Six Weeks Of CAS: From The First Screen To SEBI's Desk

CAS launched on August 3. It's now been a little over six weeks, and the story has moved through distinct phases — early data, a mechanics problem worth understanding properly, a pattern that repeated on consecutive expiries, and now a regulatory response with a comment window still open. Worth connecting those phases in one place, since each was covered as it happened rather than as a single narrative.

Week one: the first data

The first piece on this site, four days after launch, looked at what the first week of CAS actually did to the Nifty cash close, to expiry settlement, and to 0DTE options volatility. Early enough that the mechanism itself was still new to most traders reading it — the point of that piece was less "here's what's wrong" and more "here's what actually changed, mechanically, versus the system everyone had traded under until August 3."

Late August: learning to read the screen

By August 25, the more useful thing to write about wasn't what CAS was doing to prices — it was how to actually read the screen while it was happening. That piece walked through the mechanics of a call auction: NSE and BSE scanning the entire order book to find one single equilibrium price at which the maximum quantity can execute, rather than the continuous tick-by-tick matching every trader was used to. Understanding that distinction turned out to matter more than it looked like at the time, because it's exactly the mechanic behind everything that followed.

Two Thursdays, same signature

Understanding the screen made it possible to actually diagnose what started showing up on expiry days. On August 27, the Sensex's low for the CAS window came from an illiquid Indicative Equilibrium Price pinned at 74,964 — precisely the −3% floor of that session's band. The index swung 2,171 points inside the auction window. On September 3, the same pattern repeated: an illiquid IEP at 74,268, again exactly on the floor, this time a 2,658-point swing.

What made the pattern worth taking seriously rather than dismissing as noise was the contrast sitting right next to it. On August 31, the MSCI India rebalance ran roughly ₹39,000 crore through the same mechanism — an order of magnitude above a normal session — and it settled calmly. A week later, an ordinary expiry auction did the opposite on a fraction of the size: Sensex CAS turnover of roughly ₹126 crore produced a 2,000-point swing getting to its close. Size wasn't the variable. Liquidity was — and specifically, what has to settle against that liquidity: an index derivatives book running into the tens of thousands of crores, pricing off a cash auction that, most weeks, clears barely a tenth of that in matched value.

That underlying mechanism — index derivatives continuing to trade in real time while the cash market they derive value from has already stopped executing trades, for the better part of ten to fifteen minutes before the close — is also the mechanism behind SEBI's own Copthall/Mansi order of August 19: a ₹98 crore order that moved the Sensex indicative reading 362 points in two seconds, without a single share trading, timed against options positions built to benefit from wherever the close eventually landed.

The full breakdown of that pattern, and the fix it pointed toward, ran as a piece on NDTV Profit in early September.

SEBI responds

On September 12, SEBI issued a 17-page consultation paper reviewing CAS settlement methodology, market timings, and several operational details — with comments invited through October 3. Read against what had been argued through August and early September, several of the paper's own proposals land on the same structural concern: it proposes ending dissemination of the composite index-level indicative value during the auction, the exact number behind the Copthall episode, while keeping individual security-level indicative prices visible. It offers two paths for settlement methodology — a blended VWAP, or the older continuous-session VWAP kept in place until cash-auction liquidity genuinely improves. And it tightens, though doesn't eliminate, the ability to place an order well away from the reference price and cancel it before the match — the exact mechanic both the August 13 episode and the two expiry Thursdays turned on.

A full section-by-section response to that paper, including where the paper's proposals go further than expected and where real gaps remain — order types during the auction are still unaddressed, and the underlying size mismatch between ordinary CAS liquidity and the derivatives book settling against it isn't resolved by any of the current proposals — is in the NDTV piece linked above, along with the formal submission filed with SEBI directly.

Where this leaves it

Comments on the consultation paper close October 3. Whatever the final settlement methodology looks like, the mechanics covered in the second piece on this site — how a call auction actually finds its equilibrium price, and why that's a different question from what a continuous market does — aren't going away. If anything, the last six weeks have been a live demonstration of exactly why that distinction matters, on both a quiet Tuesday and a Sensex expiry Thursday.

The charts and order-flow tools referenced across these three pieces are the same ones used to pull the CAS-window data underlying all of them — worth a look for anyone trading around expiry sessions directly: charts.vtrender.com.