Summary:
SEBI may pause the Closing Auction Session for derivatives settlement and restore the 30-minute VWAP methodology by October-end 2026. The proposed change follows concerns over expiry-day volatility and could alter how final settlement prices are determined for derivatives.
What is SEBI planning to change in CAS?
India’s derivatives market could see another major change within weeks. The Securities and Exchange Board of India (SEBI) is likely to stop using the newly introduced Closing Auction Session (CAS) for determining derivatives settlement prices and return to the 30-minute volume-weighted average price (VWAP) methodology.
The proposed reversal comes barely two months after CAS was rolled out on August 3, 2026, following concerns over sharp price movements around derivatives expiry. A final regulatory circular, however, is still awaited.
Why SEBI Introduced the Closing Auction Session?
Before August, the closing price of a stock was calculated using the VWAP of trades executed during the final 30 minutes of continuous trading. SEBI replaced this mechanism for eligible F&O stocks with CAS, where orders are brought together during a separate auction to arrive at a single equilibrium closing price.
The objective was to concentrate liquidity near the market close and create a more transparent closing-price discovery process. Closing prices are particularly important because they influence index calculations, derivatives settlement and other end-of-day valuations.
CAS was introduced from August 3, 2026 for stocks with futures & options contracts. Its implementation also changed how the closing values of indices such as the Nifty 50 were determined. On the first day of implementation, the Nifty 50 rose +1.6% to 24,774.30, while the Sensex gained +0.7% to 78,639.03, highlighting an unusual divergence during the transition to the new closing mechanism.
What Could Change Under SEBI's Proposed CAS Framework?
According to the latest reports, SEBI is considering suspending the use of CAS for determining derivative settlement prices for at least one year.
The key proposed framework is:
| Parameter | Likely Framework |
| Derivatives settlement price | Last 30-minute VWAP |
| CAS for derivatives settlement | Likely paused for ≥1 year |
| Less-liquid cash market stocks | CAS may continue |
| Expected implementation | By October-end 2026 |
| Public feedback received | ~20,000 comments |
Under the proposed approach, the volume-weighted average price during the final 30 minutes of trading would once again become the reference for derivatives pricing.
Importantly, this does not necessarily mean that CAS itself will disappear from the Indian market. The auction mechanism may continue to determine end-of-day prices for stocks where liquidity in the regular cash market is relatively low.
What Is the Closing Auction Session?
The Closing Auction Session (CAS) is a separate auction mechanism introduced by SEBI for eligible stocks with futures and options contracts.
Under the mechanism, buy and sell orders are brought together during a dedicated closing auction to determine an equilibrium closing price.
Before CAS was introduced, eligible stocks used the volume-weighted average price (VWAP) of trades executed during the final 30 minutes of continuous trading for calculating the closing price.
Closing prices are important because they can influence:
- Index calculations
- Derivatives settlement
- Mark-to-market calculations
- Portfolio valuations
- End-of-day reference prices
Why Did SEBI Introduce CAS?
SEBI introduced CAS from August 3, 2026 as part of changes to the equity cash-market closing mechanism.
The objective was to improve price discovery by concentrating orders around the market close and establishing an equilibrium price through an auction mechanism.
The change also affected how closing values for major indices such as the Nifty 50 were determined.
However, concerns subsequently emerged around sharp price movements near derivatives expiry, prompting SEBI to review the settlement methodology.
Will SEBI Stop CAS Completely?
Not necessarily.
The reported proposal is a partial rollback, rather than a complete withdrawal of CAS.
SEBI is considering pausing CAS specifically for determining derivatives settlement prices, while the auction mechanism could continue for certain less-liquid stocks in the cash market.
Therefore, the proposed change should be described as a CAS rollback for derivatives settlement, rather than the complete removal of CAS from Indian markets.
Expiry-Day Volatility Triggers Review
The review follows concerns that the auction-based mechanism created unusually sharp movements in underlying stocks and, consequently, in derivative prices around expiry.
SEBI formally announced a review of derivative settlement methodology following the CAS rollout and released a consultation paper on September 12, 2026 covering the closing auction, market timings and derivatives settlement.
The consultation generated an unusually large response, with around 20,000 comments and suggestions submitted on the proposed changes.
Reports also indicate that the existing closing schedule for cash and derivatives markets is likely to be retained, while greater clarity around indicative index values during the auction process may be maintained rather than reducing the information available to investors.
What Does the CAS Change Mean for F&O Traders?
A return to the 30-minute VWAP method would spread the settlement-price calculation across a broader trading window instead of making it heavily dependent on a concentrated closing auction.
For derivatives participants, particularly on expiry days, the change could alter how final settlement prices behave and reduce the influence of abrupt price moves during a short closing window.
For now, the reported changes remain subject to SEBI’s final framework. If implemented by October-end, they would represent a partial rollback of one of the most significant changes made to India’s market-closing mechanism in 2026.
CAS vs 30-Minute VWAP: What Is the Difference?
| Feature | Closing Auction Session | 30-Minute VWAP |
|---|---|---|
| Price discovery | Separate auction | Continuous trading |
| Reference period | Dedicated closing auction | Final 30 minutes |
| Order concentration | High during auction | Spread across trading period |
| Derivatives settlement | Proposed to be paused | May become reference again |
| Cash-market use | May continue for selected stocks | Earlier methodology |
The key difference is how the closing reference price is discovered: CAS concentrates orders into an auction, while VWAP derives the price from trades executed during the final 30-minute continuous trading window.
When Could the New SEBI CAS Framework Take Effect?
The reported proposal could be implemented by October-end 2026.
However, this timeline should be treated as expected rather than confirmed until SEBI issues its final regulatory circular.
The final framework will determine:
- Effective date
- Duration of the proposed CAS pause
- Stocks/contracts covered
- Treatment of cash-market CAS
- Derivatives settlement methodology
- Any transition arrangements
Will CAS Continue for Cash-Market Stocks?
Based on the reported proposal, CAS may continue for less-liquid cash-market stocks.
This distinction is important because the proposed change is not necessarily a complete reversal of the August 2026 closing-auction framework.
The main proposed change concerns derivatives settlement prices, while CAS could remain relevant for selected stocks in the cash segment.
What Does the SEBI CAS Review Mean for the Stock Market?
The proposed change could affect the way closing prices and derivatives settlement values are determined in India's equity markets.
For investors and traders, the most important implication is that the derivatives settlement methodology could move back from CAS to 30-minute VWAP.
The change could also influence how traders assess expiry-day price movements and settlement risk.
However, the broader impact will become clearer only after SEBI publishes the final framework.
