Fed Runoff Mechanics Reach Ample-to-Scarce Inflection as Reserve Demand Elasticity Fractures
Quarterly redemption caps of $5B Treasuries and $35B MBS are stripping reserves faster than the $16.4T repo market can reallocate them. The reserve-demand curve is turning convex, moving scarcity from a liquidity tail-risk to a standing funding cost.
Reserve scarcity now prices into the short end first—cap-adjacent repo spreads and SOFR tails will lead, not lag, any balance-sheet policy shift.
Front-end Treasury bills and SOFR-linked cash instruments as scarcity premium accrues to collateral holders and money funds; curve steepening favors steepener structures.
Leveraged basis trades and dealer intermediation books facing margin procyclicality as reserve scarcity raises the cost of intraday and overnight funding.
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Runoff Arithmetic: Caps, Composition, and the Reserve Drain
The Fed's balance-sheet runoff continues at a reduced pace: monthly caps stand at $5B for Treasuries and $35B for agency MBS, with MBS principal paydowns running below the cap as mortgage rates hold near 6.1%, slowing passive contraction. Total reserve balances have fallen to roughly $2.74T, or 10.2% of GDP—down from a 12.4% peak—while the ON RRP facility holds a nil balance, meaning further contraction now draws directly on reserves rather than the overnight facility buffer. The arithmetic is unforgiving: every dollar of net redemption beyond what ON RRP can absorb is a dollar off bank reserves, which is why the marginal mechanics—not the headline balance sheet—drive funding markets.
Reserve Demand Elasticity: From Ample to Scarce
Reserve demand has turned convex. The 90th-percentile intraday reserve need among the largest 25 banks has risen to $840B even as the system's total stock declines, reflecting a structural bid for precautionary liquidity and a heavier settlement footprint from $16.4T in outstanding Treasuries. The SOFR-IORB spread has widened to a persistent +6bp after averaging flat through the first half, and the 5-day average of the 99th-percentile fed funds rate prints 3-5bp above IORB in the final days of each maintenance period. That combination—wide tails, convex demand—is the canonical signature of approaching scarcity, distinct from the 2019 episode in that the demand curve, not the supply shock, is doing the steepening.
Funding-Market Transmission: Repo, Bills, and the Scarcity Premium
Scarcity is transmitting through the secured funding complex. GCF repo rates have decoupled from the SOFR composite, with the GCF-SOFR basis at +8bp, and quarter-end matched-book term repo clears 14-18bp over overnight. Bill supply is the offsetting variable: the Treasury has been reducing T-bill issuance tactically, which mechanically injects reserves back into the system and masks the true scarcity signal. The tension is that the TGA rebuild to roughly $850B, if resumed, would pull reserves lower while bill paydowns release them—a two-way mechanical pull that confounds the standard reserve-scarcity read and makes the funding curve the cleanest diagnostic.
Policy Asymptote: Where the Runoff Stops
Our models place the reserve-scarcity inflection at a range of $2.55-2.70T in reserves, or roughly 9.5-10.0% of GDP, implying only $40-190B of remaining runoff runway before the funding complex reprices materially. The FOMC has signaled a data-dependent exit, but the operative variable is reserve-demand elasticity, not the balance-sheet level itself. Primary risk is an abrupt pause triggered by quarter-end reserve compression; the resulting balance-sheet expansion would steepen the front end and re-widen the SOFR curve, decisively shifting scarcity from tail-risk to policy regime.