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

With no annotations supplied, the description carries the full burden and does well: it discloses the paid x402 cost ($0.002 USDC on eip155:8453), the upstream source (ECB), the derivation method (direct, cross via EUR, or peg-derived) and the determinism guarantee. It omits failure behavior, caching/freshness windows and rate limits, keeping it short of 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?

A single dense sentence with the pricing/transport tag bracketed up front, followed by what is returned. Every clause earns its place, though the parenthetical '(or the official peg when CAD is pegged)' and the method list make it slightly packed for one sentence.

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 lookup, the description covers exactly what an agent needs: what rate, from which source, with what method and attribution returned, plus the payment model. Missing only freshness/latency expectations and error conditions.

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?

There are zero parameters and the schema is empty, so the baseline is 4. The description correctly implies no input is needed — the currency pair is fixed by the tool identity — and nothing about invocation is left ambiguous.

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 (SAR/CAD reference rate) and its provenance (latest ECB fixing, or peg when pegged), which makes the pair explicit beyond the tool name. It does not, however, position itself against related siblings such as fx_convert, fx_latest or fx_timeseries, so the agent has to infer the distinction from the name alone.

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: 'reference rate', 'deterministic, sourced, no estimate' signals this is for authoritative/audit-grade rates, not ad-hoc conversion. There is no explicit when-to-use or when-to-prefer-fx_convert guidance, and no mention of whether historical rates require fx_timeseries.

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