Zerodha CAS Rules Explained

SEBI’s Closing Auction Session (CAS) changed how closing prices are determined for F&O stocks starting August 3, 2026 — but the practical, day-to-day impact on your trades depends heavily on how your broker implements it.

If you trade on Zerodha, the platform has made specific adjustments to auto square-off timings, GTT triggers, and order handling to align with CAS. Here’s exactly what’s changed on Kite and what you need to watch for.

What Is CAS?

CAS is an auction-based mechanism that replaces the old VWAP-based method for determining a stock’s closing price. Instead of calculating the closing price as the average of trades in the last 30 minutes, the exchange collects buy and sell orders during a structured auction window and matches them at a single equilibrium price.

This currently applies only to Category I stocks — those with active F&O contracts. Category II stocks (everything else) continue with the old system for now.

Zerodha, like other brokers, had to rework several platform-level rules to accommodate this shift for Category I stocks specifically.

Zerodha’s Auto Square-Off Changes

This is the change most intraday traders need to know first.

• F&O stocks (Category I): Zerodha now auto-squares off open intraday positions (MIS and CO) by 3:12 PM, ahead of the 3:15 PM continuous trading cutoff. This gives a small buffer to ensure your position is closed before the auction window begins.
• Non-F&O stocks (Category II): No change. Standard intraday auto square-off remains at 3:25 PM, as it always has.

If you’re used to exiting F&O-linked intraday positions closer to 3:20–3:25 PM, that habit no longer works — Zerodha will square off the position for you before you get the chance, and you may incur square-off charges if you don’t exit manually first.

GTT Orders and Price Alerts on Kite

Good Till Triggered (GTT) orders and price alerts also follow different cutoff times depending on the stock category:

• F&O stocks: GTT orders and price alerts can trigger only until 3:15 PM, since continuous trading stops there.
• Non-F&O stocks: GTT orders and price alerts continue to trigger until 3:30 PM, the standard market close.
• F&O contracts themselves (derivatives): These can trigger until 3:40 PM, matching the extended derivatives segment close.

This means a GTT order you’ve set on an F&O stock expecting it to trigger near the old close may simply never fire, because the window has effectively shortened.

How Kite Handles Open Orders During CAS

For F&O stocks, Zerodha’s Kite platform automatically carries forward unexecuted limit orders from the regular session into the CAS window. However, this doesn’t apply to every order type. The following are excluded from carry-forward:

• Stop-loss orders
• Iceberg orders
• Orders placed outside the CAS price band (typically ±3% of the reference price)

If your order falls into one of these categories, it won’t automatically move into the auction session — you’ll need to place a fresh order during the CAS window if you still want it executed.

Placing Orders During the CAS Window

During the auction session on Kite, only limit orders are accepted — market orders are not allowed. Order entry closes at a randomized time between roughly 3:28 PM and 3:30 PM, a deliberate design choice by SEBI to prevent traders from timing orders to the exact last second. Once this window closes, the exchange matches orders and determines the closing price during the final matching phase.

Common Mistakes to Avoid on Zerodha

• Assuming the old square-off time still applies to F&O stocks — it’s now 3:12 PM, not 3:25 PM.
• Placing an order far outside the price band and expecting it to sit in the book — it gets auto-rejected instead.
• Confusing GTT cutoffs across F&O stocks (3:15 PM), non-F&O stocks (3:30 PM), and F&O contracts (3:40 PM) — these are three different timings, not one.
• Not checking whether a stock is Category I or II before assuming which rules apply — this can change if F&O contracts are added or removed from a stock.

How to Check a Stock’s Category on Kite

The simplest way is to check whether the stock has an active F&O contract listed on Kite or the NSE/BSE F&O list. If it does, Category I (CAS) rules apply. If not, the stock still follows the standard Category II timings and VWAP-based closing price.

Frequently Asked Questions

About the Author

Suyesh Gusain holds an MA in Mass Communication and is NISM-certified in Equity Derivatives and Research Analysis, with two years of hands-on experience trading Indian equity markets. He writes about SEBI regulations and market mechanics in practical, easy-to-understand terms for retail traders and investors.

Disclaimer: This article is for informational purposes only and does not constitute investment or trading advice. Broker-specific rules can change — always verify current timings and order handling directly through Zerodha’s official announcements and Kite platform notifications.