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Market mechanics · 6 min read

Closing Auction Session (CAS): what changes from tomorrow

A new SEBI rule replaces how the official closing price is calculated for F&O stocks, starting August 3, 2026.

What was the old system?

Until now, a stock's official closing price was calculated as the Volume Weighted Average Price (VWAP) of all trades executed between 3:00 PM and 3:30 PM — the last 30 minutes of the trading day. Because this method relies on scattered trades happening continuously, it's historically been seen as vulnerable to last-minute price manipulation, where a trader could push a large order right before close to nudge the closing price.

What's changing

SEBI's new Closing Auction Session works differently. Instead of using trades scattered across a half-hour window, all eligible buy and sell orders are collected into a single pool during a dedicated auction, and matched at one equilibrium price — the price at which the maximum quantity of shares can change hands. That single price becomes the official close.

Segment	Trading ends
CAS-eligible stocks (have F&O contracts)	3:15 PM, then auction
Non-F&O cash market stocks	3:30 PM (unchanged)
Index and stock derivatives (F&O)	3:40 PM

This means the market no longer closes at one single time for everyone — different segments now wind down at different points in the afternoon.

Which stocks are affected?

CAS applies to Category I stocks — those with active Futures & Options contracts, which includes Nifty 50 and Bank Nifty constituents and other major F&O-enabled names. Stocks without derivative contracts (Category II) continue using the existing VWAP method for now, though SEBI has left the door open to expanding CAS to them later.

What should you actually do differently?

• Intraday (MIS) traders: equity intraday positions in CAS-eligible stocks are typically auto-squared-off earlier in the afternoon under the new timeline — check your broker's specific cutoff.
• Pending orders: some stop-loss, iceberg, and out-of-band orders placed in CAS-eligible stocks may be cancelled automatically before the auction begins — worth reviewing any standing orders you have active near market close.
• Delivery investors, mutual funds, SIPs, ETFs: nothing changes for you — this only affects how the closing price is calculated, not how your holdings are valued or settled.

If orders during the auction window don't fully match, the calculated reference price itself becomes the official closing price — SEBI's framework is built so a CAS-eligible stock never ends the day without a valid close.

Why is SEBI doing this?

SEBI describes the goal as building a "more robust and manipulation-resistant closing price discovery mechanism" — essentially, pooling all interest into one visible auction rather than relying on scattered trades makes it harder to influence the closing print with a single large, late order. It also aligns Indian markets more closely with how several global exchanges already determine their closing prices.

What's next

CAS itself goes live August 3, 2026. A related change — aligning the pre-open auction session with the new CAS framework — is scheduled separately for September 7, 2026. The rollout is applicable across NSE, BSE, and MSEI.