How profitable scalpers get tagged as toxic flow, and what the signs are

[Shadow Trader]

Toxic flow is a dealer's term for trades that tend to be right about short-term direction at the dealer's expense. A broker that internalizes orders loses money on it, so there are systems that watch for it. Some things that tend to raise a flag, as far as is publicly discussed:

- Consistently positive markouts. After each fill, price keeps going in the trader's favor over the next seconds. This is the main signal, and it is hard to avoid if the edge is real.

- Very short holding times. Trades closed in seconds, again and again, look like latency or news trading even when they are not.

- Clustering around news. Entries right before spikes get noticed quickly.

- A high hit rate with a narrow range of outcomes, which stands out in a statistical profile.

- Fast, repeated entries on the same price level, which can resemble exploiting a lagging feed.

Consequences vary widely by broker. Some widen spreads on the account, add delay to execution, reject orders more often, move the account to a different execution model, or in the worst case close it. Others route flagged flow to external liquidity and carry on. Many do none of this and simply pass everything through.

One uncomfortable point: the label is statistical, not a judgment of behavior. An honest discretionary scalper with a real edge can look exactly like a bot exploiting latency.

Signs on the trader's side tend to be a sudden rise in average slippage, more requotes or rejections than before, and spreads that behave differently from other accounts at the same broker.

Whether it is worth worrying about depends on the broker's model, which is worth asking about directly before funding.

Have you seen execution quality change after a run of good trades, and did the broker ever explain it?