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

With zero annotations the description carries the full burden and does substantial work: it discloses per-call pricing ($0.002 USDC), the x402 payment protocol and chain (eip155:8453), the upstream data source, the peg fallback path, and determinism guarantees. It does not describe auth requirements, caching/freshness timing for the ECB fixing, or failure behavior, leaving some operational gaps.

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?

A single dense sentence pair with the payment banner first and the core purpose immediately after — every clause adds information (source, fallback, returned fields). The bracketed payment prefix is slightly noisy for selection purposes, but it is genuinely decision-relevant information, not filler.

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, no-output-schema tool, the description covers what matters: the exact pair, the authoritative source, the peg fallback, and the fields returned (rate, date, method, attribution). The only meaningful omission is freshness/cadence of the ECB fixing and any error or payment-failure behavior.

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 clarify beyond the fact that the currency pair is fixed by the tool name. Baseline 4 applies for a zero-parameter tool; the description implicitly confirms no input is required.

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 the exact resource — the official XOF/GBP reference rate — and specifies the source (European Central Bank fixing) plus the peg fallback, which pins down the tool's identity precisely. With hundreds of fx_<base>_<quote> siblings, the pair-specific wording does most of the disambiguation work, though it never explicitly contrasts with fx_convert, fx_latest, or fx_timeseries.

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 rather than stated: 'deterministic, sourced, no estimate' signals this is for authoritative spot reference rates rather than conversions or historical series, but no alternative tool is named and there is no explicit when-not guidance. An agent must infer that fx_convert is for amount conversion and fx_timeseries for history.

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