India options trading drops 27% after CAS launch

What changed on August 3

India introduced a new Closing Auction Session (CAS) on August 3. The mechanism sets the official closing price for stocks that have derivatives contracts. It replaced a half-hour volume-weighted average price method for closing prices. The revised framework applies to more than 200 stocks, with references to 208 stocks in market commentary. The change matters because index and stock derivatives settlement references the official close. Traders and brokers say the new closing window behaves differently from continuous trading. Research cited in media reports says the system is still finding its feet. The immediate effect has been most visible in the last minutes of the session.

Why options volumes dropped in the first week

Futures and options activity fell after the new closing price discovery started. A widely shared data point on social media and in reports was a 27% drop in options trading. Market participants linked the fall to uncertainty around how the closing auction would print prices. Several high-frequency trading executives said they largely stayed away from the new closing window. They cited multiple challenges, including difficulty borrowing shares to bet on declines. With fewer participants, liquidity became thinner where many strategies typically rebalance. That is especially relevant on expiry days when settlement sensitivity is high. The result was lower confidence in execution near the close.

The last 15 minutes: the biggest structural shift

Bernstein’s analysis showed a sharp change in trading concentration at the end of the day. After CAS, the final 15 minutes accounted for about 1.6% to 2.3% of daily NSE turnover. Historically, that same window contributed about 10.1% of daily turnover. Lower activity in that window can make price discovery less predictable. It can also widen the gap between expectations and the final official close. Bernstein said reduced visibility on the likely final price is a core challenge. The report linked this to sharp price swings into the close. Brokers and fund managers have flagged this as an operational risk.

What Bernstein said about volatility and price swings

Bernstein said CAS has led to sharp equity price moves in the final minutes. Those moves can flow into derivatives-linked stocks and index levels. The research firm said the transition is likely to weigh on volumes and some capital-market businesses in the near term. At the same time, Bernstein expects the market to adjust over the coming months. The firm pointed to thinner liquidity as a reason for amplified closing moves. It also noted that even when pre-auction-to-close gaps were modest, intra-window moves could be sharp. That pattern has raised questions among active participants. The overall message was that adjustment is possible, but the transition phase is disruptive.

What traders are reporting: sellers squeezed, buyers whipsawed

Bernstein said retail traders selling options are seeing erosion in the value of their positions. For option buyers, outcomes can swing quickly when the index moves during the auction. That means sharp gains or losses can appear close to settlement. This is a material change for strategies that relied on stable, liquid closing conditions. Many expiry-day approaches assume tight spreads and predictable end-session behavior. If the closing print becomes less certain, hedging can become harder and more expensive. Some participants argue the new microstructure shifts risk toward those who cannot adapt quickly. Social media comments have framed the move as unfriendly to retail derivatives traders. The complaints have intensified because the impact is visible and immediate.

Spoofing concerns and SEBI’s response

Thin auction liquidity has fueled concerns about unexplained swings in the closing window. Bernstein referenced the possibility of order “spoofing” as a worry raised by market behavior. Participants point to sharp intra-window moves as the trigger for suspicion. Against this backdrop, SEBI chairman Tuhin Kanta Pandey said the regulator sees no manipulation in the new closing auction. He also said the change enhances market transparency and efficiency. The statement has been shared widely in Reddit threads and X discussions. Even with that assurance, trading firms are still adapting their execution methods. Some traders want clearer comfort on how the new close behaves in stress. The debate is now as much about liquidity design as about intent.

Potential knock-on effects for brokers and exchanges

Several reports highlighted that the change could hit derivatives-linked businesses in the near term. Zerodha Broking estimated the new system could reduce industry-wide earnings by 1% to 5%. Jefferies suggested a larger sensitivity through expiry-day activity. It said a 10% to 20% decline in expiry-day contracts could translate into a 5% to 10% overall fall in options trading volumes. These are estimates shared in the context, not official guidance. The concern extends to exchanges because derivatives contribute meaningful revenue. The National Stock Exchange of India is also preparing for an IPO, according to the context. A sustained volume shift would matter for any exchange-dependent revenue line. For now, the market is watching whether volumes normalize as participation returns.

What the early data points show

The most discussed numbers relate to the volume drop and the shrinking end-session share. They are being used as quick indicators of whether CAS is stabilising. The table below summarises the key metrics cited in reports and social posts. It does not attempt to explain causality beyond what was reported. The numbers are being compared against historical patterns from before CAS. Traders are focusing on them because many settlement outcomes hinge on the close. If the last 15 minutes remain thin, price discovery could stay jumpy. If participation improves, volatility may ease even without rule changes. The next few expiry cycles will be watched closely.

What to watch next as the market adjusts

Bernstein expects the system to settle over the coming months. Market participants also said volumes could improve as uncertainty fades and more traders participate. A key watchpoint is whether the closing window attracts deeper two-way liquidity. Another is whether brokers see fewer client complaints about settlement swings. Regulators have already signalled confidence in the integrity of the auction, based on SEBI’s “no manipulation” comment. Separately, derivatives activity had already been weak, with July hitting a 17-month low after the central bank tightened funding norms, as cited in the context. That means CAS is landing in a market that was not at peak participation. The balance between transparency goals and tradability will remain the core discussion. Until the new microstructure becomes familiar, expiry-day strategy risk is likely to stay elevated.