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

Although annotations already declare readOnlyHint=true and idempotentHint=true, the description adds substantial behavioral context: it returns an explicit error if the investment is never recovered, discloses that cash flows after payback are ignored, and describes the return shape. This goes well beyond the annotations and helps the agent predict edge-case behavior.

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 well structured with labeled sections (formula, when to use, when not to use, behavior, returns, parameters), which makes it scannable. It is somewhat verbose and repeats the explicit-error behavior in both BEHAVIOUR and RETURNS, but every section adds useful routing or behavioral information.

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?

There is no output schema, so the description appropriately documents the return format as a JSON object with payback_period_years and inputs, plus the error case. It also covers the input format, constraints, formula, and decision-usability caveats. Nothing critical is missing for an agent to call this tool correctly.

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 schema already documents the cash_flows parameter. The description does add a clarifying example and reiterates the format and constraints, but mostly repeats what the schema states. This matches the baseline of 3 for high schema coverage.

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: 'Calculate the payback period' and defines exactly what it measures, including the fractional final year and the cumulative cash-flow recovery rule. It also distinguishes itself from time-value-aware tools by explicitly stating that it 'ignores the time value of money.' The formula and example further anchor what the tool does.

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 includes explicit WHEN TO USE and WHEN NOT TO USE sections. It positions the tool as a liquidity/risk screen, warns against using it as the sole investment criterion, and names calculate_npv and calculate_irr as alternatives for profitability and time-value considerations. This gives an agent clear routing guidance.

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