SEBI May Partly Reverse CAS Rules After Market Pushback: What the Change Means for Traders
India’s capital markets regulator, the Securities and Exchange Board of India (SEBI), is reportedly preparing to modify an important part of its recently introduced Closing Auction Session (CAS) framework after receiving substantial feedback from market participants.
The proposed change could significantly affect how settlement prices for stock derivatives are calculated. According to a Reuters report published on October 5, SEBI is likely to stop using the closing auction mechanism for determining derivatives settlement prices for at least one year. Instead, the regulator is expected to rely on the volume-weighted average price (VWAP) of trades during the final 30 minutes of the trading session.
However, this does not mean SEBI is abandoning the CAS system altogether. The closing auction is expected to remain relevant for determining end-of-day prices for certain underlying stocks, particularly those with lower liquidity.
Why Did SEBI Introduce the Closing Auction Session?
SEBI introduced the Closing Auction Session in the equity cash segment with the objective of improving the way closing prices are discovered.
The mechanism was designed to bring multiple buy and sell orders together during a dedicated closing period and establish a common closing price through an auction process. The regulator has argued that such a mechanism can improve price discovery and provide a more robust closing price, particularly in a market increasingly influenced by institutional and passive investment strategies.
The CAS framework came into operation in August 2026. However, its implementation quickly generated concerns among traders, brokers and other market participants, particularly around the way the closing price affected derivative contracts on expiry days.
Sharp Derivative Price Movements Became a Major Concern
One of the biggest issues was the connection between the CAS-determined closing price of an underlying stock and the settlement price used for related derivatives.
According to reports, the new mechanism was associated with sharp movements in derivatives prices around expiry sessions. Market participants also pointed to instances where prices during the CAS period differed significantly from levels seen during regular trading.
These concerns prompted SEBI to review the methodology rather than simply continuing with the original framework.
In September, SEBI formally indicated that it would examine the methodology used to determine settlement prices for derivative contracts following the initial experience with CAS. The regulator also sought feedback from market participants on possible changes.
30-Minute VWAP Could Become the Temporary Alternative
The most significant reported change is the possible return to a VWAP-based methodology for derivatives settlement.
Under the reported proposal, the settlement price would be calculated using the volume-weighted average price during the final 30 minutes of trading instead of relying on the closing auction price.
VWAP gives greater weight to transactions based on the quantity traded at different prices. In practical terms, this means the settlement price would reflect trading activity across a period rather than being determined primarily through a short auction process.
For traders in futures and options, this distinction can be important because the settlement price directly affects the final value of contracts reaching expiry.
SEBI is reportedly considering this approach for at least a year, giving the market more time to assess how the CAS system functions and whether the concerns surrounding derivative settlement can be addressed.
CAS Could Still Continue for Less-Liquid Stocks
The reported change should not be interpreted as a complete reversal of the CAS framework.
For underlying stocks in the less-liquid cash market, the closing auction is expected to continue being used to establish the end-of-day price. This indicates that SEBI may be separating two issues: determining an appropriate cash-market closing price and determining the settlement price of derivatives.
That distinction could become an important part of the regulator’s approach going forward.
Instead of removing CAS completely, SEBI appears to be considering whether derivatives require a different settlement methodology because of the trading behaviour and price sensitivity observed around expiry.
SEBI Received Extensive Market Feedback
The regulator’s review has also highlighted the importance of consultation when implementing major market-structure changes.
SEBI said it received thousands of suggestions in response to its consultation process. Reports have placed the number of responses at more than 3,500, while SEBI separately indicated that it had received around 20,000 suggestions to tweak the rules through its broader feedback process.
The volume of feedback demonstrates how closely the CAS changes are being watched by market participants.
Another issue discussed during the consultation involved the publication of indicative index values during the CAS period. Feedback reportedly suggested that removing the indicative index value could reduce transparency because sophisticated market participants could reconstruct the value independently.
SEBI is therefore also considering how much information should remain visible to traders during the auction process.
What It Means for Investors and Traders
For ordinary long-term equity investors, the proposed change may have limited direct impact. The more immediate implications are for participants trading futures and options, particularly those holding contracts close to expiry.
A shift to a 30-minute VWAP-based settlement could reduce the direct influence of a short closing auction on derivative settlement prices. It may also provide traders with a longer period of actual market activity from which the settlement value is calculated.
At the same time, investors should remember that the proposal is based on reported plans and that the final regulatory framework remains subject to SEBI's formal decision.
What Happens Next?
SEBI is expected to implement the reported changes by the end of October, although the final details will depend on the regulator's formal framework.
The broader development shows that India’s market structure is continuing to evolve as trading becomes increasingly sophisticated. SEBI’s challenge is to balance accurate price discovery, transparency, market stability and the practical needs of traders.
For investors, the key takeaway is simple: the CAS mechanism is not necessarily being scrapped. Instead, SEBI appears to be considering a targeted adjustment to the way derivative settlement prices are calculated, while retaining the auction mechanism for parts of the underlying cash market.
That approach could allow the regulator to address concerns raised during the early implementation period without completely abandoning the broader objective of improving closing-price discovery.
