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 declares a paid call ($0.002 USDC via x402 on eip155:8453), the upstream source (ECB), the peg fallback branch, and that values are deterministic with no estimation. What it omits is freshness/publishing cadence and error behavior, so it stops short of 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?

Payment metadata is bracketed and front-loaded, then purpose, then output shape — a sensible order with no filler sentences. The second clause stacks several parentheticals ('direct, cross via EUR or peg-derived'), making it dense, but every element is informative.

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?

There is no output schema and no annotations, so the description must describe what comes back, and it does: rate, rate date, method, source attribution. Payment cost and network are covered too. Only freshness cadence and failure modes are missing, which is minor for a zero-argument lookup.

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 for the description to disambiguate and the baseline of 4 applies. The description correctly adds no parameter guidance and instead spends its words on the output contract.

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?

It names the concrete resource (AED/NZD) and verb of retrieval ('latest European Central Bank fixing'), and adds the peg fallback, which is a genuine scope distinction. Against the hundreds of fx_*_* siblings the pair is already encoded in the name, but the description clarifies the data source and derivation rather than repeating the name. It does not explicitly separate itself from fx_convert, fx_latest, or fx_timeseries, which keeps it at 4.

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', 'deterministic, sourced, no estimate' suggests booking/audit use where an authoritative fixing is required. There is no explicit 'use this when X, use fx_convert instead when Y' routing despite obvious overlapping siblings. An agent can infer intent but must guess about alternatives.

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