Your Expiry P&L Is Now Set in a 20-Minute Auction. SEBI Wants It Back.
On 3 September the Sensex indicative close fell 2.5% inside the closing auction and put premiums jumped 400-500%. What broke in expiry settlement under CAS, what SEBI is proposing, and how to trade the next four expiries.
Since 3 August 2026, the price your expiring Nifty, Bank Nifty and Sensex options settle at is no longer the last-30-minute average. It is one number from a 20-minute auction, and your options keep trading for five minutes after that auction has already decided your P&L. On 3 September the Sensex indicative close dropped 2.5% inside the auction and some put premiums went up 400-500% before the index closed down just 0.55%. SEBI has said a consultation paper is coming this Saturday, with a VWAP-based settlement and a 3:15 expiry reportedly on the table. Until it lands, the Expiry Pressure Index is your read on how crowded the strikes near the close are.
What 3 September looked like from the option chain:
Indicative drop in CAS
-2.5%
Sensex, briefly
Some put premiums
+400-500%
in the same minutes
Actual close
-0.55%
76,152.86
Settlement window
20 min
3:15-3:35 PM
When the Closing Auction Session went live in August, most of the coverage treated it as a cash-market story. Cleaner closing prices, fewer last-tick games, good for index funds. All true. What almost nobody wrote down was the second-order consequence: every expiring index option in the country now settles to a price discovered in a thin, twenty-minute window that the options themselves are not part of.
It took exactly one month and one expiry for that to matter. We are not going to pretend we saw it coming either. We flagged the 3:40 derivatives close in our CAS explainer and moved on. This post is the one we should have written then.
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Expiry Pressure Index — how crowded is the close?
Our own metric for how much open interest is stacked at strikes near the current price going into settlement. The higher it reads, the more a small move in the auction is worth to somebody.
- Before 3 August 2026, index options settled to the volume-weighted average price of the last 30 minutes of trading. Since then they settle to a single equilibrium price from the 3:15-3:35 PM closing auction.
- On 3 September 2026 the Sensex indicative auction price briefly fell about 2.5% while some Sensex put premiums rose 400-500%; the index finally closed 0.55% lower at 76,152.86.
- Derivatives trade until 3:40 PM, five minutes after the auction that sets their settlement price has ended.
- SEBI has said a consultation paper on derivatives settlement methodology is due; reported options include VWAP across the pre-auction and auction windows, index expiry at 3:15 PM, tighter price bands and restrictions on late order entry.
- The cash needed to move a handful of heavyweight constituents inside a thin auction can be far smaller than the P&L it creates across a large book of expiring options.
What actually happened on 3 September
Thursday, 3 September was a Sensex weekly expiry. Through the continuous session the index drifted lower on a quiet day. At 3:15 PM the thirty Sensex constituents stopped continuous trading and entered the auction, as they have every day since August. Options on the index kept trading.
Inside the auction, the indicative equilibrium price for enough heavyweight names moved far enough that the indicative Sensex level printed roughly 2.5% below where it had been trading minutes earlier. Put options that had been priced for a flat-to-slightly-down close repriced for a crash. Premiums on some strikes rose four to five times. Then the auction resolved, the indicative level recovered most of the drop, and the index closed at 76,152.86, down 0.55% on the day.
If you were long those puts and sold into the spike, you had a very good afternoon. If you were short them and got stopped or margin-called on the indicative level, you had a bad one, on a day the index barely moved. Either way, the outcome was decided by a number that was not a traded price in the sense you are used to.
We are not claiming 3 September was manipulation. We do not have the order book. What we are saying is that the structure made the outcome possible, and that a structure where this is possible on a quiet day will eventually see it on a busy one.
How your expiry settlement price is calculated now
This is the part most traders have not internalised, because nothing on the option chain changed. Same strikes, same premiums, same 3:40 close for the contracts themselves. What changed is the thing those contracts resolve to.
Index option settlement, before and after CAS:
| Before 3 Aug 2026 | Since 3 Aug 2026 | |
|---|---|---|
| Settlement price | VWAP of last 30 min of trading | One equilibrium price from the auction |
| Window | 3:00-3:30 PM continuous | 3:15-3:35 PM auction |
| Price band | Normal circuit filters | Plus or minus 3% around a reference |
| Options stop trading | 3:30 PM | 3:40 PM |
| Can you trade the settlement window? | Yes, same market | No — the auction is cash-only |
Read the last two rows together. Your option trades until 3:40. The price it settles at was fixed at 3:35 by an auction you could only participate in by trading the underlying shares, not the option. For five minutes every expiry, the market is trading a contract whose value is already known.
A thirty-minute VWAP is hard to move because you have to sustain a price against everyone else for half an hour. A single auction print is a different animal. It is one number, from one window, in a session where only a subset of participants show up.
Why one auction price is dangerous for options specifically
The cash market can live with an occasional odd close. A mutual fund NAV being off by half a percent for one day is unfortunate, not catastrophic. Options are different, because they are leveraged claims on that exact print.
Consider a strike two hundred points below spot on Sensex, an hour before expiry. It is worth almost nothing. A 2.5% move in the settlement reference takes it deep in the money. The premium does not rise 2.5%; it rises several hundred percent, which is precisely what the chain showed. Now multiply that across every strike in the affected range and every open contract at those strikes.
The asymmetry is the whole problem. The amount of cash needed to shift the indicative price of a few heavyweight constituents inside a thin auction can be materially smaller than the profit or loss that shift creates across a large book of expiring options. That is not a MarketsEasy opinion; it is the risk SEBI itself is now reviewing.
See it yourself — move spot 2.5% against a near-expiry put and watch the premium:
The old VWAP was not perfect either. But it was expensive to move, and expense is the only real defence a settlement mechanism has.
What SEBI is reportedly considering
SEBI has said a consultation paper on the derivatives settlement methodology is coming. Based on what has been reported ahead of it, these are the candidates. None is final, and the paper may combine several.
Proposals on the table and what each one fixes:
| Proposal | What it changes | What it fixes |
|---|---|---|
| VWAP across pre-auction + auction | Settlement becomes an average again, over a longer window | Makes a single print far less decisive |
| Index expiry at 3:15 PM on VWAP | Options stop trading when the auction starts | Ends the five-minute gap where the outcome is already known |
| Overlapping derivatives session | Options trade alongside the auction, not after it | Lets hedgers respond in the same window |
| Tighter price bands | Narrower than the current plus or minus 3% | Caps how far an indicative print can travel |
| Late order entry restrictions | No new orders in the final minutes of the auction | Removes the last-second push |
Our read: the 3:15 VWAP expiry is the cleanest fix and the one we would bet on, because it removes the timing mismatch entirely rather than patching around it. An overlapping session is more elegant but harder to implement. Tighter bands alone treat the symptom.
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Live option chain — watch the close as it happens
Strike-by-strike OI, PCR and max pain for Nifty, Bank Nifty and Sensex, refreshing through the auction window. The tool we use to see the last thirty minutes, not just the print.
How to trade the next four expiries
The consultation paper will take weeks to become a circular, and the circular will take more weeks to take effect. Between now and then there are at least four expiries under the current rules: Nifty on 15 and 22 September, Sensex on 17 and 24 September. Here is what we are doing differently.
Adjustments that cost nothing and remove the tail:
- Close or roll short near-the-money positions before 3:15 PM. The five minutes after 3:35 are not a hedge window, they are a lottery you are already holding a ticket for.
- Stop treating the 3:15-3:35 indicative level as a price you can trade against. You cannot. It is an auction in progress.
- If you must hold into settlement, hold spreads, not naked strikes. A 2.5% indicative swing is survivable in a defined-risk structure and is not in an uncovered one.
- Watch the Expiry Pressure Index into the last hour. A crowded close is a close somebody has a large incentive to move.
None of this is advice to stop trading expiry. It is advice to stop being the counterparty to a settlement mechanism you cannot see inside.
What this does to max pain and pinning
Two of the most-read pieces on this site are our max pain backtest and the expiry pinning study, both of which measured how close the settlement landed to the strike with the most open interest. Both were run on VWAP settlements. Under CAS, the mechanism that would produce pinning has changed, and we do not yet have enough auction-era expiries logged to say whether the effect survived, strengthened or vanished.
Our honest position is that the pinning literature is now partly stale, including ours. We are logging every expiry under the new regime and will re-run the numbers once there are enough sessions to say something defensible. If SEBI moves expiry to 3:15 on VWAP, a lot of the old work becomes relevant again. If it keeps the auction, all of it needs redoing.
Read next
The two posts this one builds on: what the Closing Auction Session changed on 3 August, and the pre-open reform that landed on 7 September. For expiry-day mechanics that still apply, gamma blast — when options explode on expiry day. We will update this post the day the consultation paper is published.
Frequently Asked Questions
How is the expiry settlement price for Nifty options calculated now?
Since 3 August 2026 it is the single equilibrium price from the 3:15-3:35 PM Closing Auction Session on the index constituents, not the last-30-minute VWAP used before. Options themselves trade until 3:40 PM, after that price is fixed.
What happened to Sensex options on 3 September 2026?
The indicative Sensex level briefly fell about 2.5% inside the closing auction on a weekly expiry day, and some put premiums rose 400-500% before the index closed just 0.55% lower at 76,152.86. SEBI is reviewing the settlement methodology partly in response.
Is SEBI changing the expiry settlement price rules?
SEBI has said a consultation paper on derivatives settlement methodology is coming. Reported options include a VWAP across the pre-auction and auction windows, index expiry at 3:15 PM, tighter price bands and restrictions on late order entry. Nothing is final until the paper and a subsequent circular.
Can I trade the closing auction session?
Only in the cash market. The 3:15-3:35 PM auction is for the underlying shares. Index options are not part of it and continue trading separately until 3:40 PM, so you cannot hedge inside the window that sets your settlement price.
Should I hold options into expiry settlement under CAS?
Hold defined-risk spreads if you hold anything. Close or roll short near-the-money positions before 3:15 PM. The five minutes after the auction ends are not a hedge window because the settlement price is already fixed.
Does max pain still work after CAS?
Unknown. Every published max pain and pinning study, including ours, was run on VWAP settlements. Under a single auction price the mechanism has changed, and there are not yet enough logged auction-era expiries to say whether pinning survived.
When will the new settlement rules take effect?
Not before the consultation paper closes for comments and SEBI issues a circular, which typically takes several weeks. At least four expiries — Nifty on 15 and 22 September, Sensex on 17 and 24 September — will run under the current rules.
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