Explainer
What is bar replay, and why does forward-only replay matter?
Bar replay shows past price one candle at a time with later candles hidden; forward-only replay keeps look-ahead out of what you practise.
Updated
The short answer
Bar replay is a way to practise on history as if it were live: the chart ends at a chosen moment, and you reveal later candles only by stepping. Forward-only replay adds one more rule: you cannot move backward, so a decision you have seen the result of cannot be redone.
The point is to avoid look-ahead, which means using information you would not have had at the time. This walkthrough uses ES on Tuesday 2025-06-10 (contract ESM5) in the Trade Lab.
- Bar replay
- A tool that plays past data one bar at a time, hiding the bars after the current one.
- Look-ahead
- Using a later price, or a chart that already contains it, to judge an earlier decision.
See it work
The chart ends at the cursor
A new replay shows history up to the current candle and nothing after it. The top line gives the session, the Eastern time and the last price.
Try to look ahead
Drag the chart to the left to look for the future. There is nothing to find: the server never sends a bar later than the current one, so the space to the right stays empty.
Decide before you step
Write the decision in the journal, here Long, before you press Step. The server stamps the time and the last close on the entry.
Step, then compare
Press Step. The entry is now locked, so you cannot improve it with what you have just seen. Your next thought goes in a follow-up entry.
Why forward-only matters
A normal chart shows everything. It is easy to look at it, see where price went and call it obvious. A replay that cannot go backward removes that option. Every decision is made on the information of its own moment.
It also matters for testing. A rule judged on a chart that shows the outcome tends to look better than it would live, which is the same error as look-ahead bias in code. Forward-only replay is the manual version of avoiding it.
Jumps ask you to confirm because they reveal many candles at once and cannot be taken back. Pressing Step or a jump also pauses Play, so the pace stays yours.
Forward-only replay keeps the decision honest: you commit to what you can see, then you step.
What the Trade Lab does not do
The Trade Lab replays history. It has no live data, and no real order is ever placed. Paper fills are modelled on 1-minute bars, so they approximate real fills.
Each account can read 40 distinct 1-minute sessions and start 5 new runs per UTC day, and it needs a verified email. The Lab itself is free.
Questions and answers
What is bar replay?
A tool that shows historical price one bar at a time, with the bars after the current one hidden, so you can practise decisions as if live.
What is look-ahead bias?
Judging a decision with information from after it was made, such as a chart that already shows the outcome.
Can I rewind in the Trade Lab?
No. A replay only moves forward. You can start a new run on the same date and begin again.
Is a replay the same as a backtest?
No. A backtest runs a rule over many trades at once. A replay is you deciding one candle at a time. Our explainer compares them.
Can I see data from after the cursor with a tool or a trick?
No. The bars after the cursor are not sent to the browser at all.