The Securities and Exchange Board of India (SEBI) introduced a Closing Auction Session (CAS) to change how closing prices are determined [1].

This shift is significant because it alters the valuation process for exchange-traded funds (ETFs), index funds, and arbitrage funds. By moving away from a time-weighted average, the regulator aims to provide a more transparent price discovery mechanism and minimize end-of-day volatility [1], [2].

The new system became effective Aug. 3, 2026 [1]. Under the previous methodology, the closing price was calculated based on the average of trades from the last 30 minutes of the session [1]. The CAS replaces this with a 20-minute window from 3:15 p.m. to 3:35 p.m. [1]. During this period, the exchange pools all buy and sell interests into a single price point [2].

For investors in index funds and ETFs, the change is expected to reduce tracking error, the difference between the fund's net asset value and the actual index it follows [1]. Arbitrage funds, which rely on the spread between cash and futures markets, will also see a reshaping of their operational dynamics due to the new pricing structure [2].

Market analysts have already noted the effects of the transition. Sachin Gupta, vice president of research at Choice Broking, said, "The CAS settlement methodology has noticeably impacted market opening dynamics, especially the divergence between the Sensex and Nifty" [2].

The implementation applies to the National Stock Exchange (NSE) and the Bombay Stock Exchange (BSE) [1]. By concentrating liquidity into a single auction, SEBI intends to prevent price manipulation that can occur during the final minutes of trading [2].

“The CAS settlement methodology has noticeably impacted market opening dynamics”

The transition to a Closing Auction Session aligns India's market closing process with global standards used in many developed economies. By eliminating the 30-minute averaging period, SEBI is reducing the window for 'marking the close,' where large trades are timed to influence the final price. This creates a more accurate benchmark for fund managers, which should theoretically result in more precise valuations for passive investment products.