nestegg-calculators

Perpetuity

perpetuity

Read-onlyIdempotent

Present value of a level or growing perpetuity: payment / (rate - growth). Null when growth is not below the rate.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "payment": "<payment>",
      "ratePct": "<ratePct>"
    },
    "name": "perpetuity"
  }
}

Input Schema

Name	Required	Description
`payment`	Yes	Periodic payment.
`ratePct`	Yes	Discount rate in percent.
`growthPct`	No	Optional. Payment growth rate in percent (default 0).

Output Schema

Name	Required	Description	Default
`pv`	No	Present value (null if growth>=rate).

Schema Changelog

Changes observed during successful MCP inspections.

AddedJun 28, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations already indicate a safe, read-only, idempotent operation. The description adds valuable behavioral detail: the formula and the null condition when growth is not below the rate. This goes beyond the structured metadata.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single sentence that packs the definition, formula, and edge case. There is no wasted text; it front-loads the purpose and provides the formula efficiently.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a three-parameter calculation with a simple formula and an output schema present, the description covers the critical edge case (null when growth >= rate). It does not describe return formatting (handled by output schema) and the formula gives enough context for the calculation. Minor omission: no explicit mention of infinite horizon, but the name and formula imply it.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema descriptions cover all parameters (100% coverage), but the description's formula ('payment / (rate - growth)') adds relational meaning among payment, ratePct, and growthPct. It also clarifies the default of growthPct via the formula's context, though that is already in the schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states it computes the present value of a level or growing perpetuity, with a specific formula. It distinguishes itself from sibling tools like 'annuity-pv' and 'present-value' by explicitly targeting perpetuities.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies usage for perpetuities (infinite cash flows) and provides the calculation context, but it does not explicitly mention alternatives or conditions for when not to use it. Given the clear domain and sibling list, the context is evident without exclusions.

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

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