New 20-minute CAS mechanism begins with strong participation from 515 trading members and orders linked to 56,773 unique PANs
KD NEWS SERVICE
MUMBAI, Aug 4: The National Stock Exchange of India Limited (NSE) on Monday introduced its new Closing Auction Session (CAS) in the cash segment, ushering in a significant change in the manner in which closing prices of stocks with derivative contracts are determined.
Implemented under the guidance of the Securities and Exchange Board of India (SEBI), the new mechanism is designed to promote greater transparency, fairness and robust price discovery at the close of the trading day. The first day of the new system witnessed encouraging participation, with 515 trading members placing orders for 56,773 unique PANs, a response NSE described as particularly strong for the inaugural session.
The exchange said the new mechanism has been introduced in a phased manner, beginning August 3, 2026. Under CAS, the closing price of stocks in the cash segment on which derivative contracts are available will be determined through the closing auction mechanism. For other securities in the cash segment, the closing price will continue to be based on the volume-weighted average price (VWAP) of trades executed during the final 30 minutes of the continuous trading session.
The move assumes significance because closing prices play a critical role across India’s capital markets. They are used not only as reference points for investors and market participants but also in the valuation and settlement of derivative contracts. By introducing a structured auction process at the end of the trading day, the new framework seeks to make price formation more transparent and less susceptible to distortions arising from thin trading or isolated transactions near the market close.
A structured 20-minute closing process
The CAS is conducted as a separate 20-minute session from 3:15 pm to 3:35 pm on all trading days.
The first five minutes, from 3:15 pm to 3:20 pm, are dedicated to the calculation and dissemination of the reference price and the transition from the continuous trading session to the closing auction.
This is followed by a five-minute order-entry period from 3:20 pm to 3:25 pm, during which both market and limit orders can be entered. The subsequent five minutes, from 3:25 pm to 3:30 pm, are reserved for the entry of limit orders. During this phase, market orders cannot be modified or cancelled.
The final two minutes—from 3:28 pm to 3:30 pm—incorporate a random closure mechanism. The order-entry session can therefore close randomly at any point during these final two minutes, making the exact end of the order-collection phase system-driven rather than predictable.
Once the random closure takes place, order matching follows.
The equity derivatives segment, meanwhile, continues to operate until 3:40 pm on all trading days.
How the reference price is determined
Under the new mechanism, the reference price of a stock entering CAS is determined on the basis of the VWAP of trades executed in that stock between 3:00 pm and 3:15 pm.
If no trade takes place in the stock during that period, the last traded price (LTP) of the stock during the day becomes the reference price for the closing auction.
In cases involving a corporate action, the previous day’s closing price becomes the adjustable reference price.
The applicable price band for CAS is plus or minus 3 per cent from the reference price. The price band for stock futures between 3:15 pm and 3:40 pm is aligned with the price band applicable during the closing auction.
The exchange has also specified that only limit orders and market orders are eligible for computation of the equilibrium price, while certain order types, including Iceberg orders and Stop Loss orders, are not permitted during CAS.
Equilibrium price at the heart of the mechanism
The closing price under CAS is determined through an equilibrium price mechanism, with the equilibrium price defined as the price at which the maximum possible volume of orders can be executed.
Market orders receive priority over limit orders in this process.
Another important feature is the treatment of unexecuted orders from the continuous trading session. NSE said all unexecuted limit orders from the continuous trading session are carried forward to CAS, with exceptions including Stop Loss orders, Iceberg orders and orders carrying prices beyond the price bands applicable to the closing auction.
Limit orders carried forward from the continuous trading session retain a higher time priority than limit orders entered directly during CAS.
However, if a carried-forward order is modified during CAS, its time priority changes.
Implications for derivatives settlement
The new closing mechanism also has implications extending beyond the cash market.
NSE said settlement prices for stock derivatives and index derivatives will continue to be calculated by the clearing corporations. For index derivatives, the settlement price is the closing price of the underlying index on the expiry day, while the closing price of the underlying index is based on the closing prices of its constituent stocks.
For stock derivatives, the clearing corporations will determine settlement prices based on the volume-weighted average of the closing prices of the stock across stock exchanges.
The framework therefore seeks to create a more systematic bridge between end-of-day price discovery in the cash market and the valuation and settlement processes in the derivatives market.
NSE cautions against misreading the index graph
The exchange has also sought to clarify how investors should interpret index movements displayed during the CAS window.
According to NSE, there is no sudden change in the index at 3:30 pm. Rather, the order collection, cancellation and matching process follows a defined sequence. Since there is no continuous order matching between 3:15 pm and 3:30 pm, the index value displayed during that period remains constant because it is based on traded values.
At the same time, indicative values displayed alongside the quotes on the exchange website are continuously derived from equilibrium prices calculated during the CAS period.
The distinction is important because the visual representation of the index during the transition from continuous trading to auction-based price discovery could otherwise be mistaken for an abrupt market movement.
Separate order books may produce different prices
Another important feature highlighted by NSE is that the two exchanges maintain separate order books for the CAS, just as they do for continuous trading.
Consequently, the closing prices of individual stocks can differ between the exchanges. Since index values are calculated using the prices determined for individual constituent stocks, the resulting index values can also vary.
NSE pointed to historical evidence from pre-open call auctions to underline the possibility of such differences. Its study of opening prices based on pre-open call auction sessions during 2014–2024 found more than 8.5 lakh scrip-days in which the difference in opening prices exceeded 2 per cent.
The exchange said this historical experience provides useful context for understanding why price differences between exchanges during auction-based mechanisms should not automatically be interpreted as anomalies.
Risk management framework remains in force
The existing cash-market risk management framework will continue to apply during CAS.
Orders placed during the closing auction are subject to the applicable margin requirements at the order level, with an exception for limit orders carried forward from the continuous trading session.
However, if such carried-forward limit orders are modified during CAS, they become subject to the applicable order-level margin requirements.
The provision reinforces the exchange’s emphasis on maintaining risk controls even as the market transitions from continuous trading to auction-based price discovery.
Strong first-day participation
Perhaps the most significant early indicator of the mechanism’s acceptance was the level of participation recorded on its very first day.
NSE said 515 trading members participated in the inaugural CAS and placed orders for 56,773 unique PANs.
The participation was higher than that recorded in the day’s pre-open call auction, where 403 trading members participated and placed orders for 42,822 unique PANs.
NSE noted that the pre-open session itself has been operational for more than a decade, making the level of participation in the newly introduced CAS particularly noteworthy.
The exchange described the first-day participation as “very good” and said it expects participation to mature and increase with time.
The initial response could therefore offer an early indication that market participants are willing to adapt quickly to the new architecture for closing price discovery.
A new chapter in India’s market microstructure
The introduction of CAS represents more than a procedural adjustment to the final minutes of the trading day. It marks a deliberate move towards a more structured and transparent mechanism for determining closing prices in securities where derivatives are actively linked to the underlying stocks.
The closing price is one of the most closely watched numbers produced by a stock exchange each day. It influences portfolio valuations, derivative settlements, benchmarks and a wide range of investment and risk-management decisions. Consequently, the mechanism through which that price is arrived at is of considerable importance.
By replacing the closing-price determination for specified stocks with a defined auction process—complete with a reference price, controlled order-entry windows, random closure, equilibrium-price determination and established risk-management safeguards—NSE is seeking to make the final phase of trading more orderly and resilient.
The strong participation on day one provides an encouraging start. As traders, brokers, institutional investors and other market participants become more familiar with the new process, the closing auction could increasingly become an integral part of India’s evolving market structure.
