In a confusing sight for many traders, India’s benchmark indices – BSE Sensex and NSE Nifty 50 – closed the Monday session with a big difference in their gains. While the BSE Sensex rose 0.7% to end at 78,639.03, the Nifty 50 jumped 1.6% to close at 24,774.30.
Industry experts attributed the difference to the fact that stock exchanges introduced a new way of deciding the official closing price of stocks that have futures and options (F&O) contracts. It is the direct result of the new Closing Auction Session (CAS) introduced on August 3, 2026.
The new CAS replaces the earlier Volume Weighted Average Price (VWAP)-based closing mechanism for F&O-eligible stocks. It has been specifically designed to prevent end-of-day price manipulation.
Under the new CAS system, normal trading in F&O stocks ends at 3:15 pm. Instead of averaging prices over the last 30 minutes, the exchange collects all buy and sell orders into a blind auction. A single closing price is determined where the maximum possible quantity can be matched.
Between 3:28 PM and 3:30 PM, the exchange randomly decides the exact second when order collection stops. This prevents last-second spoofing or large institutional order manipulation.
The auction begins with reference to the 3:00 PM–3:15 PM VWAP. Further, orders outside the permitted ±3% price band are automatically rejected.
The Nifty50 consists entirely of F&O-eligible stocks. Since all these stocks shifted to the new CAS pricing mechanism simultaneously, the index reflected the new auction-based closing prices. This created temporary differences in index behaviour and unusual closing prints during Monday’s transition. These differences are expected during the initial implementation period.