PS Short Interest Jumps 183% as Borrow Cost Hits 241%

Pershing Square Inc. has added a new data point to an already bearish picture. Short interest surged 183% in a single session on October 9, more than tripling the share count on loan. That move now sits atop a borrow market that was already extreme.

The Short Side Escalates

Short interest jumped from 830,406 shares to 2.35 million on October 9, a one-day change that extends the month-long trend: shares short have risen roughly 986% since early September. Cost to borrow reached 241% on October 9, up from around 35% in mid-September. That move has been sustained across the full past week, with costs holding between 234% and 241%.

Availability stands at 11.2%, down from 14% the week before. Fewer than one share in eight in the lending pool remains unborrowed. This is consistent with the near-total exhaustion seen between September 18 and 25, when availability briefly touched its 52-week low of 0.025%. The pool has since refilled partially but remains acutely tight.

What Has Changed Since Thursday

The earlier convergence report, published October 12 at 06:14, covered the Citigroup downgrade and options positioning. The new development is the size of the short interest build. What was a flat, plateau position of 830,000 shares held for two weeks has now broken sharply higher. Short sellers are not just paying 241% to borrow: they are paying it while scaling up exposure.

The put/call ratio remains at 2.0, more than two standard deviations above its 20-day average of 1.31. Options positioning, already flagged Thursday, has not reversed.

Index Calendar Adds Context

ORTEX puts the probability of PS joining the Russell 1000 at the December 2026 Semi-Annual Review at 68%. The rank date is October 30. Tracker buying, if it materialises, would amount to roughly $112 million of forced demand over an estimated 5.6 days of normal volume. That mechanical flow would run directly against the short book being built today. The announcement date is projected at November 13, with a trade date of December 11.

Among past additions in similar circumstances, across 120 comparable events, the median stock moved minus 3.6% from announcement to effective date, and only 32% moved in the expected direction.

Data Summary

• Short interest: 2.35M shares, up 183% on October 9; up 986% over one month
• Cost to borrow: 241%, up from approximately 35% in mid-September
• Availability: 11.2%, down 19.7% week on week
• Put/call ratio: 2.0, z-score 2.08 versus 20-day average of 1.31
• Analyst consensus: 7 Hold, 2 Sell; no Buy ratings; Citigroup target $45 vs. close of $54.59
• ORTEX Russell 1000 addition probability: 68%; estimated tracker flow $112M; rank date October 30