The Closing Auction Session (CAS) is a new call-auction the exchange runs at the end of the day to decide a stock's official closing price. It is being introduced in phases, and in the first phase it applies to stocks that have F&O (derivative) contracts. For those stocks, the day's closing price comes from this auction instead of the last-30-minute average.
CAS takes effect from 3 August 2026 for stocks that have F&O contracts (the first phase). SEBI will extend it to more stocks in later phases.
What is changing?
Today, a stock's official closing price is the volume-weighted average price (VWAP) of the last 30 minutes of trading. Under CAS, for a stock that has F&O contracts, continuous trading ends earlier — at 3:15 PM — and a short auction from 3:15 to 3:30 PM then discovers a single equilibrium price that becomes the day's closing price.
Which stocks does CAS apply to?
In the first phase, from 3 August 2026, CAS applies only to cash-segment stocks that have derivative (F&O) contracts available. Every other stock trades normally until 3:30 PM and keeps the current 30-minute-VWAP closing price. SEBI will extend CAS to more stocks in later phases. (The exchange marks CAS-eligible stocks with a flag in its security file.)
When does CAS run?
For a stock that has F&O contracts:
- 9:15 AM – 3:15 PM — normal continuous trading.
- 3:15 – 3:30 PM — the Closing Auction Session, in these steps:
- 3:15–3:20 PM — transition: the reference price is calculated and no orders can be entered.
- 3:20–3:25 PM — order entry for both limit and market orders.
- 3:25 PM to close — limit orders only.
- The auction closes at a random moment between 3:28 and 3:30 PM.
- The equity derivatives segment trades a little longer, until 3:40 PM.
Non-F&O stocks are unaffected — they trade continuously until 3:30 PM as usual.
What order requests are allowed at each stage of CAS?
| Order request | 3:15–3:20 PM (transition) | 3:20–3:25 PM | 3:25 PM–random close (3:28–3:30) | After random close |
|---|---|---|---|---|
| New market order | No | Yes | No | No |
| New limit order | No | Yes | Yes | No |
| Modify price | No | Yes | Yes — limit orders only | No |
| Modify quantity | No | Yes | Yes — limit orders only | No |
| Cancel order | No | Yes | Yes — limit orders only | No |
In short: nothing can be entered during the 3:15–3:20 PM transition; both order types are open 3:20–3:25 PM; after 3:25 PM only limit orders can be placed or amended; and once the auction closes randomly (between 3:28 and 3:30 PM), no further orders or changes are accepted.
How is the closing price worked out?
- Reference price — the VWAP of trades in the stock between 3:00 pm and 3:15 pm. If there's no trade in that window, the day's last traded price (LTP) is used; if the stock didn't trade at all, the previous day's closing price.
- Price band — during CAS, orders are accepted only within ±3% of the reference price, and this band stays fixed through the auction.
- Equilibrium price — the auction finds the single price at which the most quantity can trade. That equilibrium price becomes the day's official closing price. If no equilibrium price can be found, the reference price is used instead.
Which order types can I use during CAS?
- Only limit and market orders are accepted, and both count toward the closing price.
- Not allowed: stop-loss orders, disclosed-quantity orders, IOC (Immediate-or-Cancel) orders, and Market-on-Close / Limit-on-Close orders.
- After 3:25 PM, only limit orders can be placed or modified — market orders can no longer be entered or changed.
What happens to my pending orders when CAS starts?
When continuous trading ends and the auction begins, the price band is revised to ±3% of the reference price for the auction, and orders still resting in the order book are handled against that revised band like this:
- A pending limit order priced within the revised ±3% band is carried forward automatically into the auction and may execute at the auction (equilibrium) price.
- A pending order priced outside the revised ±3% band is not carried forward — the Exchange cancels it.
- Pending stop-loss and disclosed-quantity (Iceberg) orders are cancelled by the Exchange and do not take part in the auction.
What does this mean for me?
- For F&O stocks, the closing price you see — and the end-of-day value and mark-to-market of your holdings and positions — comes from the auction, not the last-30-minute average. See Why does my P&L change after 3:30 pm?.
- Near the close, remember that stop-loss and disclosed-quantity orders won't work inside the auction window for these stocks.
Things to keep in mind
- CAS is a SEBI-mandated, exchange-wide change — it isn't a Rupeezy feature and applies to every broker.
- CAS sets the closing price of the underlying cash stock. The closing/settlement price of the F&O contracts themselves is unchanged — it stays the VWAP of the last half hour (3:10–3:40 PM) — while the futures price band is aligned to the ±3% CAS band during the window.
- This is separate from a stock's daily price band, which continues to apply as before.