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

With zero annotations, the description carries the full burden and does well: it discloses the cost and payment mechanism ($0.002 USDC via x402 on eip155:8453), the authoritative source (ECB fixing), the fallback behavior for pegged XOF, and asserts determinism/no estimation. What it omits is timing behavior — how often the fixing updates, whether the endpoint errors outside publication windows, or what a stale/pegged response looks like.

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 tightly packed sentences with no filler: the pricing/access tag first, then the substantive function. Every clause adds information (source, fallback, returned metadata, determinism) and the key promise is front-loaded.

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?

With no output schema, the description steps in usefully by enumerating what comes back — rate date, derivation method (direct, EUR cross, or peg-derived), and source attribution. That is nearly complete for a parameterless read tool; only the freshness/latency contract and error behavior are 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?

The tool takes no parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The description correctly frames a call as taking only a payment, not any currency or date arguments.

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 (official THB/XOF reference rate) and its provenance (ECB fixing, or official peg when XOF is pegged), which is far more informative than the name alone. It does not, however, distinguish itself from sibling aggregation tools like fx_latest, fx_convert, or fx_timeseries, so an agent must infer that this is the single official spot reference rather than a conversion or historical series.

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: the words 'reference', 'deterministic', and 'no estimate' signal this is for authoritative quoting/accounting rather than ad-hoc conversion, but no alternative is named. There is no explicit 'use fx_convert to convert amounts' or 'use fx_timeseries for history' routing, and no statement of freshness/cadence limits (e.g., daily ECB publication) that an agent would need to decide if the rate is current enough.

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