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

With no annotations, the description carries the burden and does substantial work: it discloses the per-call price and payment rail ($0.002 USDC via x402 on eip155:8453), the upstream source (ECB), and the derivation method (direct, cross via EUR, or peg-derived). Gaps remain on refresh cadence, behavior on weekends/holidays when no ECB fixing exists, and caching, so not 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?

Front-loaded and compact: cost/rail banner, then the substantive sentence naming pair, source, vintage, method and attribution. Slight clutter from the bracketed payment prefix and the trailing 'no estimate' fragment, but nothing is redundant.

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 stand in for return-value shape; it partially does by promising rate date, method, and source attribution. However, it never states the quoting convention (units of HKD per XOF versus the inverse) or any precision/rounding, which is the single most consequential ambiguity for an FX rate call.

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?

Zero parameters, so the schema imposes nothing to explain and the baseline of 4 applies. The description implicitly fixes the pair and source rather than exposing options, which is coherent with an empty schema.

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 and verb-equivalent ('Official XOF/HKD reference rate') and pins down the source and vintage ('latest European Central Bank fixing', 'or the official peg when HKD is pegged'). An agent can tell exactly which pair and which data source this returns, distinguishing it from the generic fx_convert/fx_latest siblings by name and scope.

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 the authoritative rate rather than a computed or aggregated one, but the description never names fx_convert, fx_timeseries, or fx_latest as alternatives, nor says when a direct pair should be preferred over a cross. An agent must infer the routing decision.

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