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

Annotations already mark this as read-only, and the description adds substantive behavioral context: it draws from the Smithsonian GVP/USGS weekly report, computes values relative to a typical week's 15-25, boosts for new activity and US alerts, and updates weekly. This goes well beyond the readOnlyHint and helps the agent anticipate staleness and data provenance. No contradiction with 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?

Three compact sentences cover output, usage, and update cadence, with all key information front-loaded. The parenthetical formula is dense but necessary for interpreting the index, and no sentence is wasted.

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, but the description enumerates the returned fields (score, trend, activity_level, new_this_week, per-volcano active list, us_alerts), names the data source, and gives selection guidance. The optional days parameter is fully documented in the schema, so the tool is contextually complete enough for an agent to invoke correctly.

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 schema covers the single optional `days` parameter fully, including meaning and the Gold-tier condition, and schema coverage is 100%. The tool description itself does not mention `days` or add interpretation beyond the schema, so the baseline of 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?

The description opens with a precise verb ('Returns') and a concrete resource ('global volcanic activity index'), and specifies the value range, formula basis, and returned fields. It clearly differentiates this tool from sibling data indices by naming volcanic activity context.

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 explicitly states when to call—'when the user asks about volcanic eruptions, unrest, ash risk, or which volcanoes are escalating'—and extends to aviation-routing, travel, and catastrophe-exposure reviews. It does not name alternative tools or exclusions, so it lacks the strongest 5-level comparison, but the context is clear and actionable.

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