never contrasts itself with the related siblings an agent might pick instead, such as fx_convert, fx_latest or fx_timeseries, so it falls short of a 5.

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 only implied: an agent infers it should call this when it needs the AUD/THB spot reference rate. There is no explicit 'use this when / don't use this when' and no named alternative (e.g. fx_convert for amount conversion or fx_timeseries for history), leaving routing to inference from the tool name.

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