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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds value beyond that by disclosing the shape of the response — multi-tier comparison rather than a scalar, plus the presence of index-level coverage notes and data_status. It stops short of describing freshness guarantees or how coverage notes should be interpreted, which keeps it off a 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?

Three sentences, each doing distinct work: purpose and scope contrast first, payload contents second, alternative routing last. No filler, no restatement of the title, and the most decision-relevant information is front-loaded.

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 must carry the return-value burden itself, and it does — naming the three tiers and the accompanying fields and notes. For a zero-parameter read tool with full annotation coverage, an agent has everything needed to select and invoke it 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 tool takes no parameters, so there is nothing for the description to disambiguate and the baseline applies. The description uses the space instead to characterize the fixed return shape, which is the relevant semantics for a zero-argument call.

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 opens with a precise statement of what is returned — 'the full index definition, not a single number' — and explicitly enumerates the payload (NCSX-IS / secured / unsecured tiers side by side, notional, constituent count, coverage notes, data_status). It is immediately distinguishable from the headline-value sibling ncsx_latest.

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 explicit when-to-use criteria with concrete example questions ('how is NCSX-IS built', 'show all tiers') and contrasts them against the case it is not for ('what is the level'). It then names the alternative tool, ncsx_latest, for the excluded case, so the routing decision is unambiguous.

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