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DELETE /v1/accounts/{accountId}/orders/{orderId}
Cancels a specific open order. If the order was partially filled before cancellation, the filled portion remains as a position — only the unfilled remainder is cancelled. Returns the order’s final state. Calling cancel on an order that has already filled, been cancelled, or expired returns the order’s terminal state without error — cancel is idempotent against terminal states. Cancel applies to limit orders that have not yet fully filled. Market orders fill atomically and are not cancellable. Response 
{
  "orderId": "ord_3e2b...",
  "predexonId": "px-abc123def456",
  "title": "Will Trump win the 2028 election?",
  "outcome": "Yes",
  "side": "buy",
  "type": "limit",
  "status": "cancelled",
  "size": "100",
  "filled": "0",
  "averagePrice": null,
  "price": "0.45",
  "fee": "0",
  "createdAt": "2026-05-06T12:00:00.214Z",
  "cancelledAt": "2026-05-06T12:01:15.882Z"
}
The example shows a fully-unfilled limit-BUY cancelled before any matches arrived — filled is "0" and there are no fills. If the order had been partially filled, filled would be the matched share count, averagePrice would reflect those fills, and metadata.fills would record where the partial fills happened.