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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description usefully adds return ordering and the size ceiling, but never says what happens when the requested window exceeds 31 days (error, truncation, or silent clamp) or how to page past 744 hours — a real gap given the limit is stated.

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?

Two tight sentences with the resource and its cardinality front-loaded, followed immediately by the use case. No filler or restatement of the title.

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, single-index time-series fetch with full schema descriptions and safety annotations, the description covers what the tool is and when to reach for it. The only meaningful omissions are window-overflow behavior and how to retrieve more than 31 days, both minor but not zero-cost for an agent planning a long-range chart.

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%: from/to carry ISO hour examples and the index enum is documented field-by-field. The description adds no syntax, timezone, or defaulting detail beyond what the schema already states, so the baseline 3 applies.

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 — 'Hourly closes of one Arc index' — plus the ordering ('oldest first') and a hard cap ('at most 744 hours per call'). It does not, however, differentiate itself from the nearest sibling arc_index_history_deep, so an agent must infer which of the two to pick from the description alone.

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?

Gives clear intended context: 'Use this for charts or changes over a window of up to 31 days.' That is a usable when-to-use signal, and the 744-hour cap implicitly bounds the tool. It stops short of naming the alternative (arc_index_history_deep) for longer windows, so the exclusion side is left to inference.

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