Four thousand six hundred and fifty-five points. That is how far the Sensex moved in the final 14 minutes of trade on September 10, more than it had moved in the previous six hours combined, when the index sat almost frozen inside a 282-point band. The culprit was a short, closing-bell auction called the Closing Auction Session (CAS), barely five weeks into its existence.
Two days later, on Saturday, September 12, the Securities and Exchange Board of India (Sebi) admitted the mechanism needs work: a fresh consultation paper proposes rewriting how CAS decides settlement prices, when derivatives trading actually ends, and what information traders are even allowed to see while the auction is live.
Why Sebi Is Reopening the CAS Playbook So Soon
CAS went live on August 3, 2026, for every stock with listed derivatives, replacing the older system in which the closing price was the VWAP of trades in the last 30 minutes of continuous trading. Under CAS, the close is instead set through an auction: buy and sell orders build up in a book and are matched at a single equilibrium price.
Sebi said feedback from exchanges, trading members, broker associations and institutional investors pointed to one specific pain point: expiry days, when derivatives trading and CAS price discovery run at the same time. Option buyers and sellers, the regulator noted, have grown uncertain about what price their contracts will actually settle at, given how active expiring index options remain right through the auction window.
Sebi chairman Tuhin Kanta Pandey had flagged the review two days earlier, telling reporters at the Global Fintech Fest in Mumbai that “CAS is here to stay,” even as swings in the benchmark indices played out live during that day’s auction. His comment framed the debate exactly where Saturday’s paper landed: not whether CAS survives, but how its expiry-day settlement price gets calculated.
Also Read: Sebi Backs Closing Auction Session: Why Nifty Jumped 200+ Points in Closing Minutes
A Fast-Moving Rulebook: CAS’s Timeline So Far
| Date | Milestone |
|---|---|
| December 2024 | Sebi floats its first consultation paper proposing a closing auction session for Indian equities |
| August 2025 | A second round of consultation refines the CAS design before launch |
| August 3, 2026 | CAS goes live for every stock with listed derivatives |
| September 10, 2026 | Sensex swings 4,655 points in 14 minutes during CAS on a weekly expiry day; Pandey says “CAS is here to stay” but flags an imminent review |
| September 12, 2026 | Sebi floats a fresh consultation paper on settlement prices, timings and IIV dissemination |
| September 24, 2026 | Sebi’s board is expected to take up the proposals |
| October 3, 2026 | Deadline for public comments on the new paper |
Barely six weeks separate CAS’s launch from Sebi already rewriting its rules — a pace that says as much about the urgency of the September 10 episode as it does about how closely the regulator is watching its own experiment.
Two Formulas, One Goal: A Fairer Settlement Price
The heart of the consultation paper is a choice between two methodologies for pricing index and single-stock derivatives on expiry day.
| Feature | Option 1: Blended VWAP | Option 2: Interim CTS VWAP |
|---|---|---|
| Price window used | Last 30 minutes of CTS plus the 10-minute CAS | Last 30 minutes of CTS only; CAS trades excluded |
| Weighting | Set by actual traded value in each window, not a fixed ratio | Not applicable — CAS contributes nothing |
| Status proposed | Sebi’s stated long-term framework | Interim; a shift to blended pricing considered only after at least a year |
| Core rationale | Reflects real trading activity across both continuous and auction liquidity | Preserves a familiar formula while participants adapt to CAS |
Under Option 1, the auction’s influence on the settlement price would rise or fall with how much money actually trades during CAS, rather than a formula fixed in advance — the more representative long-term approach, in Sebi’s own framing. Option 2 would freeze the calculation at the pre-CAS formula for now, giving auction liquidity more time to mature before it feeds into settlement prices at all.
A New Clock for Expiry Day
Alongside the settlement-price question, Sebi has proposed rewriting the sequence of sessions on expiry days.
| Element | Option A | Option B |
|---|---|---|
| Continuous trading (CAS stocks) ends | 3:30 pm | 3:15 pm (unchanged) |
| Transition period | About 1 minute | About 1 minute |
| CAS window | 3:31 pm – 3:40 pm | Roughly 3:15 pm – 3:25 pm |
| Derivatives trading ends | 3:45 pm | 3:30 pm |
Both structures cut today’s five-minute transition gap to roughly a minute, and shrink the 10-minute derivatives window that currently follows CAS down to five. Option A pushes the whole sequence 15 minutes later in the day; Option B keeps the existing 3:15 pm cash-market cut-off but tightens everything after it.
Killing the “Phantom” Index Number
One of the more technical, but consequential, proposals addresses the Indicative Index Value, or IIV — the live index number many traders watched flicker during CAS on September 10. Sebi’s paper draws a sharp line between the Indicative Equilibrium Price (IEP), an evolving, non-final price generated from the live auction order book for individual stocks, and an actual traded price. An index-level IIV built from those still-moving IEPs, Sebi argued, does not represent a level the index has actually reached — yet it can be, and evidently was, misread as one.
That distinction was on full display on September 10: indicative levels for the Sensex jumped over 1,000 points within two minutes during CAS, before the index settled far below that peak once the auction concluded. Sebi’s fix: keep publishing security-level IEPs, since those still inform trading decisions on individual stocks, but stop disseminating the index-level IIV altogether during the auction window.
How Sebi’s Fix Echoes Global Practice
Sebi’s paper notes that international closing-auction markets typically draw a firm line between indicative prices for securities actually inside the auction and the calculation of a broader index value built from those prices. That is effectively the split Sebi wants to replicate in India: keep the live, evolving IEP visible for individual stocks genuinely being auctioned, but stop projecting those still-moving numbers into a headline index figure that looks final when it isn’t. It is less a novel invention than an alignment with how mature auction markets already manage the gap between “indicative” and “traded.”
Tighter Rules on Order Cancellations
Sebi has also proposed restricting cancellation of limit orders sitting more than 1% away from the CAS reference price once the auction begins. The goal is to discourage orders placed mainly to be pulled at the last second, a tactic that can distort the emerging equilibrium price without any real intention to trade.
The Evidence Behind the Rethink
Sebi’s paper isn’t built on anecdote alone. As part of its review, the regulator compared the premium traded in expiring benchmark index options before and after CAS: 26 expiries between February and July 2026 versus five expiries between August 3 and September 3, 2026, the period since CAS went live, one input into whether CAS auctions are drawing genuine participation or simply running thinner than the continuous session they replaced.
Early reaction has leaned constructive. Uttam Bagri, managing director of BCB Brokerage, said Sebi’s paper “reflects an open mind by seeking broader market feedback” before the framework is finalised, a reading echoed by other participants who see the dual-option format as genuine consultation rather than a done deal.
The Retail Trader’s Dilemma on Expiry Day
For retail traders using weekly and monthly index options, settlement-price certainty decides whether a position expires in the money or out of it. A formula that swings on how much volume shows up in a 10-minute auction adds unpredictability that Option 2’s interim, CTS-only approach is explicitly designed to soften while the market adjusts. Until Sebi decides, every weekly expiry between now and late September effectively runs on the old, less-tested rules, which is exactly why traders are watching the next few Thursdays as closely as the consultation paper itself.
Why FIIs and DIIs Are Watching Closely
For FIIs and DIIs running index arbitrage, hedging and passive strategies, the stakes are structural rather than day-to-day. Pandey’s own comment that MSCI found its recent rebalancing went smoothly under CAS matters here, global passive funds tracking MSCI benchmarks rely on predictable, liquid closing prices during reconstitution events, and Sebi’s proposals on order-cancellation discipline and IIV transparency aim squarely at making that liquidity genuine rather than merely indicative. A settlement mechanism FIIs trust to be manipulation-resistant is, ultimately, a precondition for deeper foreign participation in Indian index derivatives.
Check Live: FII DII DATA| NIFTYTRADER
What Happens Next
Feedback stays open until October 3, 2026, but Sebi’s board is expected to take up the proposals at its September 24 meeting, well before that window closes. That leaves at least two more weekly expiries, plus the monthly cycle, to play out under today’s rules while the consultation runs.
Which way Sebi leans will say a lot about its approach to CAS itself: Option 1 signals confidence that auction liquidity is already deep enough to trust with settlement prices; Option 2 signals a more cautious regulator willing to let the mechanism prove itself first. Expect brokers and exchanges to lobby hard over the next three weeks, and expect the next few expiry sessions to be watched more closely than usual for early signs of which formula the market seems to prefer.
Read Next: RBI Rejects Tata Sons’ Exit Plan. The Listing Question Is Back
