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 real work: it discloses the paid model ($0.002 USDC per call via x402 on eip155:8453, i.e. a payment/auth prerequisite), the data provenance (ECB), the returned metadata (rate date, derivation method), and the determinism guarantee. It does not cover freshness/caching cadence, rate limits, or what happens on non-publication days (weekends/holidays), 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?

Two tightly packed sentences: the payment/transport token is bracketed up front, then the substantive definition with source, fallback and guarantee. Nothing is redundant and the key scoping information leads.

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 parameterless single-value lookup with no output schema, the description supplies what an agent needs: what the value is, where it comes from, the peg edge case, the returned fields, and the cost/payment requirement. Missing only operational details like publication cadence and failure behavior for non-fixing 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 input schema declares zero properties, so there are no parameters to disambiguate and the baseline of 4 applies. The description correctly implies a no-argument call whose currency pair is fixed by the tool name.

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 (SEK/SGD reference rate) and pins the source (latest ECB fixing, with the peg fallback for SGD), so an agent knows precisely what value it gets back. It does not explicitly contrast itself with siblings such as fx_latest, fx_timeseries or fx_convert, so the differentiation is implied rather than stated.

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 only implied: the word 'reference' and the closing phrase 'deterministic, sourced, no estimate' signal this is for authoritative/official rates rather than estimated or converted quotes. There is no explicit when-to-use, when-not-to-use, or pointer to fx_convert for actual conversion 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.