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

Annotations already declare readOnly/idempotent/openWorld, so the bar is lower; the description adds genuinely non-obvious context: the upstream source (ECB via Frankfurter) and a per-call cost of $0.35 USDC via x402 on Base or Solana. That pricing detail materially affects whether an agent should call it repeatedly.

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?

Front-loaded with the core value proposition, followed by derived metrics, source, and price. The nine-pair enumeration is long but removes ambiguity about coverage; only minor tightening (e.g. 'all 9 G10 pairs') would shorten it without loss.

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 and no parameters, the description carries the full burden of describing return contents, and it does so concretely: nine named rates, a USD strength index defined as average % change vs the G10 basket over 30 days, and a trend signal. Cost and data source are also disclosed, leaving no practical gap for invoking it.

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; baseline 4 applies. The description correctly avoids inventing parameter semantics and instead spends its words on return contents.

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 and resource ('Returns all 9 G10 FX spot rates in one call') and enumerates exactly which pairs are covered, plus two derived metrics. This clearly separates it from single-rate siblings like get_fx_rate_current and series tools like get_fx_rate_series without needing to name them.

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?

The phrase 'in one call' implies the efficiency rationale for choosing this bundle over individual FX calls, but the description never states when to prefer it versus get_fx_rate_current or get_fx_rate_series, nor any prerequisites. Usage must be inferred from scope.

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