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

With no annotations, the description carries the full behavioral burden and does well: it discloses the paid access model ($0.002 USDC/call via x402 on eip155:8453), the authoritative source (ECB fixing), the derivation method (direct, cross via EUR, or peg-derived), and the determinism guarantee ('no estimate'). What it does not cover is failure behavior, such as what happens if the fixing is unavailable or the payment is not settled.

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 preceded by a bracketed cost/payment tag, with zero filler. The cost tag is front-loaded where an agent will see it before deciding to call, and the semantic payload (pair, source, method, attribution) follows immediately.

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 annotations and no output schema, the description covers everything the agent needs: what the value is, where it comes from, how it is derived, which fields accompany it (rate date, method, source attribution), and the access cost. It even handles the INR-peg edge case, so nothing material is missing.

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 to add and the baseline of 4 applies. The description appropriately uses its space on sourcing and cost rather than inventing parameter discussion.

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 specific resource and scope: 'Official NOK/INR reference rate: latest European Central Bank fixing', which tells an agent exactly which pair and which vintage of rate it gets. It does not explicitly contrast itself with siblings like fx_convert, fx_latest or fx_timeseries, so differentiation is left to the tool name 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?

There is no explicit when-to-use or when-not-to-use guidance. The word 'latest' implies this is a spot-fixing lookup rather than a historical series (fx_timeseries) or an arbitrary amount conversion (fx_convert), but the agent must infer that from the name and must separately discover the payment prerequisite.

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