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

The annotations already declare readOnlyHint=true, so the agent knows it's a safe read. The description adds valuable context: 'Open-Meteo, hourly' data source, update frequency ('Updates: hourly'), and the calculation methodology (mean, share). This goes beyond the annotations to describe what the returned data represents and how it is derived. No contradictions 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?

The description is dense and front-loaded with the core purpose, then lists output fields and usage guidance. The first sentence is long but every clause adds meaningful detail (calculation, metros, fields, data source). The second sentence covers when to call efficiently. It is not wastefully verbose, but the structure could be slightly improved with better separation of concerns.

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, the description carries the burden of explaining return values, and it does list all output fields (heat_level, mean_peak_apparent_f, metros_over_100f, hottest_metro, per-metro table). It also provides usage context and update frequency. Some gaps remain, such as the interpretation of heat_level thresholds or how the history option interacts with the snapshot, but these are partially covered by the schema parameter description and the field names are self-explanatory. Overall, sufficiently complete for a tool with one optional parameter.

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 single optional 'days' parameter is fully described in the schema (100% coverage), including the Gold tier requirement and behavior. The description itself does not mention the parameter, but per the rubric, high schema coverage gives a baseline of 3. The description doesn't need to add anything further; the schema does the heavy lifting for this dimension.

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 US extreme-heat forecast index, specifies the exact computation (mean 7-day peak apparent temperature across 12 major metros plus share over 100°F), and distinguishes it from sibling weather/risk tools by focusing on extreme heat. The verb 'Returns' and the resource 'US extreme-heat forecast index' make the purpose immediate and unambiguous.

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 says 'Call when the user asks about heat waves, dangerous heat, or metro heat risk, or when timing staffing, energy hedging, or logistics decisions for the week ahead.' This gives clear when-to-use context, though it doesn't name specific alternative tools or when-not-to-use conditions. Missing the explicit exclusions/alternatives that would warrant a 5.

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