Borrow fee
What it costs, per year, to borrow shares so you can short them. This is the price signal from the people with actual money on the line — the stock lenders. When shorts are genuinely trapped, this price rips higher within hours, because lenders reprice scarce inventory fast.
The single most reliable real-time squeeze tell we know. A genuine squeeze prices the borrow: the fee jumps from cheap single digits into the tens of percent while availability evaporates. A stock that rallies hard while its borrow stays cheap, plentiful, and barely moves is telling you the lending market does not believe shorts are being forced out — whatever the headline Short interest (SI) says. Read it together with the Rebate (stock loan): the two sides of the same lending transaction.