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

Annotations already establish readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds genuine behavioral context beyond that: the accrual model is simple interest and scales linearly with the day count, which tells the agent what math to expect. It stops short of disclosing rounding, day-count convention, or currency units.

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?

Two sentences, the computational definition front-loaded before the use cases, with no filler. The opening clause is slightly roundabout but still efficient and earns its place.

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?

No output schema exists, so the description is the only place to learn what comes back; it implies a dollar interest figure but never states the return shape, units, or precision. For a simple three-parameter calculator with full annotation coverage this is adequate, but the missing output framing is a real gap.

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 67% (days and balance documented, annual_rate_pct undocumented). The description contributes no parameter-level detail such as whether the rate is nominal annual, percent vs. decimal, or an actual/365 basis. Names are largely self-explanatory, so the baseline 3 holds.

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 exact quantity computed (the interest a simple-interest loan accrues per day and over N days), a specific mechanism (simple interest) and a specific scope. It is clearly distinct from siblings like loan_payment or payoff_time, which compute different quantities, without needing the schema.

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 three concrete situations for use: reading a payoff letter, quantifying the gap between payoff and statement balance, and costing a skipped/deferred payment. That is strong positive guidance, but no exclusions or named alternative tool tell the agent when NOT to use it.

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