SavingsLast retirement calculators

Sustainable monthly withdrawal

sustainable_withdrawal

Read-onlyIdempotent

The largest starting monthly withdrawal, rising with inflation, that a balance can support for a chosen number of years at a given return, inflation and tax rate. Solved by bisection on the same drawdown engine.

Input Schema

TableJSON Schema

Name	Required	Description
`age`	No	Current age. Optional; enables the run-out age, the survival odds, the account rules (penalty before 59 1/2, RMDs from 73) and the Social Security start age.
`years`	No	How long the money must last, in years. Default 30.
`account`	No	Apply an account type's withdrawal rules: the 10% early-withdrawal penalty before 59 1/2 for a 401(k), 403(b), TSP, IRA, SEP or SIMPLE IRA; none for a governmental 457(b) or under the rule of 55; tax-free withdrawals for a Roth IRA; the 20% non-medical penalty before 65 for an HSA; required minimum distributions from 73 where they apply. Default none.
`balance`	Yes	Starting balance in dollars.
`tax_rate`	No	Tax on withdrawals in percent, 0 to 50. Withdrawals are grossed up so the after-tax amount matches. Default 0. Ignored for Roth and HSA accounts.
`inflation`	No	Annual inflation in percent, -5 to 15. Default 2.5. The withdrawal rises at this rate so spending power stays level.
`stress_test`	No	Apply a bad start: -20%, -10% and 0% returns in the first three years, then the average. Default false.
`annual_return`	No	Expected annual return in percent, -10 to 20. Default 5.
`spending_shape`	No	inflation: spending rises with inflation (default). level: fixed dollars. decline: rises with inflation until 75, then one point a year slower.
`pension_monthly`	No	Pension or annuity in dollars a month, netted against spending. Default 0.
`social_security_monthly`	No	Social Security benefit in dollars a month, netted against spending. Rises with inflation. Default 0.
`social_security_from_age`	No	Age the Social Security benefit starts. Needs age. Default: from now.
`pension_rises_with_inflation`	No	Whether the pension has a cost-of-living increase. Default false.

Schema Changelog

Changes observed during successful MCP inspections. Dates show when Glama detected each change.

First observedSep 8, 2026

TDQS

A3.5/5.0

Behavior3/5

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

The annotations already establish that the tool is read-only, idempotent, and non-destructive. The description adds a useful computational detail by mentioning bisection on the same drawdown engine, and it clarifies that withdrawals rise with inflation, but it does not describe the output format or numerical behavior beyond that.

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

Conciseness4/5

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

The description is short, front-loaded with the core purpose, and does not repeat the schema. The second sentence about bisection is somewhat extra but earns its place by giving a computational transparency cue without adding bulk.

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?

Given the tool's 13 parameters and the very detailed schema, the description clearly communicates the primary result: the largest starting monthly withdrawal. It does not describe the output format or explicitly mention how age, account type, pension, or Social Security feed the calculation, but the schema covers those inputs and the core return value is stated.

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?

Schema description coverage is 100%, so the schema already documents all 13 parameters in detail. The tool description adds only a high-level relationship among years, return, inflation, and tax rate, which does not materially improve on the schema's parameter explanations.

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

Purpose4/5

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

The description states a clear objective: compute the largest starting monthly withdrawal a balance can support for a chosen number of years given return, inflation, and tax assumptions. It is specific about the resource and calculation, but it does not explicitly differentiate itself from the sibling tool how_long_will_money_last, which answers the inverse question.

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

Usage Guidelines3/5

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

The intended use is implied: use this when you want the maximum sustainable monthly withdrawal for a fixed time horizon. However, the description gives no explicit guidance about when to prefer this over alternatives such as how_long_will_money_last, nor does it state 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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