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

With no annotations, the description carries the full burden and does substantial work: it discloses a per-call price and payment rail ($0.002 USDC via x402 on eip155:8453), the upstream source (ECB), the derivation method options (direct, cross via EUR, peg-derived), and asserts determinism with no estimation. It omits operational behavior such as what happens on non-publishing days (weekends/holidays) or whether results are cached, which matters for a rate feed.

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 plus a payment prefix, front-loaded with the price tag and the core rate description. Every clause carries information (cost, source, method, date, attribution), though the nested parentheticals make the middle slightly harder to scan than necessary.

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 indicate the return shape and it does: the rate, the rate date, the derivation method, and the source attribution. Combined with the disclosed payment model, an agent can call this correctly; the only gap is handling of missing fixings on non-publication days.

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 no parameter semantics to explain and the baseline applies. The pair is fully encoded in the tool name, and the description reinforces the fixed currency scope rather than introducing ambiguity.

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 (CAD/SEK) and the exact verb-less retrieval ('latest European Central Bank fixing'), and identifies the source institution, which is the key differentiator among hundreds of fx_ pair siblings. It does not, however, distinguish itself from route-level siblings like fx_convert, fx_latest, or fx_timeseries that also return rates.

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 by the pair-specific name: call this to get the CAD/SEK spot reference rate. There is no statement of when to prefer fx_convert (amount conversion) or fx_timeseries (history), and no exclusions or prerequisites beyond the payment note. An agent must infer routing from the tool name alone.

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