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

Beyond the lone readOnlyHint annotation, the description discloses the most decision-relevant trait: data is report-based and refreshes only about twice a year, not daily, and metricDate reflects the report's 'as of' date. It further enumerates the exact cases that yield null (ETNs, no first report, no EDINET filing, PDF-only foreign funds), which is precisely the failure-mode context an agent needs.

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 content is front-loaded and each English clause carries real information, but the entire passage is duplicated in Japanese, roughly doubling length without adding new facts for a single reader. The bilingual pattern is defensible for audience reach, yet it dilutes conciseness relative to a tight single-language statement.

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 available, the description carries the full burden of explaining return values, and it does so: both fields, their meaning, their timestamps, and their null conditions. For a one-parameter read-only lookup, nothing material is missing.

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% and the single symbol parameter is already documented with format and an example in the schema. The description adds no further meaning about symbol handling or ETF-vs-stock code behavior, so the baseline of 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?

The description names the resource precisely ('Net assets and distribution yield for a Japanese ETF') and enumerates the two returned fields, which separates it from equity-metric siblings like get_key_metrics or get_key_metrics_latest. It stops short of naming any sibling explicitly, so the distinction is inferred rather than stated.

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?

Usage is implied through the negative scope: NAV per unit, units outstanding, premium/discount and tracking difference are declared out of scope, which steers the agent away from misusing this tool. However, there is no explicit 'use this when… / use X instead when…' routing against list_etfs or get_key_metrics, leaving the choice to inference.

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