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

With no annotations, the description carries the full burden and does a lot of that work: it discloses the price and payment rail ($0.002 USDC/call, x402, eip155:8453), the upstream source, the inclusion of rate date, derivation method (direct, cross via EUR, or peg-derived) and source attribution, plus the guarantee that values are deterministic and not estimated. It stops short of stating failure behavior when no fixing is available or the response shape, which keeps it from a 5.

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?

The cost/rail notice is front-loaded in brackets, followed by a single dense sentence that describes the resource, source, fallback and returned fields. It is efficient, though the second sentence covers four distinct facts in one breath and could be split for easier scanning.

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, no-output-schema lookup, the definition covers the essentials: what data, from which source, with what metadata, and at what cost. The only meaningful gap is what the agent should expect when a fixing or peg is unavailable, since there is no output schema to fall back on.

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 the baseline is 4; there is nothing for the description to disambiguate. The one thing worth noting is the paid-call requirement, which is stated up front and functions as an invocation precondition rather than a parameter.

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 verb+resource pair ('Official CNY/SEK reference rate') and pins the source (latest European Central Bank fixing, or the official peg when SEK is pegged). That is enough to separate this tool from the hundreds of other fx_<base>_<quote> siblings, which differ only by currency pair.

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: the tool is for the official CNY/SEK reference rate rather than a computed quote, and 'no estimate' signals it is the authoritative figure. It never names an alternative (e.g. fx_convert for converting an amount, fx_latest or fx_timeseries for other shapes of data), so the agent must infer when this pair-specific tool beats those generic ones.

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