Explained: Why Closing Auction Session is triggering sharp market swings

India's stock market has entered a new era of price discovery after the National Stock Exchange (NSE) and BSE introduced a closing auction mechanism for more than 200 stocks with derivatives contracts. While the move is designed to make benchmark prices more robust and align India's markets with global peers, its first two trading sessions have produced unexpected price swings, leaving traders, investors and fund managers adjusting to the new regime, according to a Bloomberg report.

The official closing price plays a critical role in financial markets. It is used to calculate benchmark indices, settle derivatives contracts, value exchange-traded funds (ETFs), mutual funds and institutional portfolios, making even small changes significant for market participants.

What has changed

Until last week, the official closing price for equities was calculated as the volume-weighted average price (VWAP) of all trades executed during the final 30 minutes of continuous trading, which spanned from 9:15 a.m. to 3:30 p.m.

Under the new framework, continuous trading for eligible stocks ends at 3:15 pm, after which exchanges conduct a 20-minute closing auction, the Bloomberg report said.

During the auction window, from 3:15 pm to 3:35 pm, buy and sell orders are collected without immediate execution. The exchange then determines a single equilibrium price that maximises the number of shares that can be matched between buyers and sellers. That equilibrium becomes the stock's official closing price. Derivatives trading continues until 3:40 pm, while stocks without derivatives contracts continue to follow the earlier VWAP-based closing methodology.

Why regulator introduced the auction

The Securities and Exchange Board of India (SEBI) first proposed the closing auction mechanism in 2024 after passive fund managers sought a system that could improve benchmark tracking and reduce pricing distortions, the Bloomberg report said.

Instead of relying on thousands of trades spread over the final 30 minutes of trading, the auction concentrates liquidity into a single price-discovery event, making it more difficult for market participants to influence benchmark prices near the close.

The mechanism is already widely used by major global exchanges, including the New York Stock Exchange (NYSE), Nasdaq, the London Stock Exchange and the Australian Securities Exchange (ASX).

What happened during the first two sessions

The transition has been far from smooth. Rather than producing a stable closing price, the first two auction sessions generated sharp differences between prices at the end of continuous trading and the official market close.

On Monday, the Nifty 50's official closing level finished roughly 0.8% higher than its level at 3:15 p.m. On Tuesday, the gap remained significant at around 0.6%.

Interestingly, index futures remained largely unchanged during both sessions, suggesting the moves were driven primarily by auction dynamics rather than a broader reassessment of market valuations.

Why are prices moving so sharply

According to Goldman Sachs, many institutional investors stayed away from the first auction sessions, resulting in relatively thin liquidity. With fewer buy and sell orders in the closing auction, even modest order flows were able to move the equilibrium price sharply.

Because the auction determines the closing price by matching the maximum executable volume at a single price, large buy or sell orders can have a disproportionately high impact when participation is low.

The NSE acknowledged the issue after Monday's session, saying it expects participation in the auction process to increase over time as investors become more familiar with the new mechanism.

Why are traders confused

For decades, traders viewed the final minutes of continuous trading as the market's effective closing period. Under the new system, however, prices at 3:15 pm no longer represent the official close. Instead, the benchmark closing price is determined only after the auction concludes at 3:35 pm.

This has created confusion because stocks can finish regular trading at one level before closing significantly higher or lower following the auction. The disconnect has been particularly noticeable because derivatives prices have not always reflected the movements seen in the cash market.

Retail investors have also raised concerns over transparency, with several market participants pointing out that indicative auction prices displayed during the auction are not easily visible on some trading platforms.

Impact on market sentiment

The biggest consequence has been increased uncertainty around benchmark closing prices.

Index funds, ETFs, arbitrage strategies, derivatives settlements and institutional portfolios that rely on official closing prices have all experienced larger-than-expected deviations from prices seen at the end of continuous trading.

The divergence has also made it more challenging for traders to hedge positions before the official close and has prompted market participants to reassess how they incorporate auction dynamics into trading strategies.

Have other markets experienced similar issues?

Yes.

Closing auctions are a standard feature of most developed equity markets and initially experienced similar challenges before liquidity improved.

Australia is a key example. During the Covid-19 market volatility, the S&P/ASX 200 Index recorded a session in which nearly three percentage points of its 4.4% daily gain occurred during the closing auction, resulting in one of the index's biggest one-day advances since the 2008 financial crisis.

Over time, however, institutional participation increased and auctions became an integral part of the price discovery process.

Academic studies and institutional investors generally view closing auctions as an efficient mechanism for determining end-of-day prices because they aggregate liquidity into a single transparent process.

Market participants expect a similar evolution in India, with institutional investors gradually shifting more trading activity into the closing auction as liquidity deepens and the market adapts to the new framework.