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

With no annotations, the description carries the full burden and does well: it discloses cost and payment protocol ($0.002 USDC, x402, eip155:8453), the data source (ECB), the peg fallback when CAD is pegged, the method taxonomy, and determinism. It omits payment/auth mechanics and what happens on non-fixing days (weekends/holidays) or on error, which are the remaining behavioral 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?

One dense sentence preceded by the pricing/protocol tag; the pair, source, fallback, and returned fields are all front-loaded with no filler or repetition.

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 and no annotations, the description must describe the return shape, and it does list rate, rate date, method, and attribution. For a zero-parameter lookup this is close to complete; the missing pieces are response format/precision and the payment flow an agent must follow before the call succeeds.

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 zero parameters, so there is nothing for the description to disambiguate beyond the pair, which the name and text already fix. Baseline 4 applies for a parameterless tool.

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?

Names a specific resource (MYR/CAD reference rate) and a specific source (latest ECB fixing, with peg fallback), plus the payload contents (rate date, method, source attribution). It does not explicitly say how it differs from sibling aggregates like fx_latest, fx_convert, or fx_timeseries, so sibling differentiation is only implicit in the pair-specific name.

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?

The phrase 'Deterministic, sourced, no estimate' implies the appropriate context (use when an authoritative, non-estimated rate is required), but there is no explicit when-to-use/when-not framing and no routing to alternatives such as fx_convert for conversions or 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.