FIREproof's built-in asset classes (S&P 500, 10yr Treasury bonds, Cash) cover most plans. When you need something the historical dataset can't model, such as gold, Bitcoin, an emerging-markets sleeve, or a small-cap tilt, you can define a Custom Asset and use it inside any account's asset allocation.

When to add one

• True diversifiers - assets with low or negative correlation to the S&P 500 (gold, long Treasuries, BTC). The default cycle dataset doesn't carry these, so a custom asset is the only way to surface that diversification benefit.
• Sleeves the engine doesn't ship - small-cap, international developed, emerging markets, REITs. The Starter Templates panel includes preset values you can add with one click.
• Custom equity tilts - your own mean / volatility assumption when you don't want to ride the S&P shape.

How the historical data is consumed

Custom assets are Monte Carlo generated, not historical replay. You pick a shape (S&P 500, Bitcoin, or 10yr Bonds) that controls the texture. Calm streaks vs. stormy streaks, fat tails, asymmetry. The engine simulates paths with that shape, then rescales each path so the long-run mean and volatility hit your inputs:

r̂ₜ = (rₜ − r̄) / s × σ_target + μ_target

On top of that, the Correlation with S&P 500 setting decides how the asset moves with stocks during crashes. Setting correlation makes the asset realistic during market drawdowns; leaving it null falls back to independent Monte Carlo, which can produce unrealistically smooth diversification.

Expect extreme highs and lows

Custom assets are generated, not replayed and that means the paths you see in Proof can drift much further from the median than anything in the real historical record. Plug in a high expected return with a high volatility (BTC at 30% / 70%, for example) and across thousands of cycles you will see runs that compound into eight- or nine-figure ending balances, alongside runs that go to zero. Those tails are a property of the math, not a forecast.

How it interacts with rebalancing

Once added to the manager, a custom asset shows up as a new row in the Asset Allocation form on every account. Use the percentage column to give it a target weight. The rebalancer treats custom assets exactly like the built-ins: At year-end, holdings are nudged back toward the target allocation, factoring in the custom asset's simulated returns for that year.