datetime.day_count

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

Annotations already cover the safety profile (readOnly, idempotent, closed-world), yet the description adds substantial behavior: output precision capped at 50 significant digits with no rounding, output types (integer vs Decimal string), and the exact 30/360 variant (31st end-of-month adjustment only, no February rule) plus per-year-boundary leap denominators for ACT/ACT. These are real implementation details an agent could not infer. Minor gap: no error/edge behavior for equal or reversed dates beyond 'end must be after start'.

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, front-loaded with what is returned, followed by the convention taxonomy and the date constraint. Dense but every clause is load-bearing; the parenthetical convention qualifiers are exactly the disambiguation needed.

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 3-required-param calculation tool with an output schema (so return structure needn't be explained, though types are still noted), the description supplies output meaning, convention semantics, date format, and input constraint. An agent can call it correctly without opening anything else.

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 0% and the convention parameter has no enum, so the description carries the full burden — and it does: all four convention values are enumerated with their distinguishing semantics, the date format YYYY-MM-DD is given for start/end, and the start<end ordering is stated. Nothing about the three required parameters is left undocumented.

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 result set: computes interest-accrual day count (integer days) and annualized year_fraction (Decimal string), which is far more precise than a generic 'date difference'. It never names a sibling such as datetime.diff to draw the boundary, so it falls short of the top mark.

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 domain anchor ('이자 계산용', for interest calculation) implies the intended use case, and the ordering constraint that end must follow start is stated. However, there is no explicit guidance on when to choose this over datetime.diff or the business-day tools, and no exclusions.

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