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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint=false, so safety is covered; the description adds value beyond that by disclosing the free-vs-PRO tier split, what data each tier returns, and the freshness window ('this week'). It does not mention pagination, rate limits, or result size limits, keeping it below 5.

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 core purpose is front-loaded in the first sentence, tier behavior follows in one dense sentence, and the examples are compact and directly actionable. No filler text.

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 usefully characterizes the return shape (composition broken down by channel, origin, tier, license) and the tier-dependent depth, so an agent knows roughly what comes back. It omits any note on result size or pagination for a potentially large basket listing, which is the only meaningful 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?

Schema coverage is 100%, so the schema already documents index_code, direction and _atom_api_key; baseline is 3. The description does add meaning for _atom_api_key via the free/PRO tier explanation, but says nothing about the 'direction' enum parameter or index_code syntax beyond what the schema and examples already show.

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 states a concrete verb+resource ('what sits inside an index basket') and enumerates the returned composition (SKUs, models, vendors, split by channel/origin/tier/license), which makes it clearly distinct from a benchmarks or price-history tool. It stops short of explicitly naming a sibling (e.g. get_index_benchmarks) to route against, so it lands at 4 rather than 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 two worked examples ('What is in the flagship index?' -> index_code="FLG"', 'How much of the China index is sold through neoclouds?' -> index_code="CHN"') give clear situational context for when to reach for this tool. There is no explicit when-not or named alternative, so it is not a full 5.

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