Two different questions
Backtesting asks a question about history: given these rules, what would have happened between then and now? The answer arrives in seconds because every price it needs already exists.
Paper trading asks a question about the present: given these rules, what is happening this week? The answer arrives at the speed of the market, because the days have not happened yet. Your plan buys and sells on today's real prices using cash that is not real.
Neither replaces the other. A plan that survived five years of history but falls apart in its first live month has told you something important, and so has a plan that does well in a calm month after failing the history test.
What history is good at showing
History contains the events nobody wants to live through: sharp falls, long flat stretches, sudden reversals. A five year replay puts your plan through all of them in a few seconds.
It is also repeatable. You can run the same test twice and get the same answer, which means you can change one rule, run it again, and see exactly what that change did. Comparing two versions of a plan on identical history is the cleanest way to learn what a rule is worth.
That is the strength: breadth and repeatability. In minutes you can see a plan meet more difficult markets than most people experience in a decade.
What live practice is good at showing
A practice run catches the things history smooths over. Prices move while orders sit. Companies report earnings on a Tuesday and drop 8% by Wednesday. News arrives that no rule anticipated.
It also catches you. Watching a plan sell something you liked, in real time, on a red day, tells you whether you can genuinely leave it alone. A great many people discover during practice that they cannot, which is far cheaper to learn on simulated cash than on savings.
Stax gives six practice slots on the free tier, each trading simulated cash on real market prices, so several versions of a plan can run at once and the market itself can settle which one you were right about.
A worked comparison
Take one plan and run it both ways. The history test reports that it grew $10,000 to $13,850 over five years, fell $2,100 at its worst point, and traded 214 times.
Then it goes into a practice slot on the first of the month. Six weeks later that slot shows $10,190 on a starting $10,000, with nine trades and one holding sold because it fell past the line you set. That is a much smaller sample and it proves far less.
Here is the trap worth naming: those six weeks feel more convincing because you watched them happen. They are not. Nine trades tell you almost nothing about a rule, and one calm month is not evidence. The history test is the stronger information; the practice run is the honesty check on top of it.
The order that works
Test on history first, because it is fast and it kills weak ideas cheaply. If a plan cannot survive five years of real prices, no amount of live practice will rescue it and you have saved yourself weeks.
Then practice the survivors. A plan that passed the history test earns the right to run on simulated cash, where you find out how it behaves in the present and whether you can watch it work without interfering.
Only after both would anyone sensible think about real cash, and in Stax that is a separate deliberate step rather than something that happens by drift.
Doing both in Stax
The same plan runs through the same engine in both directions, so the rules that were tested are the rules that get practised. There is no translation step where something quietly changes.
A sensible first week: describe a plan, run the history test, read the trades, then put it in a practice slot and leave it alone for a month. Checking it daily is the impulse to resist, and noticing that impulse is part of what the exercise is for.