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

With no annotations provided, the description carries the full burden of behavioral disclosure. It effectively does so by revealing the non-obvious legal computation rule: an age is reached the day before the birthday, with a concrete example for people born on the 1st of a month. This is genuinely useful behavioral context beyond what the schema could convey.

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 description is compact and front-loaded: the first sentence states the core output, and the second paragraph adds a necessary legal nuance that directly affects correctness. Every sentence earns its place, with no fluff or repetition.

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 simple two-parameter, pure-computation tool with no output schema, the description is complete: it states what is returned (exact dates and premium start/stop), covers all relevant thresholds, and warns about the legal edge case. An agent has enough information to invoke it correctly and interpret its results.

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 description coverage is 100%, so the baseline is 3. The description references birth dates in its example but does not add extra meaning about the as_of parameter or the expected date format beyond what the schema already provides. The schema handles parameter semantics adequately.

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 specific verb and resource: 'Returns the exact date each threshold is crossed and which premium starts or stops.' It lists the exact age thresholds (40/65/70/75) and what happens at each, making the tool's purpose concrete and distinguishable from sibling calculation tools like get_insurance_rates or national_insurance.

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 gives clear usage direction: 'Compute this here rather than by subtracting years.' It explains why this tool is the correct choice by citing the 年齢計算ニ関スル法律 rule. However, it does not explicitly name alternative sibling tools or state when those should be preferred, so it falls just short of full routing guidance.

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