CAS chaos returns: Sensex puts explode up to 400% as fag-end slide jolts traders

Sensex put options shot up sharply on Thursday as another late-session swing during the closing auction session rattled expiry-day traders, bringing back concerns over volatility in the cash market close. The Sensex was around 76,510 at 3:17 pm, but slipped to nearly 74,373 by 3:20 pm, a fall of more than 2,100 points in just three minutes. The move came during expiry day, when option prices are highly sensitive to sudden changes in the underlying index.

The sharp drop triggered a massive spike in out-of-the-money put options. The 76,600 put option jumped from Rs 102 to Rs 446, a gain of nearly 335%. The 76,500 put rose from Rs 71 to Rs 346, up about 350%, while the 76,400 put moved from Rs 45 to Rs 246, a surge of around 446%. The 76,300 put climbed from Rs 31 to Rs 146, gaining nearly 370%.

CAS volatility returns


The move once again put the focus on the closing auction session, or CAS, which is used to determine the closing price of stocks and indices. In normal conditions, the mechanism helps improve price discovery at the close. But when participation is thin or order flow is one-sided, the indicative closing price can move sharply.

Thursday’s expiry-hour move was similar to the earlier CAS-led disruption, when the Sensex had plunged from around 77,200 at 3:17 pm to nearly 74,983 at 3:23 pm, wiping out more than 2,000 points within minutes before closing lower.

That earlier move had triggered anger among traders, especially those active in weekly and monthly index options. Many traders had argued that the sharp movement in the cash index during CAS distorted option prices near expiry, creating large mark-to-market swings in contracts that were close to expiry.

This time too, the put option spike showed how even a short-lived movement in the underlying index can create outsized gains and losses in options when expiry is close.

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Why put options jumped

Put options gain when the underlying index falls. On expiry day, the impact becomes sharper because time value is low and option pricing becomes heavily linked to the index’s immediate movement. That is why the fall from 76,510 to 74,373 had a strong impact on nearby put strikes. Contracts that were earlier trading as low-probability bets suddenly moved deep into profit territory, forcing a sharp repricing.

For traders who had bought puts, the move created windfall gains within minutes. But for those who had sold options, especially naked put writers or intraday sellers, the spike would have led to steep losses and margin pressure.

The 76,400 put saw the sharpest move among the strikes shared, rising more than five times from Rs 45 to Rs 246. The 76,300 put also jumped nearly five times from Rs 31 to Rs 146.

Expiry risk gets sharper

The latest move is likely to deepen the debate over how CAS affects index derivatives on expiry day. The issue is not the direction of the move alone, but the speed and timing of the swing. A sudden cash-market move in the final minutes can change the value of expiring options dramatically. Since traders have little time to react, such swings can create a feeling of disorder even if the mechanism itself is working within the market framework.

Analysts had earlier pointed out that BSE’s derivatives volumes are much larger than its cash market depth. That mismatch can become a problem on expiry day because the final index level depends on cash-market prices, while the biggest open interest and trading activity may be sitting in derivatives.

Sebi has so far not indicated any immediate rollback of CAS, but the repeated volatility around expiry is likely to keep the issue alive among traders.

The regulator had earlier said the mechanism needs wider participation and better understanding from market participants. The expectation was that as more traders and institutions participate in the closing auction, price discovery would improve and such sharp movements would reduce.

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