FIREproof

What it is

FIREproof is the second retirement simulator from the author of cFIREsim, and it exists to answer the question cFIREsim explicitly refuses: what the tax code does to the plan. The method is the same — enter a plan, replay it against every overlapping window of market history, count how many survived — but the pot of money is no longer undifferentiated. Accounts have types, withdrawals have an order, and every dollar that moves is run through brackets, capital gains with cost basis, FICA, RMDs and 72(t).

Inputs are an eight-step stepper: People, Accounts, Income, Expenses, Real Estate, Tax Strategy, Cash Flow, Parameters. Account types run from 401(k), Roth IRA, brokerage, HSA and Solo 401(k) to inherited IRAs with their own ten-year SECURE Act clocks. On top sit the pieces that decide a US early retirement — a Roth conversion optimiser that fills a bracket you name, ACA premium tax credits projected by ZIP with the subsidy cliff flagged, Medicare IRMAA tiers, a NIIT threshold cap, and a FAFSA optimiser that routes withdrawals around a child's base years. Spending can follow Guyton-Klinger, VPW, CAPE, Hebeler autopilot, or a risk-based guardrail that re-solves each check-in year against a success rate you pick. Real estate is modelled properly: mortgages year by year, appreciation, rental cash flow with depreciation, and 1031 exchanges.

Development is unusually visible. The public changelog carries 1.9.0 on 12 September 2026, two days before this card was written, and sixty-four releases in the seven months behind it.

Pricing

Basic is free and holds one saved simulation. Pro is 10 dollars a month month-to-month, or 89 a year. A Financial Planner plan is 50 a month or 399 a year and includes 20 active client seats, with 2 dollars a month for each additional claimed client; unclaimed prospects are free.

Read the feature table before assuming the free tier is a small version of the paid one. Fourteen of its sixteen rows are Pro-only, and they are the rows that describe the product — withdrawal order, the TIPS ladder, Roth conversions and 72(t), bracket-filling withdrawals, ACA optimisation, the bucket strategy, Monte Carlo custom assets, comparing two simulations, uploading a dataset, and year-by-year account statements among them. Besides the simulator, Basic shares exactly one row with Pro: Discord access.

Data & coverage

Eleven named series, each published by the app with its source and year range. The two defaults are Robert Shiller's S&P composite and the US 10-year Treasury, both starting in 1871 and carrying a 2026 entry. Most optional sleeves are shorter: Ken French's factor portfolios from 1927, NAREIT's REIT index from 1972, five-year Treasuries from 1962, Developed International from 1991, CRSP total market via VTSAX from 2001, plus synthetic TIPS-real and aggregate-bond series derived from the Treasury yield. Gold and cash are separate asset types, and Pro can upload a custom series.

The substitution rule matters more than the list. Every optional sleeve names a proxy and, before its own first year, runs that proxy instead — usually the S&P composite, and the 10-year Treasury for the bond sleeves. The app does say so where you pick the sleeve, in amber under the asset row ("Pre-1991 → S&P500 Composite"), and a "use only cycles with full data for every series" switch turns the substitution off. What it does not do is repeat any of that in the results: a portfolio with an International Developed sleeve backtested over the 1929 cycle is running US large-cap returns under an international label, and the Proof view never mentions it.

Integrations

None yet. There is no public API, no broker or bank connection and no MCP server, so every balance is typed in by hand and retyped when it moves; read-only account sync is a roadmap item. What comes out is a two-sheet Excel workbook covering every cycle and every year, on Pro, and a share link that publishes a read-only snapshot of a plan with names and account labels redacted by default.

Limitations

• The free tier is a demonstration, not a tool. One saved simulation, and none of the tax modelling. Budget 10 dollars a month or use the predecessor.
• Optional sleeves substitute another series before their start year — flagged under the asset row where you choose it, never again in the results. The honest history for anything but US large-cap and Treasuries is far shorter than 1871.
• Overlapping cycles are not independent trials. A 35-year plan runs 122 windows that share most of their years, so a success rate is not a probability.
• Monte Carlo runs per asset, not across the portfolio. Correlated whole-portfolio returns are on the vendor's own roadmap, which means success rates from that mode ignore diversification.
• Non-US support stops at the tax brackets and the account wrappers. The healthcare, Medicare and required-distribution modelling is US statute and does not travel.
• Closed source, one developer, two products. There is no repository and nothing to self-host. The author has said plainly, on a public forum in August 2026, that focusing on FIREproof is why cFIREsim's market data sat unrefreshed for some twenty months — which is the honest shape of the key-person risk in paying for this one.

Alternatives

cFIREsim is the same author's earlier and free tool, and remains the right answer if taxes are not the question. FI Calc covers withdrawal strategies against the same historical cycles, free and without an account. Boldin and ProjectionLab are the closest paid competitors for a whole-life plan with US tax modelling, both sold as annual plans rather than by the month. Portfolio Visualizer is the one to reach for if the real question is the allocation rather than the drawdown.