Expiry-day volatility in India's derivatives market poses risks to market integrity and retail investors. Examine the role of a Closing Auction Session in addressing these concerns.

On expiry day, derivative contracts are settled against the underlying stock's closing price. A few minutes of distorted trading can therefore change payouts on very large positions. SEBI's Closing Auction Session (CAS) began on 3 August 2026 for stocks that have derivatives. It sets the closing price through an auction between 3:15 and 3:35 pm, and that price is also used to settle derivatives at expiry [1][2]. CAS is a useful reform, but it does not solve the problem fully on its own.

Risks of expiry-day volatility

• Price swings: the older method closed a stock at the VWAP (volume-weighted average price) of its last 30 minutes of trades. This often caused intraday swings, especially during index rebalancing [2].
• Passive funds: passive funds hold about 30% of FPI equity AUM and 28% of domestic mutual fund equity AUM. They need an accurate closing price to avoid tracking error [2].
• Retail investors: a small move at the close can turn an option that was about to expire worthless into one worth exercising. Stock derivatives are physically settled, so a small trader may suddenly owe shares or the full cash value.

How CAS helps

• One price, more liquidity: all buy and sell orders meet in one auction, which gives a single equilibrium price, less price disruption and more certainty of execution [2].
• Fairness: CAS gives fair, equal and transparent access to every category of investor. It follows the practice of the NYSE, LSE and HKEX [2].
• Easier surveillance: with one visible window, one-sided orders stand out. SEBI issued an ex-parte interim order within days of alleged manipulation during CAS on the Sensex expiry of 13 August 2026 [3].

Limitations

• The auction can be gamed too: the same Sensex case shows that manipulators can move into the new window [3].
• Settlement risk: in the first month, most market feedback concerned derivative settlement prices that were based on the CAS close [1].

Way forward

• Separate cash closing from derivative settlement: SEBI's consultation paper offers two options. One is a blended VWAP of the last 30 minutes of continuous trading plus 10 minutes of CAS. The other is an interim VWAP of continuous trading only [4].
• Use real-time surveillance, tight price bands and investor awareness campaigns.

CAS improves price discovery and transparency, which supports market integrity. However, it works best together with a sound settlement method and strong enforcement. If SEBI calibrates it in phases, CAS can protect retail investors and keep India in line with global practice. This serves SEBI's statutory mandate of investor protection and orderly market development.