diff --git a/packages/core/src/agents/generalist-agent.test.ts b/packages/core/src/agents/generalist-agent.test.ts
index efdf705a19..510fad5673 100644
--- a/packages/core/src/agents/generalist-agent.test.ts
+++ b/packages/core/src/agents/generalist-agent.test.ts
@@ -4,13 +4,22 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { describe, it, expect, vi } from 'vitest';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { GeneralistAgent } from './generalist-agent.js';
 import { makeFakeConfig } from '../test-utils/config.js';
 import type { ToolRegistry } from '../tools/tool-registry.js';
 import type { AgentRegistry } from './registry.js';
 
 describe('GeneralistAgent', () => {
+  beforeEach(() => {
+    vi.stubEnv('GEMINI_SYSTEM_MD', '');
+    vi.stubEnv('GEMINI_WRITE_SYSTEM_MD', '');
+  });
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
   it('should create a valid generalist agent definition', () => {
     const config = makeFakeConfig();
     vi.spyOn(config, 'getToolRegistry').mockReturnValue({
diff --git a/packages/core/src/prompts/promptProvider.test.ts b/packages/core/src/prompts/promptProvider.test.ts
index f286d5bea8..2d96dee7ef 100644
--- a/packages/core/src/prompts/promptProvider.test.ts
+++ b/packages/core/src/prompts/promptProvider.test.ts
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { PromptProvider } from './promptProvider.js';
 import type { Config } from '../config/config.js';
 import {
@@ -35,6 +35,9 @@ describe('PromptProvider', () => {
 
   beforeEach(() => {
     vi.resetAllMocks();
+    vi.stubEnv('GEMINI_SYSTEM_MD', '');
+    vi.stubEnv('GEMINI_WRITE_SYSTEM_MD', '');
+
     mockConfig = {
       getToolRegistry: vi.fn().mockReturnValue({
         getAllToolNames: vi.fn().mockReturnValue([]),
@@ -60,6 +63,10 @@ describe('PromptProvider', () => {
     } as unknown as Config;
   });
 
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
   it('should handle multiple context filenames in the system prompt', () => {
     vi.mocked(getAllGeminiMdFilenames).mockReturnValue([
       DEFAULT_CONTEXT_FILENAME,