The Human HFT: What Discretionary Scalpers Do That Algorithms Can'teric

This continues the final part of the ASX order book series, after the liquidity harvester and the catalyst sniper.

When I talked through these trader profiles with a colleague, he summed up the harvester types in one phrase: human high-frequency trading machines. It's a good line, and it's mostly right. But the places where it's wrong turned out to be the most interesting part.

Where the comparison holds

The best discretionary scalpers whose journals I read share a surprising number of traits with high-frequency trading (HFT) systems:

Both are harvesting small, short-lived mispricings in the microstructure of the market, at volume. Both treat the order book as the primary source of information. Both obsess over costs. Both exit the moment the expected move fails to appear.

Where it breaks down

Speed

An HFT system reacts in microseconds. A human reacts in hundreds of milliseconds at best, more realistically seconds. Any edge that's about being first, such as racing to hit a stale quote, belongs to the machines. Humans simply can't play that game.

Taking versus providing liquidity

Many HFT strategies make money by providing liquidity: quoting both sides, earning the spread, managing inventory. The human scalpers I read about mostly took liquidity: paying the offer or hitting the bid to get in when a setup appeared, and leaning on somebody else's resting order as protection. They pay the spread rather than earn it. That's a fundamentally different business.

Context

This is the big one. An HFT market-making system typically cares about very local information: the book, recent trades, maybe related instruments. A discretionary scalper's decision at the open folds in a wide range of context:

• What did the commodity, the futures and the stock's foreign listing do overnight?
• Is this gap caused by news, or by one large seller?
• Is today a rebalance, expiry or settlement day?
• What happened the last time this stock opened like this?
• Is the sector in favour, or rotating out?
• Is the market in a regime where fades work, or where breakouts work?

None of that is about speed. All of it is about judgement at a specific moment.

Selectivity

An HFT system is always on. It quotes all day, every day, in every instrument it covers. The human scalpers' best months were the ones with the fewest trades. They made their money by declining most opportunities and pressing a few.

So where does the human edge actually live?

Putting those differences together, the remaining human edge seems to sit in a narrow but real space:

In other words, discretionary scalpers aren't competing with HFT. They're operating in the gaps HFT leaves: moments that are too infrequent, too contextual, or too lumpy for a speed-optimised system to bother with.

The edge is shrinking at the margins

The journals also showed signs of the machines encroaching. Mispriced moves that once lasted minutes being corrected in seconds as market-making algorithms stepped in. Illiquid opens snapping back to fair value faster than before. Changes to auction mechanics altering setups that had worked for years.

The human edge isn't disappearing, but it's being pushed toward the moments that are hardest to automate. The traders who did well adapted by leaning further into context and selectivity, and by adding slower styles where speed matters even less.

What this means for system builders

From a systematic trading point of view, the interesting conclusion is that the human scalper's edge is largely codifiable, just not as a speed game. The heuristics from this series can be expressed as features:

• auction mispricing versus overnight leads and sector peers;
• protection relative to expected volume;
• resistance ahead;
• persistence of large orders;
• gap cause classification from announcements and order composition;
• time stops calibrated from historical MFE.

I sketched how to compute several of these in measuring order book imbalance in code. The point isn't to out-race anyone. It's to encode the judgement and apply it consistently at the moments that matter, without the tilt, fatigue and boredom that the journals show are the human scalper's real enemies.

What this means for human traders

If you're a discretionary trader, the lesson is to stop trying to be fast and start trying to be right at the right moments:

• Don't compete in continuous, liquid trading where you're slower than everyone.
• Focus on auctions, catalysts and mechanical events where context matters more than milliseconds.
• Build your context advantage: sector knowledge, the corporate calendar, the history of the stocks you trade.
• Let the machines provide the liquidity you lean on, and pick your spots.

The phrase "human HFT" captures the intensity and the precision of the best scalpers. But the reason they survive alongside the real HFTs is that, at the moments they choose, they're doing something the machines don't.