NextFin News - India's markets regulator is set to partly reverse one of its most consequential market-structure changes in years, scaling back a new closing-price auction after traders and institutions warned it had made expiry-day derivatives riskier and pushed trading volumes to their lowest level in nearly three years. The Securities and Exchange Board of India will stop using the closing auction to calculate derivatives settlement prices for at least a year, two people with direct knowledge of the matter said, returning instead to a 30-minute volume-weighted average price while keeping the auction for less-liquid stocks.
The shift, expected to be implemented by the end of October, marks a rare public retreat by a regulator that had rolled out the Closing Auction Session, or CAS, only two months earlier on August 3. It is a direct response to a design flaw at the heart of the reform: the cash-market auction ends at 3:30 PM, but the derivatives segment keeps trading for another 10 minutes, leaving large hedgers unable to lock in a single exit price for both their stock positions and the futures or options protecting them.
The reversal is the market-structure story of the year for India, and it carries a lesson that extends far beyond Mumbai: a closing-price mechanism that looks cleaner on paper can hollow out the very liquidity that makes prices trustworthy. SEBI's climbdown is not just about fixing a 10-minute timing gap. It is a judgment that price discovery cannot be engineered by fiat when the participants who supply depth have voted with their capital and left the room.
The Retreat: What SEBI Is Reversing, And What Stays
Under the plan, the volume-weighted average price of the last 30 minutes of trading will again determine derivatives settlement prices, the sources said. The closing auction itself will not be scrapped: for underlying stocks in the less-liquid cash market, the auction will still set the end-of-day price. The partial nature of the reversal is the tell. SEBI is conceding that the auction works poorly as a settlement reference for derivatives while maintaining that it still adds value for price discovery in thin names.
The new approach would align India more closely with the United States and Europe, where derivatives settlement is commonly determined through dedicated pricing mechanisms, including volume-weighted averages over set trading periods, rather than a single closing auction. In effect, India is re-importing the mechanism it had just retired.
The timeline matters. CAS went live on August 3 after more than two years of consultation — SEBI held public consultation rounds on December 5, 2024 and August 22, 2025, and deliberated with stock exchanges, broker associations, institutional investors and other market participants. The regulator issued a fresh consultation paper on September 12 seeking feedback on seven proposals covering derivatives settlement methodology, market timings and operational aspects of the closing auction. The comment deadline was October 3. Two days later, on October 3 at 7:00 PM, SEBI posted on X that the correct number of comments received stood at 20,000 — sharply higher than the roughly 3,500 some media sections had reported earlier.
An SEBI spokesperson did not respond to a request for comment on the reversal. But the regulator had already opened the door in a statement released the previous week:
Among the issues raised, a significant area of feedback relates to the determination of settlement prices of derivative contracts on expiry based on the closing price determined through CAS.
Having considered the initial experience of CAS implementation and the feedback received from various stakeholders, SEBI said it "may be proposing certain changes in the methodology for determination of settlement prices of derivative contracts."
Why The Closing Bell Broke: The 10-Minute Mismatch
To understand why the reversal was necessary, it helps to understand what the closing price actually does. It is not merely a number printed at the end of the day. It is the reference at which every equity derivative contract linked to that underlying is marked to market and cash-settled, the anchor for index levels such as the Nifty 50 and Sensex, and the basis on which mutual fund and ETF net asset values are calculated. When that reference moves on a mechanism traders do not trust, the damage propagates through the entire chain of hedging and valuation.
Before CAS, the cash and derivatives markets closed at the same time. A trader holding a large stock position could sell futures against it and know, with reasonable certainty, that both legs would execute near the same reference price. The closing price was derived from the volume-weighted average of trades over the final 30 minutes of continuous trading — a broad sample that was hard to move with a single order.
CAS replaced that with a batch auction. From 3:15 PM to 3:30 PM, orders are collected rather than executed immediately; they are then matched at a single equilibrium price — the price at which the maximum number of shares can trade — finalized between 3:30 and 3:35 PM. Meanwhile, the derivatives segment was extended to 3:40 PM. The result is a 10-minute window in which derivatives keep trading after the cash reference price has already been fixed.
That gap is the single biggest source of concern flagged by market participants. An expiring contract can now be settled at a CAS-determined level while 10 minutes of derivatives trading still remain, opening a window of unhedgeable price risk for institutional books. Traders who operated under the old regime had price visibility and minimal uncertainty while planning hedges; with the advent of CAS and the extended derivatives session, they could no longer safely predict what might happen in the derivatives market once the auction closed.
The problem compounds on expiry days, when Nifty 50 and Sensex contracts settle and traders are simultaneously trying to maximize profits or minimize losses. With the cash auction thin, a handful of large orders can swing the reference price sharply, and that swing transmits directly into derivatives payouts.
The Evidence: Institutions Walked Away, And Volumes Followed
The data confirms the behavioral shift. Institutional investors — corporates, foreign investors, mutual funds and proprietary algorithmic traders — together account for around 60-65% of turnover in India's derivatives market. These are the participants who rely most heavily on aligned cash-and-derivatives closes to plan hedges without residual exposure. Faced with a mechanism that left them unhedged for 10 minutes each day, they did the rational thing: they sat out both sessions.
NSE's total derivatives turnover in August was Rs 34.48 lakh crore, the lowest since November 2023. Average daily options turnover fell 20% month-on-month in August.
The 18-20% decline should be viewed as the overall market-volume impact observed after CAS was introduced, rather than a measure of the volume decline specifically within the CAS window.
That distinction matters. The damage was not confined to the 15-minute auction window; it bled into the broader derivatives complex because the settlement reference itself had become unreliable. When the price at which contracts settle cannot be hedged, the entire contract becomes less useful — and traders reduce their exposure accordingly.
The retreat of institutions has a second-order cost for the retail traders the reform was partly designed to protect. Thinner institutional participation raises intraday volatility and widens bid-ask spreads on expiry days, which raises the effective cost of trading options for smaller participants. A rule sold as investor protection ended up making the market more expensive and more volatile for the very investors it was meant to shield.
Cyclical Fix, Structural Question: What This Reversal Does And Does Not Solve
The reversal is best understood as a cyclical repair of a design flaw, not a structural repudiation of closing auctions as a concept. The 10-minute mismatch is a mechanical problem with a mechanical fix: either align the session ends, or de-link derivatives settlement from the CAS reference on expiry days. Restoring the 30-minute VWAP does exactly that. It removes the unhedgeable window and should, in time, coax institutional flow back into both segments.
But the deeper question is whether volumes will fully normalize. A derivatives analyst at a domestic broking firm cautioned that such a measure may reduce hedging risks and remove some of the uncertainty but may not be enough by itself to normalize volumes to pre-CAS levels: "People and algos will need more time to adapt and come up with newer strategies."
There is a reason for that caution. Trust in a market mechanism is slower to rebuild than it is to break. Traders who pulled back in August will want to see several clean expiry cycles before recommitting capital. Algorithmic strategies that were rewritten to avoid CAS will not be rewritten again overnight. The reversal removes the structural defect; it cannot instantly restore the confidence that defect destroyed.
There is also a credibility dimension for the regulator. SEBI had indicated earlier in the consultation process that it was reluctant to restore the VWAP method in any form. Within weeks, it is preparing to do precisely that for derivatives settlement. The reversal is the right call on the merits — the market reaction proved the design unworkable — but it will invite scrutiny of how thoroughly the original reform was stress-tested before implementation.
The Counter-Thesis: Why SEBI Backed The Auction In The First Place
The strongest argument against the reversal is the argument SEBI itself made when it designed CAS. Most major markets, including the New York Stock Exchange and the London Stock Exchange, use a closing call auction to set their reference prices. A batch auction is theoretically superior to a VWAP close: it prevents last-minute price manipulation, allows large orders to be executed more efficiently without moving the market, and reduces tracking error for index funds and ETFs that must replicate closing levels. By that logic, India was simply catching up to global best practice, and the expiry-day turbulence was an implementation bug, not a design failure.
That argument is not wrong in the abstract. It fails on one count: implementation is the design. A closing auction that ends 10 minutes before the derivatives segment it settles is not the NYSE or LSE closing auction. It is a hybrid that inherited the worst of both worlds — the concentration risk of a batch auction without the synchronized close that makes batch auctions work in mature markets. SEBI's error was not adopting the auction model; it was adopting it without aligning the market's plumbing to match.
The falsifying test for this reading is straightforward. If, after the VWAP restoration takes effect, institutional participation in the cash auction recovers and expiry-day volatility compresses toward pre-August levels within two to three monthly expiry cycles, then the problem was indeed the mismatch alone and the reversal is sufficient. If volumes remain depressed and volatility stays elevated despite the aligned reference, then the damage is deeper — a loss of confidence in the regulator's market-structure process that no mechanical fix can quickly repair. The specific signal to watch: NSE's average daily options turnover returning to within 10% of its pre-CAS trend, and the bid-ask spread on Nifty options during the final 30 minutes of expiry days narrowing back toward July 2026 levels.
What Comes Next: End Of October, And Beyond
The changes are expected to be implemented by the end of October 2026, which would give traders roughly one expiry cycle to adjust before the new — or rather, restored — settlement methodology takes hold. The Reserve Bank of India's monetary policy committee meets October 5-7 in the same window, with its rate decision due October 7, but for derivatives desks the settlement change is the more consequential operational event.
Beyond the immediate fix, three questions will shape the next phase:
- Will SEBI keep the auction for less-liquid stocks? The sources say yes — the reversal is partial, and the auction's value for price discovery in thin names remains intact. That is the right call: illiquid stocks benefit most from a batch auction's order aggregation.
- Will the session-end mismatch be fully closed? Restoring VWAP for derivatives settlement removes the hedging risk, but a clean fix would also align the cash and derivatives closing times. That remains an open design question.
- What does this mean for SEBI's broader reform agenda? The regulator has been active on derivatives rules, including the October 2024 curbs that limited weekly options to one index per exchange and tightened intraday position monitoring. This episode suggests a more iterative, feedback-responsive approach — but also a need for more rigorous pre-implementation stress testing.
For investors, the practical takeaway is narrow but important: expiry-day execution should improve as institutional flow returns, and the wild premium swings that characterized August and September expiries should moderate. But the reversal does not erase the two months of disruption already absorbed by traders, nor does it guarantee a swift return to prior volume levels.
The closing bell is supposed to be the most trusted moment of the trading day. For two months in India, it became the most contested. SEBI's reversal restores the old reference, but the episode leaves a durable lesson for regulators everywhere: market structure is not a blueprint you impose. It is an ecosystem you disturb, and the participants who live in it will tell you, quickly and in capital terms, whether the design works. This time, they spoke, and the regulator listened.
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