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

Beyond the annotations (readOnlyHint=true, idempotentHint=true, destructiveHint=false), the description adds concrete behavioral guarantees: 'pure deterministic calculation', 'no side effects, no network or storage access', and 'identical inputs always produce identical outputs'. It also discloses error behavior for division by zero, non-finite inputs, and undefined mathematical combinations, which is genuinely useful.

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 (WHEN TO USE, BEHAVIOUR, RETURNS, PARAMETERS) and front-loads the core definition and formula. The PARAMETERS section is somewhat redundant with the schema's property descriptions, which adds length without adding information, but the overall organization remains clear and scannable.

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 stateless calculation tool with three primitive required parameters and no output schema, the description is complete: it covers the formula, input units and frequency matching, exclusions, deterministic behavior, error cases, and the expected return JSON shape. Nothing an agent needs to invoke it correctly is missing.

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% and the parameter guidance in the description is nearly verbatim from the schema (e.g. 'Discount rate as a decimal, e.g. 0.10...', 'Must be >= 0', 'May be negative for a future payment'). The description adds no meaning beyond what the schema already provides, so the baseline of 3 applies.

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-resource pair ('Calculate the present value of a single future amount') and reinforces it with the explicit formula PV = FV / (1 + r)^n. It clearly scopes the tool to a single lump sum, which distinguishes it from sibling tools handling multiple cash-flow streams, annuities, and perpetuities.

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?

Provides explicit WHEN TO USE and WHEN NOT TO USE sections. It names the alternatives directly (calculate_payback_period and calculate_discounted_payback_period for series recovery; perpetuity/annuity tools for streams), leaving no ambiguity about tool selection.

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