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

With no annotations, the description carries the burden well: it discloses the paid x402 model ($0.002 USDC per call on eip155:8453), the authoritative source (ECB), the peg fallback, and that results are deterministic rather than estimated. It omits failure behavior (what happens if payment or the fixing is unavailable) and freshness windows, which keeps 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?

One dense but front-loaded block: cost and payment rail first, then the resource, then the return fields. Every clause contributes, though 'deterministic, sourced, no estimate' slightly overlaps with the earlier 'Official ... reference rate' framing.

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?

Without an output schema, the description usefully enumerates what comes back — the rate, the rate date, the derivation method (direct, cross via EUR, or peg-derived), and source attribution. The one real gap is quoting convention: 'NOK/DKK' does not state which currency is the base, which matters for a rate pair.

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 fixed currency pair already encoded in the name. Baseline 4 applies; no parameter-level value could be added.

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 concrete resource and qualifier — 'Official NOK/DKK reference rate' from the 'latest European Central Bank fixing' — with a specific source and fallback path (official peg). In a sea of identically shaped fx_* siblings, the pair plus the ECB-fixing qualifier is enough to distinguish it from fx_convert, fx_latest, and fx_timeseries without opening a schema.

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: the 'latest' wording signals this is a spot-rate lookup, and 'deterministic, sourced, no estimate' hints it should be preferred over inferred rates. But it never names alternatives — an agent is left to infer that fx_convert handles conversion, fx_timeseries handles history, and fx_dkk_nok is the inverse pair.

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