New CAS Rules Explained: What Changes for Investors and How They Compare With Previous Rules

SEBI has revised the Closing Auction Session, or CAS, framework, changing market timings and how the final settlement price of expiring futures and options contracts is calculated.

Most changes will take effect on December 7, 2026. Revised rules for carrying unexecuted limit and iceberg orders into the auction will follow on February 8, 2027.

For investors, the biggest change goes beyond trading hours. A stock’s official closing price and the price used to settle its expiring F&O contracts can differ. Understanding this distinction will become important when reading closing prices, tracking positions and calculating expiry payouts.

What Is the Closing Auction Session?

The Closing Auction Session is a separate market session that determines the official closing price of eligible stocks.

During regular trading, buy and sell orders are continuously matched. In a closing auction, orders are collected and then matched at a single price that allows the maximum quantity of shares to trade, subject to the auction’s rules.

Before CAS was introduced on August 3, 2026, stock closing prices were calculated using the volume-weighted average price, or VWAP, of trades during the last 30 minutes of continuous trading. VWAP gives more weight to prices at which larger quantities of shares trade.

CAS initially applied to stocks with futures and options contracts. It replaced this average-based closing price with an auction-based price for those stocks.

One reason for introducing CAS was the growing importance of index funds and other passive investors. These funds benefit from being able to execute transactions at the official closing price, helping them follow their benchmarks more closely.

New CAS Rules Compared With Previous Rules

The latest changes revise the CAS framework introduced earlier in 2026.

The revised timings restore 15 minutes of continuous trading for CAS stocks. However, investors need to distinguish between the closing auction deadline and the trading deadline for their specific derivative contract.

How Will the New Closing Auction Work?

The revised auction has three stages:

• 3:30 pm to 3:31 pm: The exchange calculates the reference price and transitions eligible orders from continuous trading.
• 3:31 pm to 3:36 pm: Investors can enter both market and limit orders.
• 3:36 pm onwards: Only limit orders can be entered. Existing market orders cannot be modified or cancelled, and order entry closes randomly between 3:38 pm and 3:40 pm.

Order matching follows the closure of the order-entry period. The random closing time means investors should avoid assuming they can always place or change an order right up to 3:40 pm.

What Changes for F&O Expiry Settlement?

The revised framework returns expiry settlement to a calculation based on actual trades during the final 30 minutes of continuous trading.

For index derivatives, the settlement index value will use the VWAPs of constituent stocks. For stock derivatives, clearing corporations will use the underlying stock’s VWAP across exchanges during that period. CAS transactions will therefore sit outside this expiry settlement calculation.

This creates an important distinction between the official closing price and the expiry settlement price.

Consider a simplified example. Assume a stock’s final 30-minute VWAP across the relevant exchanges is ₹1,000, but its closing auction produces an official closing price of ₹1,020.

Under the revised framework, the applicable expiry settlement calculation would use the continuous-market VWAP methodology. An investor should therefore avoid assuming that the ₹1,020 closing price automatically determines the derivative’s expiry outcome.

For a call option with a ₹1,010 strike price, that difference could change whether the option finishes in the money. This illustration excludes premiums, charges and delivery obligations.

Why Did SEBI Change the CAS Rules?

The review followed concerns about the interaction between auction prices and derivatives trading near expiry.

SEBI’s September consultation noted that an indicative auction price can change as orders are entered, modified or cancelled. It is a possible execution price based on the auction order book at that moment, rather than a completed trade.

The regulator also highlighted significant movements in certain expiring index options and concerns about traders responding to changing indicative auction values.

The practical issue is straightforward. Near expiry, even a small change in the underlying price can materially affect an option’s remaining value. Linking that outcome to an evolving auction adds another factor for traders to assess.

The revised approach separates the auction’s role in setting the official close from the calculation used for expiry settlement.

What Happens to Pending Orders?

From February 8, 2027, eligible unexecuted iceberg orders will move into CAS as fully disclosed limit orders unless the participant opts out when placing the original order.

An iceberg order normally displays only part of its total quantity. Once transitioned, its entire pending quantity will be visible. Carried-forward iceberg orders will have lower time priority than other carried-forward limit orders.

For investors using such orders, the implication is practical: an order designed to reveal only part of its quantity during regular trading may reveal the full remaining quantity during the auction.

What Should Investors Watch?

• Check the correct settlement benchmark: Expiry calculations may differ from the official stock or index closing value shown on a market screen.
• Know which contract stops trading when: Expiring index contracts have a different deadline from other derivatives.
• Review pending orders before the auction: Understand whether an unexecuted order can move into CAS and how it will be displayed.
• Treat indicative auction prices carefully: They can change before matching and should not be read as completed transactions.
• Follow exchange and broker instructions: Operational details will determine how these changes appear on trading platforms.

Author’s Take

The most useful way to understand the revised CAS rules is through the two different jobs an end-of-day price performs.

An official closing price helps investors value holdings and allows funds to transact against a common benchmark. An expiry settlement price determines financial outcomes for contracts reaching their end.

My assessment is that separating these calculations gives traders a clearer basis for assessing expiry exposure while preserving the closing auction for cash-market price discovery.

However, the changes should not be treated as a guarantee of lower volatility. Their effectiveness will depend on auction participation, liquidity and how traders adapt. For investors, the immediate priority is to understand which price matters for which purpose.