diff --git a/packages/core/src/config/config.test.ts b/packages/core/src/config/config.test.ts
index 6c57d058..f1d8b965 100644
--- a/packages/core/src/config/config.test.ts
+++ b/packages/core/src/config/config.test.ts
@@ -15,6 +15,7 @@ import {
 } from '../telemetry/index.js';
 import {
   AuthType,
+  ContentGeneratorConfig,
   createContentGeneratorConfig,
 } from '../core/contentGenerator.js';
 import { GeminiClient } from '../core/client.js';
@@ -249,6 +250,7 @@ describe('Server Config (config.ts)', () => {
       // Verify that history was restored to the new client
       expect(mockNewClient.setHistory).toHaveBeenCalledWith(
         mockExistingHistory,
+        { stripThoughts: false },
       );
     });
 
@@ -282,6 +284,92 @@ describe('Server Config (config.ts)', () => {
       // Verify that setHistory was not called since there was no existing history
       expect(mockNewClient.setHistory).not.toHaveBeenCalled();
     });
+
+    it('should strip thoughts when switching from GenAI to Vertex', async () => {
+      const config = new Config(baseParams);
+      const mockContentConfig = {
+        model: 'gemini-pro',
+        apiKey: 'test-key',
+        authType: AuthType.USE_GEMINI,
+      };
+      (
+        config as unknown as { contentGeneratorConfig: ContentGeneratorConfig }
+      ).contentGeneratorConfig = mockContentConfig;
+
+      (createContentGeneratorConfig as Mock).mockReturnValue({
+        ...mockContentConfig,
+        authType: AuthType.LOGIN_WITH_GOOGLE,
+      });
+
+      const mockExistingHistory = [
+        { role: 'user', parts: [{ text: 'Hello' }] },
+      ];
+      const mockExistingClient = {
+        isInitialized: vi.fn().mockReturnValue(true),
+        getHistory: vi.fn().mockReturnValue(mockExistingHistory),
+      };
+      const mockNewClient = {
+        isInitialized: vi.fn().mockReturnValue(true),
+        getHistory: vi.fn().mockReturnValue([]),
+        setHistory: vi.fn(),
+        initialize: vi.fn().mockResolvedValue(undefined),
+      };
+
+      (
+        config as unknown as { geminiClient: typeof mockExistingClient }
+      ).geminiClient = mockExistingClient;
+      (GeminiClient as Mock).mockImplementation(() => mockNewClient);
+
+      await config.refreshAuth(AuthType.LOGIN_WITH_GOOGLE);
+
+      expect(mockNewClient.setHistory).toHaveBeenCalledWith(
+        mockExistingHistory,
+        { stripThoughts: true },
+      );
+    });
+
+    it('should not strip thoughts when switching from Vertex to GenAI', async () => {
+      const config = new Config(baseParams);
+      const mockContentConfig = {
+        model: 'gemini-pro',
+        apiKey: 'test-key',
+        authType: AuthType.LOGIN_WITH_GOOGLE,
+      };
+      (
+        config as unknown as { contentGeneratorConfig: ContentGeneratorConfig }
+      ).contentGeneratorConfig = mockContentConfig;
+
+      (createContentGeneratorConfig as Mock).mockReturnValue({
+        ...mockContentConfig,
+        authType: AuthType.USE_GEMINI,
+      });
+
+      const mockExistingHistory = [
+        { role: 'user', parts: [{ text: 'Hello' }] },
+      ];
+      const mockExistingClient = {
+        isInitialized: vi.fn().mockReturnValue(true),
+        getHistory: vi.fn().mockReturnValue(mockExistingHistory),
+      };
+      const mockNewClient = {
+        isInitialized: vi.fn().mockReturnValue(true),
+        getHistory: vi.fn().mockReturnValue([]),
+        setHistory: vi.fn(),
+        initialize: vi.fn().mockResolvedValue(undefined),
+      };
+
+      (
+        config as unknown as { geminiClient: typeof mockExistingClient }
+      ).geminiClient = mockExistingClient;
+      (GeminiClient as Mock).mockImplementation(() => mockNewClient);
+
+      await config.refreshAuth(AuthType.USE_GEMINI);
+
+      expect(mockNewClient.setHistory).toHaveBeenCalledWith(
+        mockExistingHistory,
+        { stripThoughts: false },
+      );
+    });
   });
 
   it('Config constructor should store userMemory correctly', () => {
diff --git a/packages/core/src/config/config.ts b/packages/core/src/config/config.ts
index 7c61f239..5c11667b 100644
--- a/packages/core/src/config/config.ts
+++ b/packages/core/src/config/config.ts
@@ -379,13 +379,21 @@ export class Config {
     const newGeminiClient = new GeminiClient(this);
     await newGeminiClient.initialize(newContentGeneratorConfig);
 
+    // Vertex and Genai have incompatible encryption and sending history with
+    // throughtSignature from Genai to Vertex will fail, we need to strip them
+    const fromGenaiToVertex =
+      this.contentGeneratorConfig?.authType === AuthType.USE_GEMINI &&
+      authMethod === AuthType.LOGIN_WITH_GOOGLE;
+
     // Only assign to instance properties after successful initialization
     this.contentGeneratorConfig = newContentGeneratorConfig;
     this.geminiClient = newGeminiClient;
 
     // Restore the conversation history to the new client
     if (existingHistory.length > 0) {
-      this.geminiClient.setHistory(existingHistory);
+      this.geminiClient.setHistory(existingHistory, {
+        stripThoughts: fromGenaiToVertex,
+      });
     }
 
     // Reset the session flag since we're explicitly changing auth and using default model
diff --git a/packages/core/src/core/client.test.ts b/packages/core/src/core/client.test.ts
index 5e68cfb6..9f6dcbe9 100644
--- a/packages/core/src/core/client.test.ts
+++ b/packages/core/src/core/client.test.ts
@@ -1596,4 +1596,73 @@ ${JSON.stringify(
       );
     });
   });
+
+  describe('setHistory', () => {
+    it('should strip thought signatures when stripThoughts is true', () => {
+      const mockChat = {
+        setHistory: vi.fn(),
+      };
+      client['chat'] = mockChat as unknown as GeminiChat;
+
+      const historyWithThoughts: Content[] = [
+        {
+          role: 'user',
+          parts: [{ text: 'hello' }],
+        },
+        {
+          role: 'model',
+          parts: [
+            { text: 'thinking...', thoughtSignature: 'thought-123' },
+            {
+              functionCall: { name: 'test', args: {} },
+              thoughtSignature: 'thought-456',
+            },
+          ],
+        },
+      ];
+
+      client.setHistory(historyWithThoughts, { stripThoughts: true });
+
+      const expectedHistory: Content[] = [
+        {
+          role: 'user',
+          parts: [{ text: 'hello' }],
+        },
+        {
+          role: 'model',
+          parts: [
+            { text: 'thinking...' },
+            { functionCall: { name: 'test', args: {} } },
+          ],
+        },
+      ];
+
+      expect(mockChat.setHistory).toHaveBeenCalledWith(expectedHistory);
+    });
+
+    it('should not strip thought signatures when stripThoughts is false', () => {
+      const mockChat = {
+        setHistory: vi.fn(),
+      };
+      client['chat'] = mockChat as unknown as GeminiChat;
+
+      const historyWithThoughts: Content[] = [
+        {
+          role: 'user',
+          parts: [{ text: 'hello' }],
+        },
+        {
+          role: 'model',
+          parts: [
+            { text: 'thinking...', thoughtSignature: 'thought-123' },
+            { text: 'ok', thoughtSignature: 'thought-456' },
+          ],
+        },
+      ];
+
+      client.setHistory(historyWithThoughts, { stripThoughts: false });
+
+      expect(mockChat.setHistory).toHaveBeenCalledWith(historyWithThoughts);
+    });
+  });
 });
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 96be4111..93de190d 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -164,8 +164,32 @@ export class GeminiClient {
     return this.getChat().getHistory();
   }
 
-  setHistory(history: Content[]) {
-    this.getChat().setHistory(history);
+  setHistory(
+    history: Content[],
+    { stripThoughts = false }: { stripThoughts?: boolean } = {},
+  ) {
+    const historyToSet = stripThoughts
+      ? history.map((content) => {
+          const newContent = { ...content };
+          if (newContent.parts) {
+            newContent.parts = newContent.parts.map((part) => {
+              if (
+                part &&
+                typeof part === 'object' &&
+                'thoughtSignature' in part
+              ) {
+                const newPart = { ...part };
+                delete (newPart as { thoughtSignature?: string })
+                  .thoughtSignature;
+                return newPart;
+              }
+              return part;
+            });
+          }
+          return newContent;
+        })
+      : history;
+    this.getChat().setHistory(historyToSet);
     this.forceFullIdeContext = true;
   }