diff --git a/src/genie-commands/__tests__/session.test.ts b/src/genie-commands/__tests__/session.test.ts
index ee2434214..b470007c0 100644
--- a/src/genie-commands/__tests__/session.test.ts
+++ b/src/genie-commands/__tests__/session.test.ts
@@ -7,9 +7,14 @@
  * Run with: bun test src/genie-commands/__tests__/session.test.ts
  */
 
-import { afterEach, beforeEach, describe, expect, test } from 'bun:test';
+import { afterEach, beforeEach, describe, expect, spyOn, test } from 'bun:test';
 import { existsSync, mkdirSync, readFileSync, realpathSync, rmSync, writeFileSync } from 'node:fs';
 import { basename, join } from 'node:path';
+// Import the real genie-config module so we can spyOn individual exports.
+// Using spyOn instead of mock.module avoids leaking an incomplete mock to
+// other test files (bun 1.3.x leaks mock.module across parallel workers —
+// see PR #1169). Pattern mirrors src/term-commands/init-bootstrap.test.ts.
+import * as genieConfig from '../../lib/genie-config.js';
 import { HEARTBEAT_TEMPLATE, SOUL_TEMPLATE, scaffoldAgentFiles } from '../../templates/index.js';
 import { buildClaudeCommand, getAgentsFilePath, sanitizeWindowName } from '../session.js';
 
@@ -18,6 +23,23 @@ import { buildClaudeCommand, getAgentsFilePath, sanitizeWindowName } from '../se
 // ============================================================================
 
 describe('buildClaudeCommand', () => {
+  // Pin promptMode so tests asserting --append-system-prompt-file don't depend
+  // on the host's ~/.genie/config.json. Without this, a host configured with
+  // promptMode: "system" causes buildTeamLeadCommand to emit --system-prompt-file
+  // instead, failing the append-system-prompt-file assertions below.
+  // Closes the tail surfaced during PR #1169 QA.
+  let loadGenieConfigSyncSpy: ReturnType<typeof spyOn>;
+
+  beforeEach(() => {
+    loadGenieConfigSyncSpy = spyOn(genieConfig, 'loadGenieConfigSync').mockReturnValue({
+      promptMode: 'append',
+    } as ReturnType<typeof genieConfig.loadGenieConfigSync>);
+  });
+
+  afterEach(() => {
+    loadGenieConfigSyncSpy.mockRestore();
+  });
+
   test('always contains --team-name flag', () => {
     const cmd = buildClaudeCommand('genie');
     expect(cmd).toContain('--team-name');