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diff --git a/packages/core/src/utils/generateContentResponseUtilities.test.ts b/packages/core/src/utils/generateContentResponseUtilities.test.ts
new file mode 100644
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+++ b/packages/core/src/utils/generateContentResponseUtilities.test.ts
@@ -0,0 +1,323 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import {
+  getResponseText,
+  getResponseTextFromParts,
+  getFunctionCalls,
+  getFunctionCallsFromParts,
+  getFunctionCallsAsJson,
+  getFunctionCallsFromPartsAsJson,
+  getStructuredResponse,
+  getStructuredResponseFromParts,
+} from './generateContentResponseUtilities.js';
+import {
+  GenerateContentResponse,
+  Part,
+  FinishReason,
+  SafetyRating,
+} from '@google/genai';
+
+const mockTextPart = (text: string): Part => ({ text });
+const mockFunctionCallPart = (
+  name: string,
+  args?: Record<string, unknown>,
+): Part => ({
+  functionCall: { name, args: args ?? {} },
+});
+
+const mockResponse = (
+  parts: Part[],
+  finishReason: FinishReason = FinishReason.STOP,
+  safetyRatings: SafetyRating[] = [],
+): GenerateContentResponse => ({
+  candidates: [
+    {
+      content: {
+        parts,
+        role: 'model',
+      },
+      index: 0,
+      finishReason,
+      safetyRatings,
+    },
+  ],
+  promptFeedback: {
+    safetyRatings: [],
+  },
+  text: undefined,
+  data: undefined,
+  functionCalls: undefined,
+  executableCode: undefined,
+  codeExecutionResult: undefined,
+});
+
+const minimalMockResponse = (
+  candidates: GenerateContentResponse['candidates'],
+): GenerateContentResponse => ({
+  candidates,
+  promptFeedback: { safetyRatings: [] },
+  text: undefined,
+  data: undefined,
+  functionCalls: undefined,
+  executableCode: undefined,
+  codeExecutionResult: undefined,
+});
+
+describe('generateContentResponseUtilities', () => {
+  describe('getResponseText', () => {
+    it('should return undefined for no candidates', () => {
+      expect(getResponseText(minimalMockResponse(undefined))).toBeUndefined();
+    });
+    it('should return undefined for empty candidates array', () => {
+      expect(getResponseText(minimalMockResponse([]))).toBeUndefined();
+    });
+    it('should return undefined for no parts', () => {
+      const response = mockResponse([]);
+      expect(getResponseText(response)).toBeUndefined();
+    });
+    it('should extract text from a single text part', () => {
+      const response = mockResponse([mockTextPart('Hello')]);
+      expect(getResponseText(response)).toBe('Hello');
+    });
+    it('should concatenate text from multiple text parts', () => {
+      const response = mockResponse([
+        mockTextPart('Hello '),
+        mockTextPart('World'),
+      ]);
+      expect(getResponseText(response)).toBe('Hello World');
+    });
+    it('should ignore function call parts', () => {
+      const response = mockResponse([
+        mockTextPart('Hello '),
+        mockFunctionCallPart('testFunc'),
+        mockTextPart('World'),
+      ]);
+      expect(getResponseText(response)).toBe('Hello World');
+    });
+    it('should return undefined if only function call parts exist', () => {
+      const response = mockResponse([
+        mockFunctionCallPart('testFunc'),
+        mockFunctionCallPart('anotherFunc'),
+      ]);
+      expect(getResponseText(response)).toBeUndefined();
+    });
+  });
+
+  describe('getResponseTextFromParts', () => {
+    it('should return undefined for no parts', () => {
+      expect(getResponseTextFromParts([])).toBeUndefined();
+    });
+    it('should extract text from a single text part', () => {
+      expect(getResponseTextFromParts([mockTextPart('Hello')])).toBe('Hello');
+    });
+    it('should concatenate text from multiple text parts', () => {
+      expect(
+        getResponseTextFromParts([
+          mockTextPart('Hello '),
+          mockTextPart('World'),
+        ]),
+      ).toBe('Hello World');
+    });
+    it('should ignore function call parts', () => {
+      expect(
+        getResponseTextFromParts([
+          mockTextPart('Hello '),
+          mockFunctionCallPart('testFunc'),
+          mockTextPart('World'),
+        ]),
+      ).toBe('Hello World');
+    });
+    it('should return undefined if only function call parts exist', () => {
+      expect(
+        getResponseTextFromParts([
+          mockFunctionCallPart('testFunc'),
+          mockFunctionCallPart('anotherFunc'),
+        ]),
+      ).toBeUndefined();
+    });
+  });
+
+  describe('getFunctionCalls', () => {
+    it('should return undefined for no candidates', () => {
+      expect(getFunctionCalls(minimalMockResponse(undefined))).toBeUndefined();
+    });
+    it('should return undefined for empty candidates array', () => {
+      expect(getFunctionCalls(minimalMockResponse([]))).toBeUndefined();
+    });
+    it('should return undefined for no parts', () => {
+      const response = mockResponse([]);
+      expect(getFunctionCalls(response)).toBeUndefined();
+    });
+    it('should extract a single function call', () => {
+      const func = { name: 'testFunc', args: { a: 1 } };
+      const response = mockResponse([
+        mockFunctionCallPart(func.name, func.args),
+      ]);
+      expect(getFunctionCalls(response)).toEqual([func]);
+    });
+    it('should extract multiple function calls', () => {
+      const func1 = { name: 'testFunc1', args: { a: 1 } };
+      const func2 = { name: 'testFunc2', args: { b: 2 } };
+      const response = mockResponse([
+        mockFunctionCallPart(func1.name, func1.args),
+        mockFunctionCallPart(func2.name, func2.args),
+      ]);
+      expect(getFunctionCalls(response)).toEqual([func1, func2]);
+    });
+    it('should ignore text parts', () => {
+      const func = { name: 'testFunc', args: { a: 1 } };
+      const response = mockResponse([
+        mockTextPart('Some text'),
+        mockFunctionCallPart(func.name, func.args),
+        mockTextPart('More text'),
+      ]);
+      expect(getFunctionCalls(response)).toEqual([func]);
+    });
+    it('should return undefined if only text parts exist', () => {
+      const response = mockResponse([
+        mockTextPart('Some text'),
+        mockTextPart('More text'),
+      ]);
+      expect(getFunctionCalls(response)).toBeUndefined();
+    });
+  });
+
+  describe('getFunctionCallsFromParts', () => {
+    it('should return undefined for no parts', () => {
+      expect(getFunctionCallsFromParts([])).toBeUndefined();
+    });
+    it('should extract a single function call', () => {
+      const func = { name: 'testFunc', args: { a: 1 } };
+      expect(
+        getFunctionCallsFromParts([mockFunctionCallPart(func.name, func.args)]),
+      ).toEqual([func]);
+    });
+    it('should extract multiple function calls', () => {
+      const func1 = { name: 'testFunc1', args: { a: 1 } };
+      const func2 = { name: 'testFunc2', args: { b: 2 } };
+      expect(
+        getFunctionCallsFromParts([
+          mockFunctionCallPart(func1.name, func1.args),
+          mockFunctionCallPart(func2.name, func2.args),
+        ]),
+      ).toEqual([func1, func2]);
+    });
+    it('should ignore text parts', () => {
+      const func = { name: 'testFunc', args: { a: 1 } };
+      expect(
+        getFunctionCallsFromParts([
+          mockTextPart('Some text'),
+          mockFunctionCallPart(func.name, func.args),
+          mockTextPart('More text'),
+        ]),
+      ).toEqual([func]);
+    });
+    it('should return undefined if only text parts exist', () => {
+      expect(
+        getFunctionCallsFromParts([
+          mockTextPart('Some text'),
+          mockTextPart('More text'),
+        ]),
+      ).toBeUndefined();
+    });
+  });
+
+  describe('getFunctionCallsAsJson', () => {
+    it('should return JSON string of function calls', () => {
+      const func1 = { name: 'testFunc1', args: { a: 1 } };
+      const func2 = { name: 'testFunc2', args: { b: 2 } };
+      const response = mockResponse([
+        mockFunctionCallPart(func1.name, func1.args),
+        mockTextPart('text in between'),
+        mockFunctionCallPart(func2.name, func2.args),
+      ]);
+      const expectedJson = JSON.stringify([func1, func2], null, 2);
+      expect(getFunctionCallsAsJson(response)).toBe(expectedJson);
+    });
+    it('should return undefined if no function calls', () => {
+      const response = mockResponse([mockTextPart('Hello')]);
+      expect(getFunctionCallsAsJson(response)).toBeUndefined();
+    });
+  });
+
+  describe('getFunctionCallsFromPartsAsJson', () => {
+    it('should return JSON string of function calls from parts', () => {
+      const func1 = { name: 'testFunc1', args: { a: 1 } };
+      const func2 = { name: 'testFunc2', args: { b: 2 } };
+      const parts = [
+        mockFunctionCallPart(func1.name, func1.args),
+        mockTextPart('text in between'),
+        mockFunctionCallPart(func2.name, func2.args),
+      ];
+      const expectedJson = JSON.stringify([func1, func2], null, 2);
+      expect(getFunctionCallsFromPartsAsJson(parts)).toBe(expectedJson);
+    });
+    it('should return undefined if no function calls in parts', () => {
+      const parts = [mockTextPart('Hello')];
+      expect(getFunctionCallsFromPartsAsJson(parts)).toBeUndefined();
+    });
+  });
+
+  describe('getStructuredResponse', () => {
+    it('should return only text if only text exists', () => {
+      const response = mockResponse([mockTextPart('Hello World')]);
+      expect(getStructuredResponse(response)).toBe('Hello World');
+    });
+    it('should return only function call JSON if only function calls exist', () => {
+      const func = { name: 'testFunc', args: { data: 'payload' } };
+      const response = mockResponse([
+        mockFunctionCallPart(func.name, func.args),
+      ]);
+      const expectedJson = JSON.stringify([func], null, 2);
+      expect(getStructuredResponse(response)).toBe(expectedJson);
+    });
+    it('should return text and function call JSON if both exist', () => {
+      const text = 'Consider this data:';
+      const func = { name: 'processData', args: { item: 42 } };
+      const response = mockResponse([
+        mockTextPart(text),
+        mockFunctionCallPart(func.name, func.args),
+      ]);
+      const expectedJson = JSON.stringify([func], null, 2);
+      expect(getStructuredResponse(response)).toBe(`${text}\n${expectedJson}`);
+    });
+    it('should return undefined if neither text nor function calls exist', () => {
+      const response = mockResponse([]);
+      expect(getStructuredResponse(response)).toBeUndefined();
+    });
+  });
+
+  describe('getStructuredResponseFromParts', () => {
+    it('should return only text if only text exists in parts', () => {
+      const parts = [mockTextPart('Hello World')];
+      expect(getStructuredResponseFromParts(parts)).toBe('Hello World');
+    });
+    it('should return only function call JSON if only function calls exist in parts', () => {
+      const func = { name: 'testFunc', args: { data: 'payload' } };
+      const parts = [mockFunctionCallPart(func.name, func.args)];
+      const expectedJson = JSON.stringify([func], null, 2);
+      expect(getStructuredResponseFromParts(parts)).toBe(expectedJson);
+    });
+    it('should return text and function call JSON if both exist in parts', () => {
+      const text = 'Consider this data:';
+      const func = { name: 'processData', args: { item: 42 } };
+      const parts = [
+        mockTextPart(text),
+        mockFunctionCallPart(func.name, func.args),
+      ];
+      const expectedJson = JSON.stringify([func], null, 2);
+      expect(getStructuredResponseFromParts(parts)).toBe(
+        `${text}\n${expectedJson}`,
+      );
+    });
+    it('should return undefined if neither text nor function calls exist in parts', () => {
+      const parts: Part[] = [];
+      expect(getStructuredResponseFromParts(parts)).toBeUndefined();
+    });
+  });
+});