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

With no annotations at all, the description carries the full burden and does real work: it discloses the paid x402/USDC cost and network (eip155:8453), the ECB-fixing derivation, the peg fallback, the returned metadata fields, and the determinism/no-estimate guarantee. What it omits is failure and freshness behavior — what happens when no fixing exists for today, or how stale the returned date can be.

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 bracketed payment tag, with cost/billing front-loaded before the resource description. Every clause is load-bearing, but the nested parenthetical ('or the official peg when ILS is pegged') makes it slightly harder to scan than a two-sentence split would.

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?

No output schema exists, and the description compensates by enumerating the fields returned and the derivation method. The notable remaining gap is quote direction — whether the rate is DKK per ILS or ILS per DKK — which matters for any FX consumer, plus the absence of any error/staleness contract.

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?

Zero input parameters, so there is nothing for the schema to document and the baseline applies. The description instead spends its words on return-side semantics (rate date, method, source attribution), which is the useful place to put detail for a no-arg 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 the exact resource (DKK/ILS reference rate), the authority (European Central Bank fixing), and the fallback semantics (official peg when ILS is pegged), plus what the response carries: rate date, method, source attribution. Against hundreds of sibling fx_* pair tools, the currency pair plus the 'official ECB reference rate, not an estimate' framing is enough to identify it unambiguously.

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: an agent can infer it should call this when it needs a DKK/ILS reference rate. There is no explicit routing guidance versus siblings such as fx_convert, fx_latest, or fx_timeseries, and no statement about when-not-to-use (e.g., historical series or arbitrary-amount conversion), which is the main ambiguity in a toolset this large.

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