Compare Sims is the surface where two or more saved simulations sit next to each other so you can see the same metrics on the same screen. It's the right tool for "should I retire one year earlier?" or "what if I shift this 401k contribution to a Roth?". It's the questions where you need real two-column visibility, not a single overlaid number.

What's diffable side-by-side

• Success rate - the success metric for each sim, lined up so you can read the delta directly.
• Ending-balance distribution - the min/max/mean chart for each sim, so you can compare the spread of outcomes, not just the headline number.
• Year-by-year tax - open any row in the year-by-year analysis and see the same tax breakdown for each loaded sim. Useful for spotting bracket changes between two strategies.

Recommended workflow: clone and tweak

The cleanest way to set up a comparison is to start from a known baseline:

1. Open the simulation you want as your baseline.
2. Use the duplicate-sim button in the sidebar to create a copy.
3. Change exactly one thing in the copy like retirement year, account allocation, or a Roth conversion adjustment.
4. Open Compare Sims and load both. The diff isolates the single thing you changed.

What's not diffable yet

A few surfaces remain single-sim only. Plan Diagnostics runs against the actively-selected simulation, not the comparison set, so use the Open in Proof shortcut on a row to inspect diagnostics for that specific sim. Detailed adjustment-by-adjustment diffs (a true structural input diff) are not surfaced here. For now, treat Compare Sims as an output-comparison tool and rely on the Inputs tabs for input diffing.