What is your brokerage cash sweep really paying, and does an AI trading agent make the drag worse?

There is a line we have all repeated while looking at a brokerage statement. The cash is just sitting there, doing nothing. It is the safe part, the dry powder, the part that waits. Money in the market is working, money in cash is resting. The belief shows up everywhere, from financial-planning columns to the default settings on every retail app, where uninvested balances land in something called a sweep and nobody opens that screen again.

On June 12 the market reminded us the belief is wrong. Charles Schwab stock rose about 3% after a strong monthly report, and the bear case that analysts keep circling is not trading volume or fees. It is what happens to all of that resting cash.

Where the brokerage cash sweep habit came from

The habit is not stupid. It was correct for over a decade. From the end of 2008 until early 2022, the short-term policy rate sat near zero for most of the stretch, and a sweep paying almost nothing honestly reflected a world where everything liquid paid almost nothing. A money-market fund and a bank sweep both returned a rounding error, so the rational move was to ignore the cash line and spend attention on the positions that could actually move.

So a generation of investors, me included, learned that cash is inert and the sweep is plumbing. Then rates moved and the plumbing did not. By 2024 the effective federal funds rate had climbed to 5.33%, as Bloomberg Law noted while covering the first wave of sweep lawsuits, while the default sweep mostly stayed put. That is the moment the belief and the world quietly parted ways.

What the cash sweep interest rate actually is now

Here is the part that does not match the story we tell ourselves. As of this spring, the average insured brokerage sweep was paying roughly 0.30%, per Crane Data’s brokerage sweep index, and the lowest default bank sweeps run as little as 0.01%. Over the same window a government money-market fund yielded about 3.46%, Fidelity’s flagship sweep fund was around 3.28%, and a six-month Treasury bill paid somewhere near 4.2% to 4.4%.

That gap is not a rounding error anymore. Now multiply it. Schwab reported transactional cash running near 4% of client assets, about $10,000 per account on average, against a base that just crossed thirteen trillion dollars.

Spread a few percent of thirteen trillion dollars across the gap in that table and the picture sharpens. Bloomberg Law reported in 2024 that a single firm, Ameriprise, was alleged to have made over $2.5 billion from its cash sweep program in one year. The cash was not resting. It was working a double shift, for someone else.

The net interest spread, and why it hides from every fill report

Here is the mechanism, plainly. When cash lands in a default sweep, the broker moves it into a bank, often one the broker owns or partners with. The bank puts that money to work at the prevailing short rate, pays out the sweep rate, and keeps the difference. That difference is net interest income, the gap between what the bank earns on the deposit and what it pays the customer to hold it. It is one of the largest and steadiest revenue lines a modern broker has.

The reveal is not that the spread exists. It is where it hides. Every conversation about getting a fair deal from a broker is about fills, the price we got the moment we trade, the routing, the spread we cross. The sweep drag is the mirror image of all of that. It is the yield on the cash between trades, the cost of not trading, and no fill report, no routing disclosure, and no best-execution metric captures it. It is the absence of a trade, which is why it never shows up on the page where we hunt for hidden costs.

The sweep drag is the one cost in the whole portfolio that shows up on no fill report, because it is the price of not trading.

What an AI trading agent does to your uninvested cash

Now connect it to the wave of agentic brokerage accounts that arrived this spring. An AI portfolio agent spends much of its life in cash, waiting for the next signal worth acting on. The more disciplined the agent, the fewer trades it forces and the more hours the money sits idle in whatever the account defaults to. An agent built to optimize the portfolio can therefore maximize the one cost nobody measures, unless cash is part of its mandate.

That is precisely the tension the market priced on June 12. Schwab management has spent recent weeks arguing that AI is a tailwind to its franchise. The bear case, summarized by TradingKey that day, points the other way: “concerns over Charles Schwab’s client cash monetization strategies and potential impacts from AI developments on its cash sweep model.” Both cannot be right. Either AI makes investors stop noticing the sweep, or it makes them notice it for the first time.

Read the default cash rate, compare to a T-bill

This is a rare case where the fix is short and the math is real. Open the account the agent actually trades in and read the rate on the default cash position, not the promotional rate on the homepage. Compare it to a government money fund or a Treasury bill held in the same account. If the product treats cash as a default to park rather than a position to manage, that is the tell, and moving the sweep vehicle is usually a one-time setting change that recovers a few hundred basis points on every idle dollar.

The interesting part is not that the spread exists. It always has. It is that we are about to hand the cash-holding decision to agents built to optimize trades, inside accounts whose default setting quietly profits from the cash that is not trading. The question worth sitting with is simple: does the agent’s mandate include the money it is choosing not to deploy, or only the money it does?