Overview
- The Closing Auction Session (CAS) for cash stocks with listed derivatives went live on Monday, Aug. 3, and concentrated late orders into a 20-minute auction that produced a large difference between the 3:15 pm traded levels and the auction-discovered closing value for the Nifty.
- Under the CAS, continuous trading for F&O-eligible stocks ends at 3:15 pm, a reference VWAP is calculated from 3:00–3:15 pm, and the auction runs from 3:15–3:35 pm with specific order windows, a ±3% price band and a random close before matching.
- Exchanges said the apparent late spike was a byproduct of how orders are collected, cancelled and matched in a separate CAS order book and that indicative equilibrium prices are published during the auction to help interpretation.
- Market participants and brokers are adjusting trading and risk systems because closing prices now determine index values, mutual fund NAVs and derivatives settlement, and analysts warn large institutional flows in the short auction window can amplify moves on expiry days.
- The change replaces the previous 30-minute VWAP close for F&O stocks, is being rolled out in phases, and regulators and exchanges will monitor participation and functioning as the market adapts to the new end-of-day process.