Prime Brokerage and Custody Risk

When Lehman Brothers collapsed in 2008, hedge funds that used its European arm as prime broker discovered that assets they thought of as "theirs, just held somewhere safe" were frozen in a bankruptcy proceeding for years. The lesson: a prime brokerage relationship is not neutral custody. It is a bundle of credit exposures to the broker itself, and those exposures only become visible when the broker gets into trouble.

Where the exposure comes from

A prime broker typically has the contractual right to rehypothecate — re-lend or re-pledge — a portion of a client's assets that back margin loans. In the UK and much of Europe, this can historically extend to all client assets held at the broker unless specifically ring-fenced; in the US, rehypothecation is capped, commonly near the lesser of 140% of the client's debit balance:

$$\text{Assets at risk} \approx \min(1.4 \times \text{Debit balance},\ \text{Total assets held})$$

In words: the assets a fund could actually lose if its prime broker fails are roughly capped at some multiple of what the fund owes the broker — but that multiple, and whether the cap applies at all, depends entirely on jurisdiction and the specific prime brokerage agreement signed.

Worked example

A fund has $50 million of assets at a prime broker and a $20 million margin debit balance (money it has borrowed against those assets). Under a 140%-of-debit rehypothecation cap, the broker can rehypothecate up to $28 million of the fund's assets. If the broker becomes insolvent, that $28 million is at risk of being tied up as an unsecured claim in bankruptcy proceedings, while the remaining $22 million, held in excess of the cap, is more likely to be treated as segregated client property and returned faster. Funds that manage this risk deliberately keep their margin debit low relative to total assets, specifically to shrink the rehypothecatable slice.

What this means in practice

After 2008, most institutional hedge funds moved to multi-prime setups — spreading assets and financing across two or more prime brokers — specifically so that no single broker's failure could freeze the whole portfolio. Operational due diligence on a fund now routinely asks not just "who is your prime broker" but "how much of your assets are segregated, and under which jurisdiction's rules."