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

With annotations covering the safety profile, the description adds substantial behavioral context beyond them: the free-tier 12-week delayed value with a daily live rotation, the Essential-plan requirement for live data, that change periods appear only where sufficient history exists, and that carbon data appears only where scored. These are real constraints an agent must know.

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?

A single dense sentence, but it is front-loaded with the core purpose and each parenthetical (index levels, period availability, tier gating) earns its place. Slightly heavy on nested parentheses, though justified by the amount of genuinely useful information.

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 return-value burden and does so fully: value in USD and user currency, four change periods with their history caveat, and carbon data. An agent has everything needed to call and interpret the result.

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 both parameters (code, currency) are fully documented in the schema, including the search_indices fallback and FX conversion. The description restates currency conversion ('in USD (and your currency)') but adds no syntax or format detail beyond the schema, 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 (today's value of one PackIndex code) and scopes it precisely to the L0–L3 index hierarchy. 'One' and 'Today's value' clearly separate it from get_all_indices and get_index_history.

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 establishes clear context — single live index value today — which implicitly routes the agent away from history, forecast, and bulk-list siblings. It stops short of naming alternatives or stating when-not to use it (e.g., 'for historical series use get_index_history').

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