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

With no annotations, the description carries the full burden and does so well: it discloses sentinel behavior (interestDays = -1 when settlement cannot make it), that negativeInterestDays grows around weekends, broker-specific liberation rules (SBI/GMO vs Rakuten vs au Kabucom/SMBC Nikko being out of scope), and the YYYY-MM-DD return format. It stops short of auth/permission notes, but for a pure date calculator that is a minor omission.

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 definition is long but strongly front-loaded with the one-line purpose, then organized under bracketed headings for inputs, tradeDate default, and returned fields. Every sentence carries information; the length is justified by the input-format ambiguity, though some field explanations could be trimmed.

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?

Given an output schema exists, the return-value explanations are strictly redundant, yet the description still supplies the domain complexity an agent needs: input normalization rules, the tradeDate default, and broker-conditional liberation schedules. It is essentially complete for correct invocation, only mildly over-explaining outputs.

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 already 100%, so the baseline is 3, but the description adds real meaning beyond the schema: it enumerates accepted vestingDate formats ("2026-09-30", "9月末", "09", "9", "9月20日", "0920") and explains that month-only inputs resolve to the next upcoming cycle, plus the exact default rule for the omitted tradeDate.

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 opening sentence gives a specific verb+resource: takes a vesting date and returns the dates/days needed for a cross trade (クロス取引). Scope is clear and it implicitly separates itself from siblings by naming estimate_cross_fee's domain (fees) and explaining how the prevYear field pairs with get_benefit, though it never explicitly contrasts the siblings' purposes.

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 concrete when-to-use conditions: tradeDate defaults to 'now' (before/after 15:30 JST, weekends/holidays rolling forward), and prevYear is meant to be matched against get_benefit's negativeInterests / remainingHistories prev data. It does not state when NOT to reach for this tool versus estimate_cross_fee, so it falls short of the explicit-exclusion bar.

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