Highlights
- CAS has also triggered unusually large divergences between opening and closing levels, particularly in the NSE Nifty 50.
- NSE Nifty caught Dalal Street off guard on Monday, August 3, surging nearly 200 points in the closing minutes of trading despite the absence of any major trigger.
- Market participants reported significant discrepancies between derivative pricing and the final cash-market settlement, fuelling debate over the new framework.
The first three trading sessions on the NSE and BSE, following the implementation of the new Closing Auction Session (CAS) framework by SEBI, have highlighted a striking shift in how benchmark indices behave between the opening bell and the official close, with the NSE Nifty50 and BSE Sensex frequently ending the day far from where regular cash-market trading suggested they would.
Effective Monday, August 3, the new mechanism replaced the earlier volume-weighted average price (VWAP)-based closing price for F&O stocks with an auction-discovered closing price. While the reform aims to improve price discovery and curb end-of-day price manipulation, the CAS has also triggered unusually large divergences between opening and closing levels, particularly in the NSE Nifty 50.
The phenomenon was visible from Day 1 itself.
Stock market on August 3 – The day CAS was implemented
NSE Nifty caught Dalal Street off guard on Monday, August 3, surging nearly 200 points in the closing minutes of trading despite the absence of any major trigger. However, the sharp late-session rally was not driven by fresh buying momentum, but rather the initial operational impact of the Securities and Exchange Board of India’s (SEBI) newly launched Closing Auction Session (CAS) mechanism for F&O-eligible stocks.
At 3:15 pm, when continuous regular trading drew to a close, the Nifty was hovering near the 24,573-mark. Just minutes later, however, the benchmark’s official closing price was discovered at 24,774.30 through the new closing auction process, lifting its intra-day gain to 390.70 points, or 1.60 per cent. On the other hand, BSE Sensex settled 544.39 points, or 0.70 per cent, higher at 78,639.03.
Stock market on August 4 – The second day of CAS implemention
Tuesday’s trading session amplified the trend. Although the cash market had already shut for CAS-eligible stocks, the combination of the closing auction and weekly Nifty derivatives expiry led to sharp adjustments in the benchmark’s official closing value, surprising many traders who were tracking only regular market prices.
While Sensex jumped 544.39 points, or 0.70 per cent, to settle at 78,639.03, Nifty Nifty climbed 390.70 points, or 1.60 per cent, to end at 24,774.30.
Market participants reported significant discrepancies between derivative pricing and the final cash-market settlement, fuelling debate over the new framework.
Stock market on August 5 – The third day after CAS implementation
The divergence was seen on Wednesday as well. While the Sensex rallied more than 450 points during regular trading, the Nifty remained comparatively subdued, reflecting the continuing adjustment of market participants to the auction-based closing mechanism and differing stock weightages between the two benchmark indices.
On Wednesday, August 5, Sensex gained 152.05 points or 0.19 per cent to close at 78,581. The NSE Nifty went up by 9.75 points or 0.04 per cent to close at 24,624.65.
CAS rollout faces ‘teething issues’
The implementation of the CAS mechanism has triggered concerns among market participants after the first few trading sessions witnessed sharp divergences between regular market prices and official closing prices, particularly in the Nifty 50.
While experts broadly backed the objective of improving price discovery through an auction-based closing mechanism, they said the exchange and regulator should closely examine the initial anomalies to reassure investors that the new system is functioning fairly.
Samir Arora, Founder and Fund Manager at Helios Capital Management, advised the exchange should proactively analyse trading activity during the closing auction to dispel concerns over potential market abuse.
“There might be teething issues and learning issues with the way NSE is conducting CAS but it would enormously help the market and its own credibility if NSE would analyze all the big participants in CAS and analyse their other trades during the day and confirm (formally and in black and white) that they do not see any attempt at manipulation by any market participant,” Arora said in a post shared from his X handle.
Vinod Nair, Head of Research at Geojit Investments Limited, said the overlap of Tuesday’s weekly expiry with the new closing auction mechanism distorted market behaviour and amplified volatility.
“Tuesday’s weekly expiry, combined with the implementation of the new mechanism for determining F&O closing prices, has led to a distortion in market trends. The significant gap between the 3:30 p.m. and 3:40 p.m. closing prices of Nifty stocks and the index, along with the divergence with Sensex, suggests that the new system is not functioning as intended, resulting in heightened price volatility. This has triggered forced square-offs of positions, particularly among retail investors, ahead of the 15 minutes blind derivatives window closing session,” Nair stated.
He further stated that the volatility forced several market participants, especially retail investors, to square off positions before the 15-minute blind derivatives trading window ended.
“These appear to be initial teething issues in the new system and the exchanges and market regulator need to address the discrepancies. Currently, the impact is limited to the F&O segment of trading stocks and main indices. Importantly, these are not fundamental structural concerns and are likely to be brought under control. The broader economic and financial outlook remains solid and does not alter the view of long-term investors. The current volatility is expected to reverse as the exchanges return to their normal operating structure,” he added.
ET NOW Digital – etnownews.com – spoke to Sachin Gupta, Vice President – Research at Choice Broking to understand the CAS framework, its role in closing trading sessions, who will be benefitted from it and other key details.
Here’s the edited excerpts from the interview:
What role did CAS play in Sensex, Nifty closing?
According to Sachin Gupta, Vice President – Research at Choice Broking, the implementation of the CAS settlement methodology had a noticeable impact on today’s market opening, particularly on the divergence between the Sensex and Nifty.
“Since the Sensex opening price is now derived from the previous day’s closing auction prices of its constituent stocks rather than the first traded price, several heavyweight stocks opened closer to their CAS-derived closing levels, reducing the gap-down impact on the index,” Gupta stated.
The Nifty, however, continued to reflect broader market weakness through the opening trades of its constituents, resulting in a sharper decline. “This difference in opening price calculation, coupled with varying stock weightages and constituent overlap, led to the unusual situation where the Sensex appeared relatively flat while the Nifty opened significantly lower,” he added.
Overall, the new CAS framework influenced price discovery at the opening and contributed to the temporary divergence between the two benchmark indices, Gupta said.
What changes with CAS implementation?
Effective August 3, the introduction of the Closing Auction Session (CAS) framework marks a significant shift in the Indian equity market’s price discovery process.
“The closing auction price will gain greater significance, as it will serve as the official reference price for several market functions, enhancing the reliability and consistency of closing prices,” Gupta said, adding the move is expected to improve market efficiency, reduce the possibility of price manipulation near the close, and provide a more transparent benchmark for index calculations, portfolio valuations, ETFs, mutual funds, and institutional trades.
Over time, market participants are also likely to witness higher participation during the closing auction window, leading to better liquidity and more efficient execution of large orders at the end of the trading session. Overall, the introduction of CAS is aimed at strengthening market integrity and making the price discovery process more robust, the analyst stated.
Better price discovery over time
The new framework is designed to improve price discovery rather than introduce additional volatility, Gupta noted.
He said the closing price is determined through a call auction mechanism rather than the last traded price. “During the closing auction window, the exchange collects all buy and sell orders without executing them immediately. It then identifies a single equilibrium price at which the maximum number of shares can be matched while minimizing any unmatched orders. This equilibrium price becomes the official closing price of the stock,” he stated.
If more than one price satisfies the matching criteria, the exchange applies predefined tie-breaker rules to determine the final closing price. “This auction-based process ensures that the closing price reflects overall market demand and supply, making it more transparent, efficient, and less vulnerable to last-minute price manipulation,” Gupta said.
Has the new CAS mechanism improved price discovery, or has it introduced fresh volatility?
While the transition to the new mechanism may initially alter trading behaviour around the close, Gupta believes the new CAS framework will ultimately contribute to lower volatility over the long run.
By determining the closing price through an auction that matches the maximum number of buy and sell orders at a single equilibrium price, the process reflects broader market consensus instead of the last individual trade. This makes the closing price more transparent and less susceptible to end-of-day price manipulation,” he said.
While the initial implementation may lead to occasional adjustments as traders adapt their execution strategies, any short-term fluctuations are expected to be temporary.
Over the medium to long term, higher participation in the closing auction should improve liquidity, enhance the quality of price discovery, and result in more stable and reliable closing prices, Gupta added.
Who stands to benefit the most from CAS?
Gupta believes the CAS is expected to benefit institutional investors, mutual funds, ETFs, index funds, and foreign portfolio investors (FPIs) the most, as these participants often execute large trades at the market close and rely on an accurate closing price for portfolio valuation and benchmarking.
“A transparent auction-based closing price helps reduce execution costs and minimizes the impact of end-of-day price manipulation. Retail investors also benefit indirectly through fairer price discovery, improved market integrity, and more reliable closing prices,” he said.
Overall, CAS enhances confidence in the market by ensuring that the closing price better reflects genuine supply and demand rather than isolated last-minute trades, the analyst stated.
Will CAS ultimately reduce or increase volatility in the long run?
In the long run, CAS is expected to reduce volatility rather than increase it. “By determining the closing price through an auction that aggregates all buy and sell orders, CAS produces a price that better reflects the true market consensus instead of being influenced by a few last-minute trades,” he said, adding this improves the quality of price discovery, reduces the scope for end-of-day price manipulation, and enhances liquidity during the closing session.
Gupta further stated that although there may be some short-term adjustments as market participants adapt to the new framework, the overall impact is likely to be a more stable, transparent, and efficient market with greater confidence in the closing price as a benchmark for trading, valuation, and index calculation.
