diff --git a/core/src/exchanges/gemini-titan/auth.ts b/core/src/exchanges/gemini-titan/auth.ts
index 6546c134..bb96a1d0 100644
--- a/core/src/exchanges/gemini-titan/auth.ts
+++ b/core/src/exchanges/gemini-titan/auth.ts
@@ -63,11 +63,11 @@ export class GeminiAuth {
     /**
      * Build WebSocket handshake authentication headers.
      *
-     * WebSocket auth uses a time-based nonce (seconds since epoch)
+     * WebSocket auth uses a time-based nonce (milliseconds since epoch)
      * as the payload, not a JSON object.
      */
     buildWsHeaders(): Record<string, string> {
-        const nonce = Math.floor(Date.now() / 1000).toString();
+        const nonce = Date.now().toString();
         const b64Payload = Buffer.from(nonce).toString('base64');
         const signature = crypto
             .createHmac('sha384', this.apiSecret)
diff --git a/core/test/exchanges/gemini-titan-auth.test.ts b/core/test/exchanges/gemini-titan-auth.test.ts
new file mode 100644
index 00000000..7a72b2f9
--- /dev/null
+++ b/core/test/exchanges/gemini-titan-auth.test.ts
@@ -0,0 +1,37 @@
+import crypto from 'crypto';
+import { GeminiAuth } from '../../src/exchanges/gemini-titan/auth';
+
+// Milliseconds matter here: Gemini's WebSocket API requires the handshake
+// nonce to be a Unix timestamp in milliseconds.
+const FROZEN_NOW_MS = 1755878400000; // 2025-08-22T16:00:00.000Z
+
+describe('GeminiAuth WebSocket headers', () => {
+    afterEach(() => {
+        jest.restoreAllMocks();
+    });
+
+    it('uses a millisecond nonce for the WebSocket handshake', () => {
+        jest.spyOn(Date, 'now').mockReturnValue(FROZEN_NOW_MS);
+        const auth = new GeminiAuth({ apiKey: 'test-key', apiSecret: 'test-secret' });
+
+        const headers = auth.buildWsHeaders();
+
+        expect(headers['X-GEMINI-NONCE']).toBe(String(FROZEN_NOW_MS));
+    });
+
+    it('signs the same millisecond nonce it sends', () => {
+        jest.spyOn(Date, 'now').mockReturnValue(FROZEN_NOW_MS);
+        const auth = new GeminiAuth({ apiKey: 'test-key', apiSecret: 'test-secret' });
+
+        const headers = auth.buildWsHeaders();
+
+        const payload = Buffer.from(headers['X-GEMINI-PAYLOAD'], 'base64').toString('utf8');
+        expect(payload).toBe(String(FROZEN_NOW_MS));
+
+        const expectedSignature = crypto
+            .createHmac('sha384', 'test-secret')
+            .update(headers['X-GEMINI-PAYLOAD'])
+            .digest('hex');
+        expect(headers['X-GEMINI-SIGNATURE']).toBe(expectedSignature);
+    });
+});