SEBI Closing Auction: How CAS Created a 300 bps Market Risk

When the Securities and Exchange Board of India (SEBI) introduced the Call Auction Session (CAS) to determine closing prices, the goal was straightforward: reduce pre-close spoofing and address small structural inefficiencies. However, early market behavior suggests that the new mechanism may be creating larger distortions, with stock prices moving sharply in the final minutes of trading.

What Is the Call Auction Session?

The CAS is designed to determine closing prices through an auction rather than continuous trading. In theory, concentrating orders should improve price discovery. In practice, an auction needs sufficient liquidity and broad participation to produce a reliable clearing price.

The Liquidity Problem Behind CAS

Early observations point to a significant decline in closing-session liquidity. Auction volumes on the National Stock Exchange (NSE) are reportedly around 1% of total daily turnover, compared with approximately 12% under the former 30-minute Volume-Weighted Average Price (VWAP) system.

On the Bombay Stock Exchange (BSE), relatively small order imbalances of around ₹12–15 crore may be enough to influence the Sensex closing price. When market depth is limited, a small number of orders can have an outsized impact on the final print.

Five Risks Created by the Current Closing Auction Design

1. The ±3% price band may amplify distortions

In a shallow auction, a large order imbalance may push the clearing price toward the price-band limit. Instead of simply containing volatility, the ±3% boundary can become a target for price movement when available liquidity is insufficient.

2. Low liquidity can create a feedback loop

Participants may hesitate to provide liquidity to an auction they consider unpredictable. Lower participation then produces thinner order books, more volatile closing prices, and even less willingness to participate.

3. Predictable reversals may transfer wealth

If sharp closing moves regularly reverse at the next market open, investors who trade or receive valuations based on closing prices may face unnecessary execution and valuation risk. This is particularly relevant for retail investors and mutual funds whose transactions use end-of-day prices or net asset values.

4. Expiry days increase basis and execution risk

Late auction volatility can leave futures positions and hedges misaligned. The uncertainty surrounding the final uncrossing period also creates jump risk, which market makers may reflect through wider bid-ask spreads.

5. Closing-price distortions spread across the financial system

Closing prices influence mutual fund NAVs, portfolio margins, collateral valuations, and index-derivative settlements. A distorted final price can therefore affect more than the last few minutes of one trading session.

Three Possible Fixes for Indian Markets

1. Use a 3:15 PM reference price for expiry derivatives

Using an earlier reference price for expiry settlements could reduce the impact of late-session auction volatility and improve hedging reliability.

2. Create one common NSE–BSE closing auction

Pooling order flow across exchanges could reduce venue arbitrage and give the auction a deeper liquidity base. Any such change would require careful coordination between exchanges and regulators.

3. Incentivize designated liquidity providers

Designated liquidity providers could help maintain two-sided markets during the closing call. Possible incentives include exchange-fee waivers or Securities Transaction Tax (STT) offsets, subject to regulatory approval.

Conclusion

Closing auctions can improve price discovery when they are supported by deep liquidity, transparent rules, and broad institutional participation. If the current CAS design continues to produce thin order books and sharp price gaps, SEBI and the exchanges may need to adjust the mechanism before temporary volatility becomes a structural market problem.

Note: The market observations and proposed reforms in this article should be independently verified against official exchange and regulatory data. This article is for informational purposes only and is not investment advice.