The Securities and Exchange Board of India (SEBI) is rolling out a major shift in how stock closing prices are determined for foreign and domestic equity markets. The traditional Volume-Weighted Average Price (VWAP) method is being replaced by a dedicated Closing Auction Session (CAS) for stocks traded in the Futures & Options (F&O) segment.
Here is a complete breakdown of what CAS is, how the auction mechanics work, and the exact timeline of the new closing procedure.
What is the Closing Auction Session (CAS)?
CAS is an auction-based trading window designed to determine the official closing price of F&O-enabled stocks.
Unlike the Continuous Trading Session (CTS)—where buy and sell orders execute instantly whenever prices match—CAS collects all eligible buy and sell orders during a specific window first.
Why the change?
An auction-based system ensures the official closing price reflects true, aggregated market demand rather than being skewed by a few large trades executed in the final seconds of market hours.
How Does the Equilibrium Price Algorithm Work?
The core principle behind CAS is finding the price that executes the highest volume of shares.
- Buyers willing to pay ₹1,010 will happily accept ₹1,005 (or any price below their limit).
- Sellers willing to accept ₹1,000 will happily take ₹1,005 (or any price above their limit).
By aggregating cumulative buy and sell interest across price bands, the exchange calculates the Executable Volume:
|
Price Level |
Cumulative Buy Qty |
Cumulative Sell Qty |
Executable Volume |
|
₹1,000 |
1,500 |
200 |
200 |
|
₹1,002 |
1,400 |
500 |
500 |
|
₹1,005 |
1,200 |
900 |
900 (Highest) |
|
₹1,008 |
800 |
1,150 |
800 |
|
₹1,010 |
500 |
1,300 |
500 |
In this scenario, ₹1,005 becomes the official Equilibrium/Closing Price because it results in the maximum executable volume (900 shares).
Tie-Breaker Rules
If two prices result in the same executable volume, the exchange resolves the tie by:
- Choosing the price with the smaller order imbalance (unmatched buy/sell quantity).
- If still tied, choosing the price closest to the Reference Price.
Order Matching Priority
Once the price is set, orders execute at the single equilibrium price in this order:
- Market Orders vs. Market Orders (matched by time priority).
- Unmatched Market Orders vs. Limit Orders (matched by price-time priority).
- Limit Orders vs. Limit Orders (matched by price-time priority).
Breakdown of the CAS Timeline (3:00 PM – 3:35 PM)
- 3:00 PM – 3:15 PM: Pre-Calculation Window: Continuous Trading Session (CTS).
Continuous trading runs normally. During these 15 minutes, the exchange calculates the initial VWAP, which serves as the Reference Price for the upcoming auction.
- 3:15 PM – 3:20 PM: Market Transition & Filter: No New Orders Allowed.
Continuous trading stops for Category I stocks. The exchange sets a price band of ±3% around the Reference Price.
- 3:20 PM – 3:25 PM: Order Entry Session I: Market & Limit Orders Accepted.
Traders can place both market and limit orders. The exchange streams real-time indicative data:
- 3:25 PM – 3:30 PM: Order Entry Session II: Limit Orders Only.
Only limit orders can be placed. Existing market orders cannot be modified or cancelled. To prevent last-minute order gaming/flooding, this window closes randomly between 3:28 PM and 3:30 PM.
- 3:30 PM – 3:35 PM: Price Discovery & Matching: Auction Execution.
Order entry closes completely. The exchange executes the matching algorithm, sets the official closing price, and executes all matched trades simultaneously.
Summary Takeaways for Traders
- Reduced End-of-Day Manipulation: Expiry-day volatility and last-minute price ramping will drop significantly.
- Price Certainty: Institutional traders get clearer execution at a fair market-clearing price.
- Order Caution: Ensure your limit orders during 3:15–3:30 PM stay strictly within the ±3% reference price band to prevent automatic cancellation.
