For years, the last few minutes of the trading day were the Wild West of Indian markets – a narrow window where a handful of well-timed trades could quietly tilt the closing price for everyone else. SEBI just tried to fix that. And for a few days in August, it looked like the fix itself might be the problem.

What Happened?

India’s stock markets just went through a quiet but significant plumbing change. On August 3, SEBI rolled out the Closing Auction Session (CAS) – a new 20-minute window at the end of the trading day, used specifically to determine the official closing price for the Sensex and Nifty. Instead of the old method (a VWAP-based calculation using trades from the last few minutes), CAS collects buy and sell orders separately and matches them to arrive at a closing price – the same way stock exchanges in Japan, Hong Kong, the US, Germany, and Australia already do it.

Naturally, the first session didn’t go smoothly. There were noticeable price differences between the CAS-derived closing price and what the old system would have produced. Traders noticed. Social media noticed. And within days, questions started swirling about whether something was off with the mechanism itself.

Why This Came Into the Limelight?

Because closing prices matter more than most people realise. They decide the NAV of every mutual fund, they’re the reference point for countless derivative contracts, and they set the benchmark that index funds track. A wobble in how that number gets calculated isn’t a small technical footnote – it ripples through the entire market.

So when the very first CAS session on August 3 produced price gaps that felt unusually large, it triggered exactly the kind of chatter regulators dread: whispers of manipulation, concerns that a handful of large trades could be swinging the closing price, and brokers scrambling to explain a mechanism their own trading algorithms weren’t even built for yet.

What Is the Issue?

At its core, the problem wasn’t really manipulation – it was unfamiliarity. SEBI Chairman Tuhin Kanta Pandey was candid about this on the sidelines of the Global Commodity Conclave in Mumbai. Most trading algorithms and systems, he pointed out, were designed around the old closing-price process. CAS is structurally different, and the industry simply hadn’t caught up.

The other issue was visibility. CAS is meant to be a transparent auction – participants are supposed to see indicative prices building up in real time as orders come in, which helps everyone understand roughly where the close is heading before it actually happens. But in the early days, most traders couldn’t see this indicative pricing at all, because most brokers hadn’t added it to their platforms yet. Without that visibility, the auction looked opaque even though, by design, it wasn’t supposed to be.

This is exactly the kind of gap that matters more than people realise – when a broker’s platform doesn’t surface a new data point on day one, retail traders are left guessing at a mechanism the regulator insists is fully transparent.

And underneath both of these was a third, quieter problem: low participation. A closing auction only works well if enough people actually place orders in it. Thin participation means a small number of trades can swing the price more than they should – not because of manipulation, but simply because there isn’t enough volume on the other side to absorb it.

What Is the Solution?

SEBI’s response wasn’t to scrap or overhaul CAS – it was to let the market catch up to the tool. A few things are already in motion:

Indicative pricing on broker platforms: Zerodha and Upstox have already added indicative CAS prices to their web platforms, with app versions expected soon. ICICI Securities, Groww, and Angel One are expected to roll out the same feature by August 14. Once every major broker shows this data, participants get real-time visibility into where the closing price is heading – no more flying blind in the last 20 minutes.

This is also where a platform like Alice Blue matters to its own users: as more brokers add indicative CAS pricing to their dashboards, traders on platforms that integrate it cleanly get a much clearer read on where the close is heading, instead of discovering the final number only after the auction has ended.

A random closing mechanism: Built directly into CAS, this prevents any single last-second order from disproportionately swinging the final price – addressing the exact vulnerability that existed in the old VWAP system.
Encouraging mutual fund and broader participation: SEBI is actively pushing more institutional and retail participation into the auction window itself, since a deeper, more liquid auction is the most reliable defence against price distortion.

Pandey’s framing was refreshingly practical: this is a “microstructure reform,” not a controversy. It’s the kind of infrastructure upgrade that most large economies have already gone through, and its teething issues – real as they are – reflect the market adjusting to a new format, not any deeper flaw in the system.

What Does SEBI Say?

SEBI’s public position is measured but firm. Pandey stated plainly that the regulator has not found any evidence of manipulation in CAS so far. He noted that the price gaps seen during that first session on August 3 have already narrowed in subsequent sessions – a sign the market is adapting, not breaking.

SEBI’s bigger point was really about mindset, not mechanics.

The bigger picture: “CAS is a very big microstructure reform, and we are actually behind it,” Pandey said, pointing out that Japan adopted a similar system in 2024, while Hong Kong, the US, Germany, and Australia already run something comparable.
The real priority: Not fixing CAS, but getting more people comfortable using it. “The issue now is that participation in CAS should increase,” he said. “It’s a transparent market; pricing is always visible.”
What SEBI’s doing about it: Staying in touch with brokers, exchanges, and market participants – listening to feedback and weighing whether any tweaks are genuinely needed, without rushing to change course too soon.

Conclusion

The CAS rollout is a reminder that market infrastructure changes rarely go perfectly on day one – and that’s fine, as long as regulators and market participants treat early friction as a signal to adapt rather than a reason to panic. SEBI’s read is clear: no manipulation, just a market still learning a new rhythm. As indicative pricing spreads across brokers like Alice Blue and participation deepens, the price gaps that sparked concern in early August should continue to narrow on their own.

For traders, the real takeaway isn’t to distrust the closing price – it’s to make sure the platform they’re using is keeping pace with how that price now gets made, so the last 20 minutes of the trading day feel like clarity rather than a guessing game.