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

With no annotations, the description carries the full burden and does well: it discloses cost and payment rail ($0.002 USDC/call, x402, eip155:8453), the authoritative source, the peg-fallback behavior, the derivation method reported alongside the rate, and a determinism/no-estimate guarantee. It does not address behavior on non-publishing days (weekends/TARGET holidays) or staleness handling, which keeps it short of 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?

Two tight sentences: the payment/cost bracket is front-loaded, then the data description. Every clause carries information (source, fallback, method, attribution, no estimates) with no restatement of the tool name.

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?

With no output schema, the description must convey what comes back, and it does: rate, rate date, method (direct/cross via EUR/peg-derived) and source attribution. Only the concrete field shape and freshness/edge-case behavior are left unspecified, which is a minor gap for a zero-parameter single-value tool.

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 to disambiguate and the baseline is 4. The description correctly adds no parameter noise and instead spends its words on the return semantics.

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 precise resource (AUD/CHF reference rate), a precise source (latest European Central Bank fixing), and the fallback basis (official peg when CHF is pegged). An agent can distinguish this from the hundreds of sibling fx_<ccy>_<ccy> tools and from fx_convert/fx_latest without opening anything else.

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 by 'latest ECB fixing' — i.e. use it when you need the current official reference rate for this specific pair — but the description never states when to prefer it over fx_convert, fx_latest or fx_timeseries, nor any exclusion such as historical lookups. The context is inferable but not spelled out.

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