(Chapter 6)
Not everyone seems happy with the Closing Auction Session. They wonder about the reasons that prompted SEBI to introduce it. We will discuss that, but first, the effect of the Closing Auction Session (CAS) in the week it was introduced.
Closing Auction Session on First Day
At 3:15 PM on August 3, continuous trading for F&O-eligible stocks ended. The Nifty was trading around 24,580. This was also the range where it had traded in the past 15-20 minutes. Under old VWAP system that would have been the average price of index. Now the auction price mechanism started.
By the time the auction completed and the official closing price was confirmed, Nifty had settled at approximately 24,774. India’s benchmark index rose by roughly 194 points, in less than twenty minutes. No new information entering the market and no trade happened on the underlying in a normal continuous session in this 20 minutes. The aggregate effect was a 194-point gap between where the market was trading at 3:15 PM and where it was officially declared to have closed.
For technical analysis, Nifty had crossed the 200-day simple Moving Average and was officially in a bull market. But the problem was that it did not reflect a surge in demand. It reflected the mechanics of how auction prices feed into index calculations.
The Nifty is not a traded instrument with its own closing auction. It is a derived calculation from the closing prices of its constituent stocks. Each of those constituent stocks went through its own CAS. Each stock’s auction produced an equilibrium price. Those equilibrium prices, weighted by market capitalisation, produced the Nifty’s official close.
The exchange confirmed it was not an error. It was, in the most literal sense, the system working exactly as designed. Question is: Designed for whom?
Black Jack Closing
If Day 1 was a warning, Day 2 was a demonstration of consequences. August 4 was a weekly expiry day for Nifty options. Nifty opened 71 points down at 24,703, which also remained the high of the day. Its low was 24,427 and it remained near 24,500 till 3:15 PM.
On Day 1, the artificial CAS close caused confusion, discomfort, and notional dislocations. On Day 2, with options expiring at the official close, the CAS delivered real, immediate, and irreversible financial damage to traders who had done nothing wrong.
By 3:15 PM, when continuous trading ended, Nifty stood at 24,463.45, down 310.85 points on the day, a fall of 1.25%. Then the auction began.
By approximately 3:28 PM, before the auction order entry window had even closed, a roughly 150-point upward move was embedded in the Indicative Equilibrium Price visible to participants. The official closing price, confirmed post-auction, was 24,614.90, a decline of just 159.40 points, or 0.64% on the day. It is summed up as below:
| Event | Time | Nifty Level | Change |
|---|---|---|---|
| Regular Trading Ends | 3:15 PM | 24,463.45 | -310.85 pts (-1.25%) |
| CAS Jump Embedded | ~3:28 PM | N/A | ~150-point rise during auction |
| Official Closing Price | Post-CAS | 24,614.90 | -159.40 pts (-0.64%) |
The traders who bought puts with strikes below 24,600 before 3:15 PM were wiped out as worthless. Worthless calls from 24,500 up to 24,600 suddenly had value. It was an upset never seen before in 15 minutes.
Fifteen minutes produced the numbers which were to decide the fate of option holders. Speculation had risen to another level. It was no different from a Blackjack game of cards in a casino.
A trader with the right market direction, holding a position that was in-the-money at 3:24 PM, was wiped out by a 151-point CAS-driven repricing that occurred in a fifteen-minute window they could neither trade against nor exit from efficiently. These are not theoretical dislocations. These are actual traders, actual positions, actual losses, on an expiry day when every rupee of movement becomes settlement.
Rajesh Baheti, Managing Director of Crossseas Capital, put it plainly: “Continuing CAS in the way it is designed right now is simply dangerous.” His specific concern is structural. The derivatives session should expire before the cash CAS, not after. The current design creates a blind window where option settlement prices are determined by an auction with lower participation and without active market makers providing price discipline.
The NSE confirmed that the observed price movements were not a technical glitch. They were the result of the new system’s design.
Now let us revisit the Jane Street manipulation. What it did was legitimate till it was held to be illegitimate.
Jane Street Manipulation
Jane Street is a massive, highly secretive Wall Street quantitative trading firm and market maker. They use ultra-fast high-frequency trading (HFT) algorithms, colocation servers at exchange data centers. They use complex mathematical models to trade global equities, ETFs, and derivatives.
Jane Street allegedly generated over Rs. 43,289 crore in index options profits by moving the closing prices of Bank Nifty constituents on expiry days through what regulators termed an “injection spike,” a sudden and artificial price movement in the final hour of trading. Jane Street used a 4-step execution strategy to game this VWAP calculation:
[Step 1: Morning/Early Afternoon]
Accumulate massive, cheap Out-of-the-Money (OTM) Index Options
│
▼
[Step 2: 3:00 PM – 3:30 PM]
Flood Cash Market with aggressive Buy orders in underlying bank stocks ("Injection Spike")
│
▼
[Step 3: 3:30 PM Close]
Cash buying pushes up stock prices ➔ VWAP rises ➔ Index Closing Price rises artificially
│
▼
[Step 4: Expiry Settlement]
Options expire Deep In-the-Money (ITM) ➔ Massive exponential options payout!
(Options profit massively outweighs cash buying loss)It was termed as “marking the close.” It was a market manipulation where a participant buys scrips aggressively to artificially jack up the prices of derivatives. Buying is not with genuine investment intent, but solely to manipulate the settlement price of a related derivative contract.
But the intent was inferred post facto from the result. Jane Street was permitted to operate in Cash Market as well as Derivatives as per rules. The fact they could pump in large enough money to buy scrips, that it would suck up the liquidity of a small size of market was the real manipulation. Why was there no commensurate limit on one party taking such a large position?
This pertinent question was never raised at SEBI and of course no one answered it.
Now look up what is happening with CAS. Is it not an action replay of same mechanism? The index is jacked up by a few individual players in the last 15 minutes and they know the directions in which to buy the derivatives/options to make it profitable.
The Design
It appears that the design itself is the problem.
The official close of 24,774 became the reference for mutual fund NAVs calculated on August 3. It became the settlement reference for derivative contracts. It became the closing level that GIFT Nifty futures would trade against overnight. All of these downstream calculations rested on a number that was not determined by normal channels of price discovery.
GIFT Nifty’s response on August 3 was immediate and unambiguous. It fell by over 140 points in after-hours trading. The foreign market, without the constraint of accepting SEBI’s auction arithmetic as the valid close, priced in what it expected the corrected open to reflect: that 24,774 was not a real price.
In the past week, the result has been the same every day, albeit the gap is sometimes shrinking. Today, on 12 August 2026, the actual close was 24,361, but CAS shot it up to 24,435. How is the market taking it? Traders are up in arms, but the overall market is trying to grapple with the situation.
The Market
There is a saying that the market knows everything. Thus, the markets are efficient at one thing above all else: exposing fiction.
On August 4, Nifty opened at 24,703. That is 71 points below the CAS-manufactured close of 24,774. In a single opening print, the market delivered its verdict on the previous day’s closing price. When an official closing price is repudiated by the opening price of the very next session, the closing price has failed its fundamental purpose.
A close that the next open immediately reverses is a close that no one trusted enough to hold overnight.
The Institutional Footprints
There is, however, a silver lining on this dark cloud, provided retail investors understand how to read it.
Under ordinary market conditions, official data on net institutional activity (such as FII and DII buying or selling) is published by the exchanges late in the evening or the following morning. On August 4, for instance, FIIs net bought Rs. 2,446 crore while DIIs net sold Rs. 936 crore. During the regular session up to 3:15 PM, retail participants have no direct visibility into these net institutional flows.
The CAS auction window changes this dynamic. Because high-volume institutional orders dominate the 3:15 PM to 3:35 PM auction pool, the resulting price movement serves as an immediate, real-time footprint of institutional bias. The 150-point surge during the August 4 auction was, in effect, the market reflecting net institutional buying hours before the official figures were released.
For alert retail investors, the CAS price movement offers a real-time proxy for institutional direction. But a vital caution is necessary. Relying solely on auction price jumps to gauge institutional sentiment is dangerous.
Auction dynamics can be influenced by index rebalancing, options settlement hedging, or single-stock order imbalances that do not reflect broad market conviction. Moreover, institutions aware that retail treats the CAS close as a directional signal could theoretically game the order entry window to create deceptive price prints before the random cutoff.
The CAS footprint can serve as a useful secondary input when evaluated alongside broader technical structure, volume profiles, and trend analysis. It is a silver lining for observant investors, but relying on it without confirmation from other market signals turns an informative footprint into another trap.
Why?
The biggest question is why SEBI did it? Official version is given in the link given below in References section.
Let me tell a story. There was a friend who was selling products used in the manufacture of cigarettes. Advertising such products is prohibited, but inside office posters can be put up. So in his office, there was a big banner which said “!@#$%^& Filter”. It was about the filter used in cigarettes, which he was selling.
“Is the !@#$%^& filter something new?” I asked my friend.
“No, everyone uses it. It is the only kind of filter used in all cigarettes,” he replied. “But others do not name it while selling. We do,” he added.
My friend was a marketing graduate and from a political family. This effortless marketing was a fusion of the two streams.
This is the kind of logic SEBI has for introducing “Closing Auction Session.” If you do not understand, continue to read the next chapter 7 where we shall discuss it in detail.
References:
-
Ventura Securities: The CAS Conundrum: Why Nifty Fell Nearly 200 Points While Sensex Stayed Flat.
-
SEBI: Can it Prevent Investor Losses in Financial Market?
SEBI: Can it Prevent Investor Losses in Financial Market?
-
SEBI Curbing Stock Exchange “Injection Spike” Fraud
SEBI Curbing Stock Exchange “injection spike” fraud.