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

With no annotations, the description carries the full burden and does substantial work: it discloses the price and payment protocol ($0.002 USDC/call, x402, eip155:8453), the authoritative source, the fallback behavior when CAD is pegged, and the resolution method (direct, cross via EUR, or peg-derived). It omits failure behavior for non-publishing days (weekends/holidays) and any error semantics, which keeps it below 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?

The cost/protocol tag is front-loaded and clearly bracketed, followed by one dense but readable sentence covering source, fallback, method and return fields. Every clause carries information; no filler. It is slightly over-packed, with method and attribution details competing in the same sentence, but nothing is wasted.

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 partially does (rate date, method, source attribution, plus a determinism guarantee). The critical gap is quote direction: 'THB/CAD' does not establish whether the value is THB per CAD or CAD per THB, and the numeric field itself is never named. For a paid, zero-parameter endpoint this ambiguity forces the agent to infer the semantics of the returned value.

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 there is nothing for the description to disambiguate. Baseline of 4 applies for a zero-parameter definition.

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: it returns the official THB/CAD reference rate, sourced from the ECB's latest fixing, with a peg fallback. The pair is named explicitly and the tool is distinguished from an estimate or conversion tool by the phrase 'reference rate' and 'no estimate'. It does not, however, disambiguate against near-identical siblings such as fx_cad_thb (inverse pair), fx_convert or fx_latest.

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?

There is no explicit when-to-use or when-not-to-use statement, and no sibling is named as an alternative. Usage is only implied by the qualifier 'Deterministic, sourced, no estimate', which suggests it is the right choice when an authoritative reference is required rather than a computed or estimated rate. The agent must infer the rest.

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