Frequently Asked Questions on Closing Auction Session
CAS currently applies only to cash segment for F&O linked equity stocks. For these stocks, the closing price will now be decided through the Closing Auction Session.
CAS is a 20‑minute session from 3:15 pm to 3:35 pm on every trading day.
- 3:15 pm – 3:20 pm: Transition from regular trading to CAS, and publication of the reference price.
- 3:20 pm – 3:25 pm: Order Entry Session 1 – market and limit orders allowed.
- 3:25 pm – 3:30 pm: Order Entry Session 2 – only limit orders allowed, order entry closes at a random time between 3:28 pm and 3:30 pm.
- 3:30 pm – 3:35 pm: Order matching and closing‑price discovery.
No change for now. Non‑F&O stocks will continue to trade in the normal way till 3:30 pm, and their closing price will still be based on the VWAP of trades from 3:00 pm to 3:30 pm until CAS is extended to them in a later phase.
SEBI has given a phased timeline: CAS in the cash segment: effective August 3, 2026.
- Alignment of the pre‑open auction with CAS rules: effective September 7, 2026.
No. CAS will first apply only to F&O linked equity stocks, where the closing price will be decided through the auction.
The reference price for CAS in each stock is:
First preference: VWAP of all trades between 3:00 pm and 3:15 pm in that stock is taken as the reference price.
If there are no trades in this window: The Last Traded Price (LTP) during the day is taken as the reference price.
If there is no trade at all during the day: The previous day’s closing price (adjusted for any corporate action) becomes the reference price.
This reference price is also used to set the ±3% price band for CAS orders.
Allowed in CAS (within the price band):
- Market orders – buy/sell at whatever price the auction discovers (only between 3:20 pm and 3:25 pm).
- Limit orders – buy/sell at or better than a specific price (3:20 pm–3:25 pm and 3:25 pm–3:30 pm).
- IOC (Immediate or Cancel) orders are permitted as long as they are market/limit orders within the band.
Not allowed in CAS:
- Stop‑loss (SL) orders.
- Disclosed‑quantity orders (where only part of the quantity is visible).
- Orders outside the ±3% band.
Brokers may choose to block some algo orders during CAS, but this is at the broker’s discretion.
During the transition at 3:15 pm:
- Limit orders within the CAS price band are carried forward into CAS.
- Limit orders outside the band are cancelled.
- Stop‑loss and disclosed‑quantity orders are cancelled.
- Limit orders marked as End of Session or End of Day & remain in the book if they are within the band.
From 3:15 pm to 3:20 pm (transition), you cannot place or modify orders, but you can cancel orders that are still eligible.
Yes, with some conditions:
- 3:20 pm–3:25 pm (Order Entry Session 1):
- You can place, modify and cancel both market and limit orders.
- 3:25 pm–random close (between 3:28 pm and 3:30 pm) (Order Entry Session 2):
- You can place, modify and cancel limit orders only.
- Market orders cannot be placed, modified or cancelled.
If the exchange cancels orders due to regulatory / system reasons (for example, member suspension, market halt), both market and limit orders are cancelled immediately throughout CAS.
Random close means the exchange stops order entry at a random time between 3:28 pm and 3:30 pm. Traders do not know the exact second when the window will close, which makes it harder to bunch large orders at the very last moment to push the closing price.
During CAS, each eligible stock has a fixed price band of ±3% around its reference price.
- Any order below the lower limit or above the upper limit is automatically rejected / cancelled by the exchange.
- The band does not change dynamically during CAS. It stays fixed for that session.
At the equilibrium price, matching happens in this sequence:
- Market orders vs market orders, in time priority.
- Any leftover market orders are matched first with the best-priced limit orders, and if prices are the same, the earlier order is matched first.
- Limit orders vs limit orders, in price‑time priority.
Market orders are handled first, while limit orders let you set the price you want. If several orders are at the same price, the one placed earlier is handled first.
If two orders are at the same price, the one placed earlier is matched first.
Yes. If you modify an order, it is treated as a new order and is added in the order queue.