Why it matters
FOK avoids partial-fill complexity. It’s useful when partial exposure creates operational risk or when you need a clean all-or-nothing execution.
Operational use: this is how the market taxes urgency. When it appears, slow down or size down.
Common trap
Using FOK in thin markets and getting repeatedly cancelled, then chasing with market orders in frustration.
Example
You try to enter size with FOK; it keeps cancelling. Instead of resizing, you hit the market. The eventual fill is worse than a planned stagger would have been.
Operational cue: if you can’t point to it on the chart, in the DOM, or in your broker logs, treat it as a story and trade smaller.
Notes
Quick rule
- If you see this repeatedly, you are in a different regime than your model assumes.
- Regime change usually means position size change.
- When in doubt: trade smaller, or don’t trade.