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

Annotations already declare the full safety profile (readOnlyHint, idempotentHint, destructiveHint=false, openWorldHint=false), so the bar is low. The description adds value by disclosing what the payload contains (index closes plus 24 h change base and decay parameters), though it says nothing about refresh cadence, staleness, or pagination.

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 sentences, no filler, and the most decision-relevant information (which indices and what timestamp granularity) is front-loaded before the usage cue.

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 zero-parameter, read-only snapshot tool with no output schema, the description supplies enough about the returned content (three indices, 24 h change, decay parameters) to call it correctly. The domain jargon ('24 h change base', 'decay parameters') is left unexplained, which is the only real gap.

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 tool takes zero parameters, so there is nothing to document and the baseline is 4. The description correctly implies a no-argument snapshot call rather than a query requiring inputs.

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 names the exact resource and scope: the hourly close of the Arc Builders, Integrations and Ecosystem indices (ABI, AII, AEI), with explicit abbreviations. 'Latest' cleanly separates it from the arc_index_history / arc_index_history_deep siblings, so an agent can pick it without opening a schema.

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?

It gives a concrete trigger: 'Use this when asked how Arc app activity is trending overall.' That is clear context, but it names no alternatives or exclusions (e.g. when to prefer arc_index_history for trends over time or arc_app for single-app detail).

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