Closing Auction Session (CAS): What Changed on 3 August 2026 and How It Affects Your Trading Day

From 3 August 2026, NSE determines the closing price of F&O-eligible stocks through a Closing Auction Session (CAS) rather than the traditional volume-weighted average. The change is small on paper and significant in practice: the official close of an F&O stock is now set at 3:15 PM, while its derivative keeps trading until 3:40 PM.

This guide covers what changed, the exact session timeline, which orders the auction accepts, and the three practical consequences most intraday traders will notice first.

What actually changed

Before CAS, the closing price of a stock was the volume-weighted average price (VWAP) of trades in the last 30 minutes. That method is still used — but only for non-F&O stocks.

The inversion is the part worth internalising. For an F&O stock, the underlying's official close is determined before the derivatives market shuts. The cash close is no longer the last price of the day.

The CAS timeline, phase by phase

3:00 – 3:15 PM — reference price window

Continuous trading runs normally. The VWAP of every trade in this 15-minute window becomes the auction's reference price. If a stock did not trade in this window, its last traded price is used; if it did not trade at all that day, a theoretical futures price is used.

3:15 – 3:20 PM — transition

Continuous trading closes for CAS-eligible stocks. The exchange revises the price band to ±3% of the reference price. No orders can be entered in the auction book yet.

3:20 – 3:25 PM — order entry, limit and market

Both limit and market orders can be placed, modified or cancelled. The Indicative Equilibrium Price (IEP) becomes visible — the price at which the auction would currently settle.

3:25 – 3:30 PM — limit orders only

Only limit orders can be entered, modified or cancelled from here. Market orders already in the book can no longer be modified or cancelled. Critically, this window ends at a random moment between 3:28 and 3:30 PM — a deliberate anti-gaming measure. Do not plan to place an order at 3:29:59.

3:30 – 3:35 PM — matching

All orders match at a single equilibrium price. That price becomes the official close. Matching runs in sequence: market against market by time priority, then remaining market against limit by price-time priority, then limit against limit.

3:50 – 4:00 PM — post-close

The post-close session runs as before, executing at the determined closing price.

How the closing price is chosen

The exchange picks the price that satisfies these rules in order:

Which orders the auction accepts

Allowed: limit orders throughout, and market orders between 3:20 and 3:25 PM. A market order placed in the 3:25–3:30 window is converted to a limit order at the IEP.

Rejected outright: stop-loss orders of both kinds, disclosed-quantity orders, IOC orders, and super orders carrying targets or stop-losses.

That rejection list is the one to remember. A stop-loss is not merely unlikely to fill in the auction — it is not accepted.

Revised MIS auto square-off

Note what this implies: an intraday equity position is squared off at 3:10 PM, five minutes before the closing price of that stock is even determined. If you have been holding intraday positions to the close, that habit no longer does what you think it does.

Square-off timings are set by your broker and may differ. Check your own broker's published policy.

Three things that change in practice

1. "Buying at the close" is now an auction, not a trade

For F&O stocks you are no longer taking a price from the order book at 3:29. You are submitting into an auction whose clearing price you cannot see with certainty until it matches.

2. Your stop-loss will not protect you into the close

Because stop-loss orders are rejected by the auction, positions carried into the CAS window are not protected by a resting stop. Plan the exit before 3:15 PM.

3. The cash close and the derivative close are different moments

An F&O stock settles at 3:15 PM; its future trades until 3:40 PM. Any analysis comparing a stock's close to its future's close is now comparing two different points in time.

What this means for your journal

If you journal intraday trades, your entry and exit timestamps around the close now describe a different market structure than they did before 3 August. Trades tagged as "closing move" or "last-hour" before and after that date are not directly comparable. Worth a note in your journal so you do not read a behavioural change as a strategy change.

Frequently asked questions

Does CAS apply to all stocks?

No. Only F&O-eligible stocks. Non-F&O stocks continue with continuous trading until 3:30 PM and the traditional VWAP close.

What is the price band during CAS?

±3% of the reference price, applied from 3:15 PM through the auction phases.

Can I place a stop-loss during the closing auction?

No. Stop-loss orders, disclosed-quantity orders, IOC orders and super orders with targets or stops are not accepted in CAS.

What happens if there is not enough volume to find a price?

If no equilibrium price can be discovered, the reference price — the 3:00–3:15 PM VWAP — becomes the closing price.

When exactly does the order window close?

At a random instant between 3:28 and 3:30 PM. The randomisation is intentional, to prevent last-second gaming of the auction.

Do index options change?

The options methodology is unchanged. Stock futures see a ±3% band reset from 3:15 to 3:40 PM, with dynamic price-band flexing paused during that window.

This article is educational. ArthaLearn is not a SEBI-registered investment adviser or research analyst. No buy or sell recommendations are given and no returns are promised. Verify all timings with your broker and the NSE circular before changing how you trade.