Summary of this article
Sebi has changed how F&O stock closing prices are calculated
Experts say the market is adjusting to the new system
NSE expects participation to increase as traders get familiar with CAS
The fag-end jump in the NSE Nifty's official closing level on August 3, 2026 caught many market participants off guard. The benchmark appeared to surge nearly 200 points after the regular trading session had ended, prompting confusion over whether fresh trades had taken place after 3:15 PM and raising questions over the Securities and Exchange Board of India’s (Sebi’s) newly introduced Closing Auction Session (CAS).
At 3:15 PM on August 3, when the regular trading session ended, the Nifty was trading around 24,573. But when the official closing price was determined through the new closing auction mechanism, the index settled at 24,774.30, taking its gain for the day to 390.70 points, or 1.60 per cent. The Sensex, in comparison, ended the session 544.39 points, or 0.70 per cent, higher at 78,639.03, making the gap between the two benchmark indices unusually pronounced.
A day later, the confusion had not entirely faded. On August 4, the Sensex ended 210.08 points, or 0.27 per cent, lower at 78,428.95. The Nifty, meanwhile, fell 159.40 points, or 0.64 per cent, to close at 26,614.90, as market participants continued to assess how the new closing auction mechanism could influence end-of-day price movements.
NSE Steps In After Confusion
Following the market close on August 3, the National Stock Exchange (NSE) issued a clarification explaining that there was no continuous order matching between 3:15 PM and 3:30 PM under the new framework. During this period, the index shown on trading screens remains unchanged because trades are not executed. Instead, exchanges continuously calculate indicative equilibrium prices, which are displayed alongside market quotes. The official closing price is determined only after order matching takes place once the auction window ends.
The clarification came after many traders interpreted the sharp jump in the Nifty’s official close as a sudden burst of buying after market hours.
What Changed Under CAS
From August 3, closing prices of stocks with listed derivatives are no longer based on the volume-weighted average price (VWAP) of trades executed during the final 30 minutes of trading.
Instead, these stocks now go through a separate 20-minute CAS between 3:15 PM and 3:35 PM, where buy and sell orders are collected and matched to discover a single equilibrium price.
The official closing price is the level at which the maximum executable trading volume can be matched. Market orders receive priority over limit orders.
Only futures and options (F&O) stocks are covered under the new closing auction system. For the rest of the cash market, closing prices continue to be calculated using the existing VWAP method. Equity derivatives, meanwhile, remain open until 3:40 PM.
Adjustment Phase Or Genuine Volatility
The late-session swings in benchmark indices after the introduction of CAS have fuelled concerns that the new mechanism could make markets more volatile. However, market analysts say the first few trading sessions are more reflective of an adjustment period than a flaw in the system.
Piyush Jhunjhunwala, founder and CEO of Stockify, a Sebi-registered broker, said the initial confusion should not be mistaken for evidence that the market has become more unstable.
“The implementation of CAS represents a significant shift in how closure prices are established in Indian stock exchanges. Though the first few days did raise confusion among many in the markets along with increased volatility in indices, it is still too early to say that the system has made the markets more unstable,” he said.
According to Jhunjhunwala, structural market reforms typically go through an adjustment period before participants adapt their trading behaviour. He said CAS is designed to improve the accuracy of closing prices by matching buy and sell orders through an auction, thus reducing the scope for end-of-day price distortions and providing more reliable benchmarks for index calculations, mutual fund valuations and derivatives settlement.
He, however, acknowledged that heightened activity during the auction could temporarily increase volatility, particularly in heavily traded stocks.
"In the short run, the substantial activity in the closing session can lead to greater volatility, particularly in the shares with huge trading volumes, but eventually, the increased engagement of the players will stabilise the situation,” he added.
Jhunjhunwala further said that investors should view CAS as “a long-term reform process of the market, which aims at ensuring price discovery and integrity of the market”, rather than as a source of market chaos.
Why Sebi Changed The System
The confusion over the new closing mechanism has overshadowed the problem Sebi was trying to solve. According to market participants, the regulator’s objective was to make closing prices less vulnerable to large end-of-day trades and strengthen the integrity of price discovery.
Vedant Gupte, co-founder and CEO of the Sebi-registered investment platform Trackk, said closing price has far greater significance than many investors realise because it determines derivatives settlement, the net asset value (NAV) of mutual funds, portfolio valuations, and index rebalancing.
“Every trading day ends with one number that matters more than the rest: the closing price,” Gupte added.
According to Gupte, under the earlier system, the final 30 minutes of trading had increasingly become vulnerable to large orders that could influence the closing price, especially during MSCI index rebalancing.
Quoting Sebi's own analysis, Gupte said volatility during the final half hour had reached as much as 3.30 times the level seen earlier in the trading session on MSCI rebalancing days, while foreign passive funds underperformed their benchmarks by an average of 3.39 basis points.
He said the auction mechanism addresses that weakness by pooling all buy and sell orders into a single order book and discovering one equilibrium price instead of averaging scattered trades.
“The old system took an average. The new one holds an auction,” he further said.
Confusion Versus Chaos
Gupte said the anxiety visible on D-Street reflects operational changes rather than a flawed market structure. He said the design of CAS is intended to reduce end-of-day volatility rather than increase it, while the confusion stems from the market now closing at different times for different segments. Continuous trading in F&O stocks ends at 3:15 PM, the auction concludes around 3:35 PM and derivatives trading continues until 3:40 PM.
“What Dalal Street is actually feeling is confusion, not chaos,” Gupte added.
He nevertheless cautioned that traders may shift their activity to an earlier part of the session, meaning volatility may not disappear but simply move to a different time. “There is one honest caveat. Volatility has not been switched off. It may simply have moved earlier, as traders rush to exit before the shortened window,” he further said.
According to Gupte, the first monthly derivatives expiry and the next major index rebalancing under CAS will offer a better assessment of whether the new framework has achieved its objective.
NSE Sees Encouraging Participation
The exchange, in its clarification, also said participation in the first CAS was encouraging. According to NSE, 515 trading members placed orders for 56,773 unique Permanent Account Numbers (PANs) on the first day of implementation, exceeding participation in the pre-open call auction session despite CAS being introduced only this week. The exchange said participation is expected to improve further as market participants become familiar with the new system.
