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Why your next-day opening no longer matches yesterday's chart — and how to adjust your intraday, scalping and BTST rules for the Closing Auction Session era.
Quick answer: Since 3 August 2026, SEBI's Closing Auction Session (CAS) decides the "official" close of F&O stocks and indices between 3:15 PM and 3:35 PM using a matched auction price — not the last traded price from regular trading. This means Bank Nifty can print a closing price 100–600+ points away from where price action actually stopped at 3:15 PM, and the next session often opens closer to that CAS close, not your last candle. Traders now need separate rules for reading the close, placing intraday stops, deciding BTST, and choosing between option scalping and full intraday holds.
SEBI's Closing Auction Session (CAS) is a dedicated 20-minute window from 3:15 PM to 3:35 PM that decides the official closing price for stocks with active F&O contracts — and by extension, indices like Bank Nifty and Nifty 50 that are built from those stocks. Instead of the closing price being the volume-weighted average of the last 30 minutes of normal trading (the old VWAP method), CAS collects buy and sell orders into a single pool during this window and matches them at one equilibrium price — the price at which the maximum number of shares can trade. That single matched price becomes the day's official close, confirmed a few minutes after 3:30 PM.
This rolled out on 3 August 2026 for Category I stocks (those with F&O contracts). It replaced the earlier method where the last 30 minutes of continuous trading decided the close.
This is the single biggest change for chart-based traders. You now have two different "closes" to think about:
| Term | What it means | Why it matters |
|---|---|---|
| Price Action Close | Where the last regular candle actually stopped before 3:15 PM, based on real continuous trading | This is what your CPR, support/resistance, and pattern levels are still built on |
| CAS Rule Close | The official close fixed by the 3:15–3:35 PM auction, which can be well above or below the price action close | This becomes tomorrow's reference level, previous-day CPR pivot input, and the base for gap-up/gap-down opening |
Real example: On CAS's first live day, Nifty was trading near 24,573 when regular trading ended at 3:15 PM, but the auction fixed the official close at 24,774 — a jump of nearly 390 points that happened with no visible trading on the live chart. Bank Nifty has shown similar post-3:15 PM swings on expiry and high-volume days.
Practical takeaway: when you mark your CPR (Central Pivot Range) or daily pivot for the next session, decide in advance whether you are calculating it off the price action close or the CAS close — and be consistent. Most traders should shift to using the CAS close for pivot calculations, since that is now the exchange's official reference, while still watching the price action close as an intraday support/resistance zone.
Because CAS is now the officially recorded close, it is also the base the market "resets from" overnight — global cues, F&O positioning, and gap calculations all reference the CAS close, not the last traded price you saw on your chart at 3:14 PM. This is why you'll often see Bank Nifty open the next day at a level that looks like a gap from your final candle, even when nothing dramatic happened overnight.
Regular continuous trading effectively pauses for CAS-eligible instruments once the auction window opens. Your last few minutes of "normal" price movement now end at 3:15 PM, not 3:30 PM. Adjust your intraday square-off and trailing stop-loss discipline to this earlier cutoff rather than the old 3:20–3:25 PM habit.
If you use Central Pivot Range or classic pivots, recompute them using the CAS close as the previous day's close input. Continuing to use the pre-auction price action close will misplace your pivot, BC/TC, and R1/S1 zones for the new session.
Since this window is a matched auction and not continuous trading, normal price-action signals (breakouts, candle closes, momentum) don't behave the same way here. Avoid opening fresh intraday positions once CAS begins; manage only existing positions and let the auction complete.
A large gap between price action close and CAS close (as seen in Bank Nifty's post-CAS sessions) often reflects genuine institutional order flow that showed up only in the auction. Use that gap size as an early input into your next-day directional bias, alongside your usual CPR width and narrow/wide CPR reading.
CAS changes the risk profile of the last 20 minutes differently for scalpers and intraday option holders.
| Approach | Before CAS | After CAS (from Aug 2026) |
|---|---|---|
| Option Scalping (last 15–20 min) | Continuous price ticks, tight scalps possible until 3:28–3:29 PM | Underlying stops updating continuously by 3:15 PM; scalping the index into the close is far less reliable — spreads can widen and option premiums can lag or overshoot the CAS move |
| Full Intraday Option Buying | Held and squared off anytime up to 3:30 PM based on live price | Square off core intraday option positions by 3:10–3:15 PM unless deliberately holding through the auction on a strong conviction trade; premium behaviour during CAS is choppier and less liquid |
Rule of thumb: If your option-buying edge depends on reading live tick-by-tick price action, that edge effectively disappears at 3:15 PM now, not 3:30 PM. Shift your scalping session mentally to end 15 minutes earlier.
BTST (Buy Today, Sell Tomorrow) traders rely on the closing price to judge overnight risk and next-day continuation. With CAS in place, the level that matters for your BTST decision has changed.
CAS (Closing Auction Session) is SEBI's new mechanism, effective 3 August 2026, that fixes the official closing price of F&O-eligible stocks and related indices through a 20-minute matched auction from 3:15 PM to 3:35 PM, replacing the earlier last-30-minutes VWAP method.
CAS directly applies to the individual stocks that have F&O contracts, which includes Bank Nifty's constituent banking stocks. Since Bank Nifty is built from these stocks, its official closing level is effectively shaped by their CAS auction prices too.
Because the CAS auction can fix the closing price well away from where regular trading last stood at 3:15 PM, the next session's opening reference shifts to that CAS level, creating what looks like a gap from your last visible candle.
Use the CAS close, since it is now the exchange's official closing price and the base the next day's pivot and CPR levels are effectively built from. Keep the price action close as a secondary intraday reference.
Scalping into the last 15–20 minutes is riskier now because continuous price discovery for CAS-eligible underlyings effectively ends at 3:15 PM. Plan to end scalping activity earlier and avoid fresh scalp entries during the auction window.
Confirm the official CAS close before finalising a BTST position, check the gap between price action close and CAS close as a directional signal, and keep stop-losses slightly wider to account for auction-window volatility.
Join the CPR Brahmastra Strategy Sunday Live Webinar & Mentorship Program to see how Trading Direction adapts CPR levels, intraday rules and options strategy for the CAS era — live, on the chart.
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