SEBI Closing Auction Session is barely a month old, and the regulator is already reviewing one of its consequences. On September 3, the Securities and Exchange Board of India said it would examine the methodology used to determine settlement prices for derivative contracts after receiving feedback following the August 3 rollout of the new closing mechanism. The review is timely. CAS was introduced to produce a better cash-market closing price. Yet that closing value also feeds into the settlement of expiring futures and options. A mechanism designed for the cash market can therefore have consequences far beyond it.
How the Closing Auction Session works
Until CAS was introduced, the closing price of a stock was based on the volume-weighted average price of trades during the final 30 minutes of continuous trading. SEBI replaced this for shares with derivative contracts by a separate auction running from 3.15 pm to 3.35 pm. The session includes calculation of a reference price, order entry and matching, after which the auction determines the closing price.
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There is nothing experimental about a closing auction as such. SEBI pointed out when announcing the framework that major markets use auctions to establish closing prices. Pooling orders at the close can improve execution for institutions that need to transact near benchmark prices. It can also produce a closing value based on concentrated demand and supply rather than an average of trades over the preceding half-hour.
The case for CAS is therefore sound. The difficulty arises from the place occupied by the closing price elsewhere in India’s market structure.
At NSE, the final settlement price of index futures and options is the closing value of the underlying index on expiry day. For derivatives on individual securities, the relevant closing price of the underlying stock is used. BSE’s derivatives framework similarly ties final settlement to the underlying closing value. The cash-market close is therefore more than a number printed at the end of the trading session. On expiry days it helps determine gains and losses on outstanding derivative positions.
Why expiry days have become more sensitive
The first month of CAS has shown how much can happen when price discovery is concentrated near the close.
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On September 3, during the weekly Sensex derivatives expiry, the index’s indicative auction close at one point showed a fall of about 2.5%. Premiums on some Sensex put options jumped 400% to 500% before the auction settled. The Sensex eventually closed 0.55% lower. The Nifty, whose movement was less pronounced, ended 0.17% down.
That episode does not establish that the auction produced an incorrect price. An auction may reveal information or order imbalances that were not visible during continuous trading. Nor should divergence between two indices, with different constituents and weights, automatically be treated as evidence of market failure.
The episode does show how an auction movement can be transmitted immediately into derivative settlement. Traders carrying large positions into expiry have to respond to a closing value being established within a concentrated period. Hedging demand can then interact with the auction itself, particularly where liquidity in the relevant underlying stocks is limited.
CAS had an encouraging test as well. The August 31 MSCI rebalancing generated about $4.1 billion of trades during the auction window at NSE, nearly 40 times the average auction turnover since CAS began. Despite large movements in some affected stocks, the market absorbed the flows without a comparable disruption to the benchmark indices.
The lesson from the first month is therefore more complicated than the early volatility suggests. Closing auctions can handle substantial institutional business. Expiry-day derivative settlement creates a separate set of incentives that deserves examination.
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Falling derivatives turnover needs careful reading
Trading activity has fallen since CAS began. Average daily turnover in equity derivatives dropped 22% from July to ₹346.9 trillion in August, a 14-month low and the sharpest monthly decline since December 2024. Cash-market turnover, by comparison, was broadly stable. Reuters separately reported a roughly 20% fall in average daily options turnover during the first month, with some algorithmic trading firms cutting activity more sharply.
Those numbers warrant attention, but they do not establish that CAS caused the entire fall. India’s derivatives market was already being reshaped by regulatory measures intended to reduce speculative trading. SEBI has raised contract sizes, reduced the number of weekly expiries and tightened risk management over the past two years. Its latest study found that the number of active individual derivatives traders fell by about a fifth in FY26, while 87.7% of individual traders still lost money.
A decline in speculative turnover is therefore not, by itself, an argument against CAS. SEBI should be more concerned if uncertainty around the auction discourages market-making, raises hedging costs or reduces liquidity needed for efficient price discovery.
This distinction matters for the regulator’s review. The success of a closing auction should not be measured by whether derivatives turnover returns to its earlier level. It should be measured by whether the auction produces a credible closing price without creating avoidable distortions elsewhere in the market.
SEBI should separate two pricing functions
There is no compelling reason to abandon CAS after one difficult month. Its objective remains worthwhile. India needs a closing mechanism capable of absorbing institutional flows and producing a price that reflects demand and supply at the end of the trading day.
The more useful question is whether the same auction price should automatically determine the final settlement of a much larger derivatives ecosystem.
SEBI can retain CAS as the mechanism for establishing the official cash-market close while examining whether derivative settlement should draw on a broader price window or another methodology that reduces dependence on the outcome of a single auction. Any alternative would need to reduce expiry-day sensitivity without reopening opportunities for manipulation or creating additional basis risk for hedgers.
That is a narrower reform than dismantling CAS, and a more defensible one. The closing auction and derivatives settlement perform different functions. Linking them mechanically makes the design of one responsible for risks created in the other.
SEBI’s decision to review the settlement methodology recognises the problem. India can keep the gains from a modern closing auction while reducing the leverage that a few minutes at the end of an expiry day can exert over the derivatives market.