India’s equity cash market introduced a Closing Auction Session, or CAS, on August 3, 2026, for a defined group of shares: stocks in the cash segment on which derivative contracts are available.
The phased rollout does not cover every listed security. Under SEBI’s January 16 closing-auction circular, the remaining cash-market securities continue to use the volume-weighted average price, or VWAP, of trades during the final 30 minutes of continuous trading to determine their closing price.
What changed on August 3
For stocks covered by CAS, the official closing price is now produced through a single-price auction rather than the previous final-30-minute VWAP method.
The auction seeks the price at which the largest volume can be executed. If more than one price meets that condition, the framework uses order imbalance and proximity to the reference price as tie-breakers. If no equilibrium price is discovered, the reference price becomes the closing price.
This change is intended to gather closing orders into one session and produce a common auction price for covered stocks. It is a change to the closing-price process, not a blanket extension of trading for every security.
How the 20-minute closing auction works
NSE’s current CAS timetable divides the session into four stages:
- 3:15–3:20 p.m.: The reference price is calculated while the market transitions from continuous trading to CAS.
- 3:20–3:25 p.m.: Limit and market orders may be entered, modified or cancelled.
- 3:25–3:30 p.m.: Only limit orders may be entered, modified or cancelled. Market orders cannot be entered, modified or cancelled. The system closes order entry at a random point between 3:28 and 3:30 p.m.
- 3:30–3:35 p.m.: Orders are matched and trades are confirmed.
The reference price is normally the VWAP of trades between 3:00 and 3:15 p.m. If there were no trades during that period, the last traded price for the day is used. If the stock did not trade during the day, the previous trading day’s closing price—adjusted where required for a corporate action—becomes the reference.
CAS operates within a price band of 3% above or below that reference price.
What does not move to the same closing time
Cash-market securities outside CAS continue in continuous trading from 9:15 a.m. to 3:30 p.m. Their closing prices continue to use the final-30-minute VWAP method.
Equity derivatives continue to trade until 3:40 p.m. SEBI’s circular expressly describes this as a continuation, not a newly extended derivatives session.
The transition from CAS to the post-close session runs from 3:35 to 3:50 p.m. The cash-market post-close session then operates from 3:50 to 4:00 p.m.
The pre-open change starts September 7
The changes to the morning pre-open auction do not take effect on August 3. SEBI set a separate implementation date of September 7, 2026.
The pre-open session will remain a 15-minute window from 9:00 to 9:15 a.m., but its internal stages will be:
- 9:00–9:05 a.m.: Market and limit orders may be entered.
- 9:05–9:10 a.m.: Order entry is limited to limit orders; market orders cannot be modified or cancelled. Order entry closes randomly between 9:08 and 9:10 a.m.
- 9:10–9:12 a.m.: Orders are matched.
- 9:12–9:15 a.m.: Orders transition from the pre-open session to continuous trading.
The September 7 timetable is therefore distinct from the August 3 closing-auction launch.
What happened on the first day
Reuters’ August 3 launch report confirmed that the mechanism went live for stocks with traded futures and options contracts.
Reuters reported a delay in publication of the Nifty closing value and an unusual divergence between the Nifty and Sensex. A source familiar with the matter told Reuters there was no trading glitch. These are narrow first-day observations; they do not establish that CAS caused the index divergence or indicate how the mechanism will perform over time.
This article explains exchange timings and market structure. It does not provide investment or trading advice.
