diff --git a/packages/agents-a365-tooling-extensions-openai/src/McpToolRegistrationService.ts b/packages/agents-a365-tooling-extensions-openai/src/McpToolRegistrationService.ts
index e6edb0e4..a35562c4 100644
--- a/packages/agents-a365-tooling-extensions-openai/src/McpToolRegistrationService.ts
+++ b/packages/agents-a365-tooling-extensions-openai/src/McpToolRegistrationService.ts
@@ -174,8 +174,15 @@ export class McpToolRegistrationService {
       orchestratorName: toolOptions?.orchestratorName ?? this.orchestratorName
     };
 
-    // Convert OpenAI messages to ChatHistoryMessage format
-    const chatHistoryMessages = this.convertToChatHistoryMessages(messages);
+    let chatHistoryMessages: ChatHistoryMessage[];
+    try {
+      // Convert OpenAI messages to ChatHistoryMessage format
+      chatHistoryMessages = this.convertToChatHistoryMessages(messages);
+    } catch (err: unknown) {
+      // Convert errors from message conversion into a failed OperationResult
+      const error = err instanceof Error ? err : new Error(String(err));
+      return OperationResult.failed(new OperationError(error));
+    }
 
     // Delegate to core service
     return await this.configService.sendChatHistory(
diff --git a/tests/tooling-extensions-openai/sendChatHistoryMessagesAsync.test.ts b/tests/tooling-extensions-openai/sendChatHistoryMessagesAsync.test.ts
index 16acf680..633ad405 100644
--- a/tests/tooling-extensions-openai/sendChatHistoryMessagesAsync.test.ts
+++ b/tests/tooling-extensions-openai/sendChatHistoryMessagesAsync.test.ts
@@ -242,6 +242,36 @@ describe('McpToolRegistrationService - sendChatHistoryMessagesAsync', () => {
       expect(result.errors).toHaveLength(1);
     });
 
+    it('EH-05: should return failed when message conversion throws unexpected error', async () => {
+      const messages = createMixedMessages();
+
+      // Mock convertToChatHistoryMessages to throw an error
+      jest.spyOn(service as any, 'convertToChatHistoryMessages').mockImplementation(() => {
+        throw new Error('Conversion error');
+      });
+
+      const result = await service.sendChatHistoryMessagesAsync(mockTurnContext, messages);
+
+      expect(result.succeeded).toBe(false);
+      expect(result.errors).toHaveLength(1);
+      expect(result.errors[0].message).toBe('Conversion error');
+    });
+
+    it('EH-06: should handle non-Error thrown values gracefully', async () => {
+      const messages = createMixedMessages();
+
+      // Mock convertToChatHistoryMessages to throw a non-Error object
+      jest.spyOn(service as any, 'convertToChatHistoryMessages').mockImplementation(() => {
+        throw 'String error'; // eslint-disable-line no-throw-literal
+      });
+
+      const result = await service.sendChatHistoryMessagesAsync(mockTurnContext, messages);
+
+      expect(result.succeeded).toBe(false);
+      expect(result.errors).toHaveLength(1);
+      expect(result.errors[0].message).toBe('String error');
+    });
+
     it('should re-throw validation errors from core service', async () => {
       const messages = createMixedMessages();
       // Remove conversation ID to trigger validation error