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

The description adds important behavioral details beyond the annotations: it specifies the calculation is 'interpolated within the crossing period' and returns 'Null if never'. This discloses edge-case behavior and the interpolation method, which is valuable context that annotations do not provide. Since annotations already declare read-only and idempotent, the description still adds meaningful behavioral information.

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 a single, front-loaded sentence that communicates the core concept, the calculation method, and the null edge case. Every clause earns its place with no redundancy or filler. It is concise and well-structured.

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 two-parameter tool with an output schema, the description is nearly complete. It covers the calculation method, interpolation, and null behavior. It does not explicitly address the expected order or sign of cashflows, but the schema and context imply this. The existence of an output schema means return values need not be described. Minor gaps exist (e.g., what happens with empty cashflows) but are not critical for a simple calculator.

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?

The input schema has 100% coverage with clear descriptions for both parameters ('Upfront cost.' and 'Inflow each period.'). The description does not significantly expand on parameter semantics; it reinforces that cashflows are inflows and initial cost is upfront, but the schema already conveys this. Baseline of 3 is appropriate.

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 clearly states the tool's function: 'Simple payback period: periods until cumulative cashflows recover the initial cost, interpolated within the crossing period. Null if never.' It uses a specific verb ('recovers') and resource ('cumulative cashflows'), and distinguishes from the sibling 'discounted-payback' by explicitly labeling this as 'Simple'.

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 implies the tool is for undiscounted payback calculations, but it does not explicitly state when to use it versus alternatives like 'discounted-payback'. There is no 'use when' or 'use instead' guidance, though the term 'Simple' hints at a differentiation. This is a clear but under-specified usage context.

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