Closing Auction Session: Kamath flags market gaps

Sharp end-of-day price moves have become a talking point as India prepares to shift how closing prices are set for certain stocks. The debate has focused on the new Closing Auction Session (CAS), which becomes effective from August 3, 2026, and applies to stocks with futures and options (F&O) contracts. Zerodha founder and CEO Nithin Kamath has argued that CAS itself is not the root cause of recent dislocations. In posts on X, he described closing auctions as a globally accepted mechanism used by large markets. He also said the mechanism can improve price discovery and help execute large institutional orders more cleanly. At the same time, Kamath acknowledged the rollout will change investor workflows and create operational complexity for brokerages. NSE, meanwhile, has said a recent CAS ran as intended and that there was no technical issue.

Why the market close is suddenly under scrutiny

The close matters because the day’s final price is used as a reference point by many market participants. Recent sharp moves into the close have led to questions about whether the new closing method is amplifying volatility. The discussion has intensified because CAS is a visible, structural change that coincides with these moves. Kamath’s view is that the auction is revealing limitations that already exist, rather than creating them. He has framed CAS as a tool designed to make price formation more robust, not weaker. The debate has also been shaped by concerns around execution for passive funds that track indices. Those funds often need to trade near the close to match benchmark closing prices. When large orders hit late, prices can move quickly and tracking error can rise.

How closing prices were calculated before CAS

Kamath noted that the earlier closing price was based on the volume-weighted average price (VWAP) of trades in the last 30 minutes of trading. That method ties the close to continuous trading activity late in the session. It also means a sequence of trades near the end can influence the final price, especially if liquidity is uneven. Under that approach, investors watching the last half hour could often infer where the close might land. It also created a familiar, single continuous window for most cash market participants. The VWAP-based close did not require a separate auction phase for eligible stocks. For many retail investors, the process was simple because it matched the visible market tape. The change to CAS alters that end-of-day routine and may require new expectations around how the final print is formed.

What the Closing Auction Session changes from August 3, 2026

SEBI has introduced CAS as a new way to calculate the closing price of a stock, effective August 3, 2026. Instead of looking at trades over the last 30 minutes, CAS pools buy and sell orders at the end of the day and determines a single equilibrium price. The equilibrium is the price where the maximum number of shares can be traded, and that becomes the closing price. The stated aim is better price discovery and making end-of-day manipulation more difficult. Kamath also pointed to a practical reason: passive funds often execute large orders near the close, and pooling can reduce abrupt prints. He said that because CAS matches orders at a single price, influencing the close becomes harder. The new framework is being introduced by Indian exchanges for stocks with F&O contracts, replacing the earlier closing mechanism for that set. Over time, the market will have to adapt to auction-based closing for these names.

New closing times: three different cut-offs

Kamath warned that the new schedule creates staggered market closing times that may confuse investors initially. Stocks with F&O contracts will stop continuous trading at 3:15 pm and then move into the Closing Auction Session. Stocks without F&O contracts will continue trading until 3:30 pm under the regular cash market timetable. Index and stock futures and options contracts will continue trading until 3:40 pm. The CAS itself has been described as a structured 20-minute window for eligible stocks from 3:15 pm to 3:35 pm. A transition period calculates reference prices until 3:20 pm, followed by order entry that shifts to limit-orders only at 3:25 pm. To reduce last-second gaming, the order entry session closes at a randomized time between 3:28 pm and 3:30 pm. Kamath said “things get a little more complicated” from Monday, August 3, and flagged higher customer support load for brokers.

Snapshot table: old close vs CAS, and timing changes

Kamath’s core argument: CAS is not the main problem

Kamath’s central point is that closing auctions work well when markets have deep liquidity and a diverse ecosystem. He listed market makers and arbitrageurs as important participants in that ecosystem. In his framing, CAS is a mechanism that assumes the presence of these stabilising forces. When those forces are weaker, an auction can surface bigger gaps between supply and demand at the close. That can look like a CAS problem even if it is actually a market-structure issue. He said “CAS itself is not a bad idea” and noted that most large global markets have some form of closing auction. NSE has also said a recent CAS functioned as intended, reinforcing that the debate is not about a technical breakdown. Kamath still allowed that refinements may be needed, but he pushed the conversation toward deeper reforms. The message is that design tweaks alone may not solve what he sees as structural constraints.

Liquidity, market makers, and why auctions need depth

Kamath said closing auctions work best when liquidity is deep and participation is broad. A larger, more varied set of participants can narrow imbalances and make pricing more resilient. Market makers can supply liquidity when one side dominates, and arbitrageurs can keep prices aligned across instruments. In a thinner ecosystem, pooled orders can produce a sharper clearing price because fewer participants can step in. Kamath’s posts imply that the mechanism is exposing that vulnerability. He also noted distortions between instruments as an area that needs attention, suggesting misalignments can become more visible around the close. For investors, the practical takeaway is that the closing print may reflect more than just the day’s last traded momentum. It can reflect who is able to respond during the auction window. Strengthening the ecosystem, in his view, would make the auction outcome less sensitive to imbalances.

The shorting constraint and Kamath’s “upward pressure” point

A key structural issue Kamath highlighted is the difficulty of exploiting arbitrage opportunities in India. He linked this to the challenge of taking short positions in the cash market. In his words, unless borrowing stocks and shorting them becomes easy, there is bound to be structural upward pressure in the markets. This argument positions shorting and securities lending as market-balancing tools rather than speculative add-ons. If investors cannot easily short, negative views may not translate into trades that offset buy-side pressure. In that setting, a closing auction can still clear, but the price may skew if sell-side participation is constrained. Kamath stressed that CAS is not the reason these limitations exist, but that it makes them more apparent. He also pointed to “reducing distortions between instruments” as part of the fix, which aligns with making arbitrage more effective. His suggested direction is to make it easier for different market views to express themselves through trades.

Rollout timing and the liquidity backdrop Kamath cited

Kamath noted that the CAS rollout has coincided with the Reserve Bank of India’s new norms on banks’ capital-market exposure. He said this overlap further impacts liquidity, which matters because auctions depend on participation and available capital. The point is not that RBI norms caused CAS, but that the market is absorbing multiple changes at once. When liquidity is affected, price discovery at the close can become more sensitive to order imbalances. This can feed the perception that the auction itself is driving volatility. Kamath’s framing suggests investors should separate the mechanism from the environment it is operating in. If liquidity improves and participation broadens, the same auction design may produce smoother outcomes. If liquidity tightens, even a well-designed auction can produce more visible dislocations. This context is part of why he argued the bigger challenge is strengthening the overall ecosystem.

What may change next and what investors should prepare for

Kamath said some refinements to CAS may be required, while keeping the focus on broader structural improvements. He specifically called out making shorting and securities lending easier and encouraging genuine market-making. He also mentioned reducing distortions between instruments, which would help arbitrage keep markets aligned. On the operational side, he expects confusion initially because different segments will close at different times. He also said brokerages are likely to see more customer queries as investors adjust to the new timetable. Separately, he acknowledged a business impact, saying the change could reduce Zerodha’s brokerage income by around 1-5 percent. That estimate was presented as a modest hit, but it reflects lower activity or different trading behaviour around the close. For investors, the immediate practical adjustment is understanding when continuous trading ends for the instrument they trade and how the closing price will be formed. The broader debate, as Kamath framed it, is whether India can build the depth and participation needed for closing auctions to consistently deliver stable price discovery.