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

With no annotations supplied, the description carries the full disclosure burden and does substantial work: it reveals the payment scheme and price, the upstream authority (ECB), the deterministic/no-estimate guarantee, the peg fallback, and what accompanies the rate (rate date, derivation method, source attribution). It stops short of stating update cadence, whether the fixing is a mid rate, and what happens on non-publication days.

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?

A single dense sentence plus a bracketed payment prefix; nothing is padded and the core resource statement is front-loaded after the cost tag. The clause about peg handling could arguably be split out for scannability, but no sentence is wasted.

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 zero-parameter tool with no output schema, the description adequately tells the agent what comes back: a rate plus rate date, derivation method, and source attribution. It lacks concrete field names and format, but the conceptual return contents are covered, which is what matters when no output schema exists.

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 for the description to add beyond the input schema, which is trivially complete at 100% coverage. Baseline for a parameterless tool applies.

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 precise resource, the official SAR/DKK reference rate, and specifies the sourcing basis (latest ECB fixing, or the official peg when DKK is pegged). The pair is unambiguous thanks to the tool name, but it does not explicitly separate itself from related siblings such as fx_convert, fx_latest, or fx_timeseries, which an agent could also plausibly reach for.

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: an agent infers this is for obtaining the SAR/DKK reference rate, and the bracket note tells it a call costs $0.002 USDC via x402 on eip155:8453. There is no explicit when-to-use versus fx_convert (for actual conversion) or versus a reverse pair like fx_dkk_sar, and no note on when the timestamped series tool should be preferred.

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