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 well: it discloses the price and payment protocol ('$0.002 USDC/call, x402, eip155:8453'), the authoritative source (ECB fixing), the peg-fallback behavior for pegged currencies, and the determinism guarantee. It stops short of stating update cadence, behavior on non-publication days, or what happens if payment is missing.

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 sentences, front-loaded with the payment model, then the substance of the response. It is dense but every clause (rate date, method, attribution) contributes; only the parenthetical method list is slightly compressed for readability.

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 what comes back: rate date, derivation method (direct / cross via EUR / peg-derived) and source attribution. Combined with the disclosed cost and source, an agent has enough to call it correctly; only freshness/latency expectations are left unstated.

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; the baseline of 4 applies. The description correctly implies the pair and data source are fixed at the endpoint level.

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 and scope: 'Official NOK/IDR reference rate' from the European Central Bank, and rules out the reverse-sibling ambiguity by fixing the pair direction in the name and text. It also states the three possible derivation methods, so an agent knows exactly which datum this returns versus the hundreds of other fx_* pair tools.

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?

There is no explicit when-to-use / when-not-to-use statement and no named alternative (e.g. fx_convert for conversions, fx_latest for a multi-pair snapshot). The tagline 'Deterministic, sourced, no estimate' implicitly positions it for audit-grade needs over estimated rates, but the agent must infer that routing without help.

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