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 x402 payment model, the exact price ($0.002 USDC/call), the network (eip155:8453), the source attribution (ECB), the derivation method (direct, cross via EUR, or peg-derived), and that the value is deterministic with no estimation. It stops short of stating failure behavior — e.g. what is returned on non-fixing days or weekends — which matters for a daily-fixing tool.

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 that front-loads the cost/access model before the substantive description, which is the right order for a paid tool. No filler sentences, though the pricing prefix, while useful, occupies roughly a fifth of the text and could be considered metadata rather than description.

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 actually enumerates the return payload — rate, rate date, method, source attribution — which is more than most siblings do. It is nearly complete; only edge-case behavior (pegged periods, stale fixings, non-publication days) is left undefined.

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 parameters, so the baseline of 4 applies; there is nothing for the description to disambiguate. The currency pair is baked into the tool name and the description confirms it, leaving no parameter-level ambiguity.

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?

States a specific verb+resource: 'Official AUD/SEK reference rate: latest European Central Bank fixing', with the pair and the data source named. An agent can identify the pair unambiguously, but the description never contrasts this with the aggregating siblings fx_convert, fx_latest, or fx_timeseries, so the differentiation is implied by the naming convention 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?

Usage is only implied: 'reference rate' and 'no estimate' hint that this is for authoritative single-pair lookups rather than conversions, but there is no explicit when-to-use statement and no exclusion pointing to fx_convert for conversions or fx_timeseries for history. The agent must infer the routing from the name and the word 'reference'.

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