Closing Auction Session (CAS) is a hot topic among
traders and equity market participants. Many traders feel uneasy, predicting
volatility and uncertainty, especially since their strategies have
traditionally revolved around VWAP (Volume Weighted Average Price) for all 0DTE
(0 days to expiry) strategies.
Recent Market Behavior and Concerns Around CAS
- In
the last two expiry cycles, option prices spiked and then sharply dropped
after the closing price was discovered.
- Many
traders faced losses, even when their thesis was correct, because
stop-losses triggered amid uncertainty.
Key Concerns About CAS:
- Derived
CAS prices are not visible on usual apps and terminals.
- Uncertainty
about the closing price creates fear.
- Suspicions
exist that closing prices can be manipulated.
- Implied
Volatility (IV) in options spikes on expiry and becomes highly volatile
before close, with no exit for traders on the wrong side post-CAS.
- High
risk of fat finger trades distorting stock prices and the index closing
price.
- Index
tends to close higher because many hold liquid cash rather than stocks.
- Fear
of CAS has reduced option volumes, decreasing liquidity.
- SEBI
regulations have tightened, limiting weekly expiries, increasing margins,
and imposing other restrictions that impact the options market.
- MWPL
(Market-Wide Position Limit) based on stock delta creates uncertainty on
position adjustments, forcing traders to close full positions.
Understanding SEBI's Perspective and Market Evolution
- SEBI
introduced CAS after thorough data analysis, global best practices review,
and stakeholder feedback.
- The
consultation paper was released for public comments in August 2025.
- SEBI’s
past rules have increased exchange volumes, reduced defaults, enhanced
technological use, and improved market transparency.
- Market
microstructures evolve with regulatory changes, as seen in prior changes
like migration from American to European options.
Current Realities and Future Expectations
Transparency of CAS Prices
- Currently,
CAS prices are not widely available except on select terminals (NEAT,
BOLT, in-house CTCL using TBT data).
- Over
time, software vendors and data providers will incorporate CAS price and
anomaly data for better decision-making.
Uncertainty and Liquidity
- Closing
prices can vary due to last-minute buy/sell orders or order withdrawals.
- Such
outlier price movements will reduce as liquidity in CAS improves.
Potential for Price Manipulation
- Price
rigging is possible now due to low liquidity.
- SEBI
and exchanges actively monitor to prevent unfair manipulation.
Option Pricing and Volatility
- High
option prices on 0DTE and subsequent theta decay happen post-CAS price
discovery.
- With
reduced uncertainty, option pricing will stabilize and follow its normal path.
Price Deviations in CAS
- Stock
prices can deviate up to 3% during CAS.
- Large
deviations often coincide with new fundamental information announced
during CAS.
Index Closing Price Bias
- Index
may close higher statistically due to liquid cash presence.
- Increasing
liquidity is expected to balance this.
CAS Participants and Liquidity Providers
- CAS
was introduced primarily for institutional players to adjust
portfolios and manage delta.
- Other
participants include domestic institutional investors (DII), foreign
institutional investors (FII), arbitrage mutual funds, proprietary
desks, and family offices.
Role of Arbitrageurs and Institutions
- Arbitrageurs
buy underlying shares and sell futures to optimize returns.
- They
provide liquidity by entering bids/asks in CAS and hedging simultaneously
in futures markets.
- Institutions
with large positions adjust their portfolios during CAS to reduce tracking
errors.
Algorithmic Trading and Market Access
- SEBI
permits algo trading and DMA (Direct Market Access) for institutional
clients.
- Participants
use low latency software to capitalize on arbitrage opportunities across
equity, derivatives, CAS, and SLBM segments.
How Liquidity Will Emerge in CAS
Scenario 1: CAS Price Above Reference Price
- Arbitrageurs
offer ask in the cash market, hedge by buying stock futures.
- DIIs
with short futures positions covers by squaring off in CAS.
- Increased
futures prices attract sellers, providing liquidity in Futures markets.
- SLBM (Securities Lending and Borrowing Mechanism) also facilitates short selling.
Scenario 2: CAS Price Below Reference Price
- Arbitrageurs
bid in the cash market, hedge by selling futures.
- DIIs with arbitrage mandates participate similarly.
- Falling
futures prices encourage short sellers to cover positions, adding
liquidity.
Price Deviation Expectations
- Rules
allow stock and index to close within ±3% of settlement price.
- Arbitrageurs
enter when deviations exceed approximately 0.50%, covering transaction cost, brokerage and finance costs.
- Index deviation of 0.50% translates to ±120 points in Nifty and ±400 points in Sensex, which is significant but unlikely to persist without major news.
Thus, institutions will
provide quotes in CAS and they will get liquidity in Futures market from
speculators and from people who have offsetting long positions.
With Falling deviation between Closing price and Reference price, We will see return of volumes and sanity in option pricing of Stock and Index on Expiry day also.
Summary Flowcharts
1. CAS Price Discovery and Liquidity Flow
2. Market Participant Interaction in CAS
