diff --git a/src/exchanges/paradex/gateway.ts b/src/exchanges/paradex/gateway.ts
index 236bfd0..81578e2 100644
--- a/src/exchanges/paradex/gateway.ts
+++ b/src/exchanges/paradex/gateway.ts
@@ -602,7 +602,10 @@ export class ParadexGateway {
       // Only omit amount for MARKET close-position orders; STOP requires explicit size
       const shouldOmitAmount = isClosePosition && type === "market";
       const amountArg: any = shouldOmitAmount ? undefined : amount;
-      if (!shouldOmitAmount && amountArg != null && extraParams.size === undefined) {
+      // Paradex/ccxt may require `size` even when amount is omitted for closePosition MARKET orders.
+      if (shouldOmitAmount && amount != null && extraParams.size === undefined) {
+        extraParams.size = amount.toString();
+      } else if (!shouldOmitAmount && amountArg != null && extraParams.size === undefined) {
         extraParams.size = amountArg.toString();
       }
       const order = (await this.exchange.createOrder(
diff --git a/tests/paradex-gateway.test.ts b/tests/paradex-gateway.test.ts
new file mode 100644
index 0000000..51afdbe
--- /dev/null
+++ b/tests/paradex-gateway.test.ts
@@ -0,0 +1,56 @@
+import { describe, expect, it, vi } from "vitest";
+import { ParadexGateway } from "../src/exchanges/paradex/gateway";
+
+describe("ParadexGateway createOrder", () => {
+  it("sets `size` for closePosition MARKET orders even when amount is omitted", async () => {
+    const createOrder = vi.fn(async (symbol, type, side, amount, price, params) => ({
+      id: "1",
+      symbol,
+      type,
+      side,
+      status: "open",
+      price: price ?? 0,
+      amount: amount ?? Number(params?.size ?? 0),
+      filled: 0,
+      stopPrice: undefined,
+      timestamp: Date.now(),
+      lastUpdateTimestamp: Date.now(),
+      info: {
+        reduceOnly: params?.reduceOnly,
+        closePosition: params?.closePosition,
+      },
+    }));
+
+    const gateway = new ParadexGateway({
+      symbol: "BTC/USDC",
+      displaySymbol: "BTC/USDC",
+      privateKey: "test",
+      walletAddress: "test",
+      logger: () => {},
+      usePro: false,
+    }) as any;
+
+    gateway.exchange = { createOrder, markets: {}, symbols: [] };
+    gateway.initialized = true;
+    gateway.marketSymbol = "BTC/USDC";
+
+    await gateway.createOrder({
+      symbol: "BTC/USDC",
+      side: "SELL",
+      type: "MARKET",
+      quantity: 1.23,
+      reduceOnly: "true",
+      closePosition: "true",
+    });
+
+    expect(createOrder).toHaveBeenCalledTimes(1);
+    const [_symbol, _type, _side, amountArg, _price, extraParams] = createOrder.mock.calls[0]!;
+    expect(amountArg).toBeUndefined();
+    expect(extraParams).toMatchObject({
+      closePosition: true,
+      reduceOnly: true,
+      size: "1.23",
+    });
+  });
+});
+