nestegg-calculators

Portfolio Longevity

portfolio-longevity

Read-onlyIdempotent

How many years a balance lasts while withdrawing from it: the balance grows each year, then the withdrawal (optionally stepping up) is taken. Returns the depletion year, or sustainable=true when it outlasts 200 years.

Input Schema

TableJSON Schema

Name	Required	Description
`balance`	Yes	Starting balance.
`annualRatePct`	Yes	Annual portfolio growth in percent.
`annualWithdrawal`	Yes	Amount withdrawn in the first year.
`withdrawalGrowthPct`	No	Optional. Yearly step-up of the withdrawal in percent (default 0).

Output Schema

TableJSON Schema

Name	Required	Description	Default
`years`	No	Depletion year (null if sustainable).
`sustainable`	No	True if it outlasts 200 years.

Schema Changelog

Changes observed during successful MCP inspections.

First observedJun 28, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Beyond the annotations (read-only, idempotent), the description discloses the order of operations (growth before withdrawal), the optional step-up, and the 200-year threshold for 'sustainable'. This adds meaningful behavioral context.

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?

Two sentences front-load the core purpose and return behavior without wasted words.

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

Completeness5/5

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

The description covers the calculation logic, return behavior, and cap condition. With an output schema and full parameter descriptions, no critical information is missing for an agent to invoke this tool.

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

Parameters3/5

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

The input schema fully describes all four parameters, so the description need not repeat them. It adds a small detail about the optional step-up ('optionally stepping up') which aligns with withdrawalGrowthPct, but overall the schema already covers parameter meaning.

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 defines the tool's function: calculating how many years a balance lasts under withdrawals, with growth applied first. It distinguishes from related siblings like fire-number by focusing on longevity rather than required balance.

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 gives clear context for when to use the tool (to determine portfolio depletion time) but does not explicitly mention alternatives or exclusion cases. Still, the context is unambiguous enough for an agent to select it appropriately.

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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