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

With readOnlyHint=true already indicating a safe read operation, the description adds valuable context such as 'Keyless; refreshed hourly' and the data source (NOAA SWPC planetary Kp composited with live solar wind). It does not describe error handling or rate limits, but for a read-only index tool this is sufficient.

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 three sentences, front-loaded with the return value and use cases. 'Updates: hourly' is redundant with 'refreshed hourly,' but the overall structure is efficient and every other sentence earns its place.

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 simple read-only tool with no output schema, the description thoroughly covers the output fields, use cases, authentication requirements (keyless), and refresh frequency. The optional 'days' parameter is fully documented in the schema, so the description is complete enough for an agent to select and invoke the tool 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 100% of the single optional parameter 'days' with a full description, so the baseline is 3. The description does not add parameter-specific semantics beyond the schema, but that is acceptable given the complete schema coverage.

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 clearly states the tool returns a 0-100 geomagnetic storm intensity index and lists the output fields (g_scale, kp_last_24h, storm_trend). It also provides a specific resource (geomagnetic storm intensity) and associated use cases, but it does not explicitly differentiate from sibling tools by naming alternatives, so it falls short of a 5.

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 provides explicit 'Call when' guidance covering user queries about solar storms, aurora visibility, space weather, and disruption risks, as well as operational use cases like satellite maneuvers and power-grid maintenance. It does not mention when not to use the tool or alternative tools, but the context is clear and specific.

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