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

With no annotations, the description carries the full burden and does so well for a paid endpoint: it discloses price ($0.002 USDC/call), payment protocol (x402, eip155:8453), provenance (ECB fixing), fallback behavior (official peg when CNY is pegged), and determinism ('no estimate'). It omits rate limits and what happens on days the ECB does not publish, so it stops 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?

One dense sentence plus a bracketed payment tag. Cost and protocol are front-loaded, then the core value proposition, then the return details. Slightly run-on, but every clause (source, peg handling, rate date, method, attribution) contributes distinct information.

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?

No output schema exists, and the description compensates by naming the returned fields - rate date, method (direct/cross via EUR/peg-derived) and source attribution. Combined with the payment terms and determinism claim, an agent has enough to decide and call correctly; only freshness/staleness behavior on non-publishing days is left open.

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; the schema-vs-description question is moot. Baseline 4 applies for a no-parameter tool.

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 concrete resource - the SGD/CNY reference rate - and pins down the source (European Central Bank fixing) and the peg fallback. The pair-named tool is easily distinguishable from other fx_* siblings by currency codes, though the description itself never contrasts it against fx_latest, fx_convert or fx_timeseries.

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: 'official ... reference rate' signals this is for reference/accounting purposes rather than conversion, but no explicit when-to-use or when-to-prefer-an-alternative guidance is given. The peg note hints at an edge case but does not route the agent to a different tool for any scenario.

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