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

Annotations already declare readOnlyHint=true, and the description adds meaningful behavioral context beyond that: 'Updates: daily,' the optional days parameter returning a daily history series, and the Gold-tier requirement for history. This gives the agent useful operational expectations beyond the structured annotations.

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 description is dense but efficient: it leads with the core output, then the calculation methodology and components, then use cases, then update cadence. Every sentence adds value and there is no filler or repetition.

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 tool with one optional parameter and no output schema, the description is complete: it names the return fields (score, trend, regime, per-pair vol ranking, freshness, methodology_version), explains the default snapshot vs. history behavior, and notes the update frequency. An agent can reasonably decide to invoke it without additional clarification.

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?

Schema description coverage is 100%: the days parameter is fully described in the schema, including that it returns a history series, accepts up to 5 years, and requires Gold tier. The tool description does not add additional parameter meaning beyond what the schema already provides, so the baseline 3 is appropriate.

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?

Description opens with a specific verb and resource: 'Returns a 0-100 major-currency FX volatility index' and enumerates the exact currency pairs and calculation basis. This clearly distinguishes it from the many opaque sibling tools by identifying its unique subject matter.

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 description gives explicit call-when guidance: 'Call when the user asks about currency-market stress, FX volatility, dollar-pair turbulence, or macro risk-on/risk-off, or when timing hedging, carry, or forward-contract decisions.' It lacks explicit when-not-to-use or mention of alternatives, so it falls just short of a 5.

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