diff --git a/packages/core/src/core/client.test.ts b/packages/core/src/core/client.test.ts
index aac4ee84a1..6487e04e80 100644
--- a/packages/core/src/core/client.test.ts
+++ b/packages/core/src/core/client.test.ts
@@ -166,6 +166,7 @@ describe('Gemini Client (client.ts)', () => {
       generateContent: mockGenerateContentFn,
       generateContentStream: vi.fn(),
       batchEmbedContents: vi.fn(),
+      countTokens: vi.fn().mockResolvedValue({ totalTokens: 100 }),
     } as unknown as ContentGenerator;
 
     // Because the GeminiClient constructor kicks off an async process (startChat)
@@ -902,6 +903,75 @@ ${JSON.stringify(
       });
     });
 
+    it('should use local estimation for text-only requests and NOT call countTokens', async () => {
+      const request = [{ text: 'Hello world' }];
+      const generator = client['getContentGeneratorOrFail']();
+      const countTokensSpy = vi.spyOn(generator, 'countTokens');
+
+      const stream = client.sendMessageStream(
+        request,
+        new AbortController().signal,
+        'test-prompt-id',
+      );
+      await stream.next(); // Trigger the generator
+
+      expect(countTokensSpy).not.toHaveBeenCalled();
+    });
+
+    it('should use countTokens API for requests with non-text parts', async () => {
+      const request = [
+        { text: 'Describe this image' },
+        { inlineData: { mimeType: 'image/png', data: 'base64...' } },
+      ];
+      const generator = client['getContentGeneratorOrFail']();
+      const countTokensSpy = vi
+        .spyOn(generator, 'countTokens')
+        .mockResolvedValue({ totalTokens: 123 });
+
+      const stream = client.sendMessageStream(
+        request,
+        new AbortController().signal,
+        'test-prompt-id',
+      );
+      await stream.next(); // Trigger the generator
+
+      expect(countTokensSpy).toHaveBeenCalledWith(
+        expect.objectContaining({
+          contents: expect.arrayContaining([
+            expect.objectContaining({
+              parts: expect.arrayContaining([
+                { text: 'Describe this image' },
+                { inlineData: { mimeType: 'image/png', data: 'base64...' } },
+              ]),
+            }),
+          ]),
+        }),
+      );
+    });
+
+    it('should estimate CJK characters more conservatively (closer to 1 token/char)', async () => {
+      const request = [{ text: '你好世界' }]; // 4 chars
+      const generator = client['getContentGeneratorOrFail']();
+      const countTokensSpy = vi.spyOn(generator, 'countTokens');
+
+      // 4 chars.
+      // Old logic: 4/4 = 1.
+      // New logic (heuristic): 4 * 1 = 4. (Or at least > 1).
+      // Let's assert it's roughly accurate.
+
+      const stream = client.sendMessageStream(
+        request,
+        new AbortController().signal,
+        'test-prompt-id',
+      );
+      await stream.next();
+
+      // Should NOT call countTokens (it's text only)
+      expect(countTokensSpy).not.toHaveBeenCalled();
+
+      // The actual token calculation is unit tested in tokenCalculation.test.ts
+    });
+
     it('should return the turn instance after the stream is complete', async () => {
       // Arrange
       const mockStream = (async function* () {
diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 95988e14e7..df42b1694e 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -56,47 +56,10 @@ import { handleFallback } from '../fallback/handler.js';
 import type { RoutingContext } from '../routing/routingStrategy.js';
 import { debugLogger } from '../utils/debugLogger.js';
 import type { ModelConfigKey } from '../services/modelConfigService.js';
+import { calculateRequestTokenCount } from '../utils/tokenCalculation.js';
 
 const MAX_TURNS = 100;
 
-/**
- * Estimates the character length of text-only parts in a request.
- * Binary data (inline_data, fileData) is excluded from the estimation
- * because Gemini counts these as fixed token values, not based on their size.
- * @param request The request to estimate tokens for
- * @returns Estimated character length of text content
- */
-function estimateTextOnlyLength(request: PartListUnion): number {
-  if (typeof request === 'string') {
-    return request.length;
-  }
-
-  // Ensure request is an array before iterating
-  if (!Array.isArray(request)) {
-    return 0;
-  }
-
-  let textLength = 0;
-  for (const part of request) {
-    // Handle string elements in the array
-    if (typeof part === 'string') {
-      textLength += part.length;
-    }
-    // Handle object elements with text property
-    else if (
-      typeof part === 'object' &&
-      part !== null &&
-      'text' in part &&
-      part.text
-    ) {
-      textLength += part.text.length;
-    }
-    // inlineData, fileData, and other binary parts are ignored
-    // as they are counted as fixed tokens by Gemini
-  }
-  return textLength;
-}
-
 export class GeminiClient {
   private chat?: GeminiChat;
   private sessionTurnCount = 0;
@@ -493,11 +456,12 @@ export class GeminiClient {
     // Check for context window overflow
     const modelForLimitCheck = this._getEffectiveModelForCurrentTurn();
 
-    // Estimate tokens based on text content only.
-    // Binary data (PDFs, images) are counted as fixed tokens by Gemini,
-    // not based on their base64-encoded size.
-    const estimatedRequestTokenCount = Math.floor(
-      estimateTextOnlyLength(request) / 4,
+    // Estimate tokens. For text-only requests, we estimate based on character length.
+    // For requests with non-text parts (like images, tools), we use the countTokens API.
+    const estimatedRequestTokenCount = await calculateRequestTokenCount(
+      request,
+      this.getContentGeneratorOrFail(),
+      modelForLimitCheck,
     );
 
     const remainingTokenCount =
diff --git a/packages/core/src/core/geminiChat.test.ts b/packages/core/src/core/geminiChat.test.ts
index bafc12dcae..96db6f214e 100644
--- a/packages/core/src/core/geminiChat.test.ts
+++ b/packages/core/src/core/geminiChat.test.ts
@@ -174,15 +174,15 @@ describe('GeminiChat', () => {
         { role: 'model', parts: [{ text: 'Hi there' }] },
       ];
       const chatWithHistory = new GeminiChat(mockConfig, '', [], history);
-      const estimatedTokens = Math.ceil(JSON.stringify(history).length / 4);
-      expect(chatWithHistory.getLastPromptTokenCount()).toBe(estimatedTokens);
+      // 'Hello': 5 chars * 0.25 = 1.25
+      // 'Hi there': 8 chars * 0.25 = 2.0
+      // Total: 3.25 -> floor(3.25) = 3
+      expect(chatWithHistory.getLastPromptTokenCount()).toBe(3);
     });
 
     it('should initialize lastPromptTokenCount for empty history', () => {
       const chatEmpty = new GeminiChat(mockConfig);
-      expect(chatEmpty.getLastPromptTokenCount()).toBe(
-        Math.ceil(JSON.stringify([]).length / 4),
-      );
+      expect(chatEmpty.getLastPromptTokenCount()).toBe(0);
     });
   });
 
diff --git a/packages/core/src/core/geminiChat.ts b/packages/core/src/core/geminiChat.ts
index 3e48bcfcb9..a62fa73108 100644
--- a/packages/core/src/core/geminiChat.ts
+++ b/packages/core/src/core/geminiChat.ts
@@ -46,6 +46,7 @@ import { handleFallback } from '../fallback/handler.js';
 import { isFunctionResponse } from '../utils/messageInspectors.js';
 import { partListUnionToString } from './geminiRequest.js';
 import type { ModelConfigKey } from '../services/modelConfigService.js';
+import { estimateTokenCountSync } from '../utils/tokenCalculation.js';
 
 export enum StreamEventType {
   /** A regular content chunk from the API. */
@@ -213,8 +214,8 @@ export class GeminiChat {
     validateHistory(history);
     this.chatRecordingService = new ChatRecordingService(config);
     this.chatRecordingService.initialize(resumedSessionData);
-    this.lastPromptTokenCount = Math.ceil(
-      JSON.stringify(this.history).length / 4,
+    this.lastPromptTokenCount = estimateTokenCountSync(
+      this.history.flatMap((c) => c.parts || []),
     );
   }
 
diff --git a/packages/core/src/services/chatCompressionService.test.ts b/packages/core/src/services/chatCompressionService.test.ts
index 84d91ff192..4a27a72510 100644
--- a/packages/core/src/services/chatCompressionService.test.ts
+++ b/packages/core/src/services/chatCompressionService.test.ts
@@ -154,6 +154,9 @@ describe('ChatCompressionService', () => {
         generateContent: mockGenerateContent,
       }),
       isInteractive: vi.fn().mockReturnValue(false),
+      getContentGenerator: vi.fn().mockReturnValue({
+        countTokens: vi.fn().mockResolvedValue({ totalTokens: 100 }),
+      }),
     } as unknown as Config;
 
     vi.mocked(tokenLimit).mockReturnValue(1000);
@@ -286,6 +289,11 @@ describe('ChatCompressionService', () => {
       ],
     } as unknown as GenerateContentResponse);
 
+    // Override mock to simulate high token count for this specific test
+    vi.mocked(mockConfig.getContentGenerator().countTokens).mockResolvedValue({
+      totalTokens: 10000,
+    });
+
     const result = await service.compress(
       mockChat,
       mockPromptId,
diff --git a/packages/core/src/services/chatCompressionService.ts b/packages/core/src/services/chatCompressionService.ts
index 817f6cf48a..b3dbf74512 100644
--- a/packages/core/src/services/chatCompressionService.ts
+++ b/packages/core/src/services/chatCompressionService.ts
@@ -14,6 +14,7 @@ import { getResponseText } from '../utils/partUtils.js';
 import { logChatCompression } from '../telemetry/loggers.js';
 import { makeChatCompressionEvent } from '../telemetry/types.js';
 import { getInitialChatHistory } from '../utils/environmentContext.js';
+import { calculateRequestTokenCount } from '../utils/tokenCalculation.js';
 import {
   DEFAULT_GEMINI_FLASH_LITE_MODEL,
   DEFAULT_GEMINI_FLASH_MODEL,
@@ -195,12 +196,10 @@ export class ChatCompressionService {
     // Use a shared utility to construct the initial history for an accurate token count.
     const fullNewHistory = await getInitialChatHistory(config, extraHistory);
 
-    // Estimate token count 1 token ≈ 4 characters
-    const newTokenCount = Math.floor(
-      fullNewHistory.reduce(
-        (total, content) => total + JSON.stringify(content).length,
-        0,
-      ) / 4,
+    const newTokenCount = await calculateRequestTokenCount(
+      fullNewHistory.flatMap((c) => c.parts || []),
+      config.getContentGenerator(),
+      model,
     );
 
     logChatCompression(
diff --git a/packages/core/src/utils/tokenCalculation.test.ts b/packages/core/src/utils/tokenCalculation.test.ts
new file mode 100644
index 0000000000..7e1eae3e88
--- /dev/null
+++ b/packages/core/src/utils/tokenCalculation.test.ts
@@ -0,0 +1,130 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi } from 'vitest';
+import { calculateRequestTokenCount } from './tokenCalculation.js';
+import type { ContentGenerator } from '../core/contentGenerator.js';
+
+describe('calculateRequestTokenCount', () => {
+  const mockContentGenerator = {
+    countTokens: vi.fn(),
+  } as unknown as ContentGenerator;
+
+  const model = 'gemini-pro';
+
+  it('should use countTokens API for media requests (images/files)', async () => {
+    vi.mocked(mockContentGenerator.countTokens).mockResolvedValue({
+      totalTokens: 100,
+    });
+    const request = [{ inlineData: { mimeType: 'image/png', data: 'data' } }];
+
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+
+    expect(count).toBe(100);
+    expect(mockContentGenerator.countTokens).toHaveBeenCalled();
+  });
+
+  it('should estimate tokens locally for tool calls', async () => {
+    vi.mocked(mockContentGenerator.countTokens).mockClear();
+    const request = [{ functionCall: { name: 'foo', args: { bar: 'baz' } } }];
+
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+
+    // Estimation logic: JSON.stringify(part).length / 4
+    // JSON: {"functionCall":{"name":"foo","args":{"bar":"baz"}}}
+    // Length: ~53 chars. 53 / 4 = 13.25 -> 13.
+    expect(count).toBeGreaterThan(0);
+    expect(mockContentGenerator.countTokens).not.toHaveBeenCalled();
+  });
+
+  it('should estimate tokens locally for simple ASCII text', async () => {
+    vi.mocked(mockContentGenerator.countTokens).mockClear();
+    // 12 chars. 12 * 0.25 = 3 tokens.
+    const request = 'Hello world!';
+
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+
+    expect(count).toBe(3);
+    expect(mockContentGenerator.countTokens).not.toHaveBeenCalled();
+  });
+
+  it('should estimate tokens locally for CJK text with higher weight', async () => {
+    vi.mocked(mockContentGenerator.countTokens).mockClear();
+    // 2 chars. 2 * 1.3 = 2.6 -> floor(2.6) = 2.
+    // Old logic would be 2/4 = 0.5 -> 0.
+    const request = '你好';
+
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+
+    expect(count).toBeGreaterThanOrEqual(2);
+    expect(mockContentGenerator.countTokens).not.toHaveBeenCalled();
+  });
+
+  it('should handle mixed content', async () => {
+    vi.mocked(mockContentGenerator.countTokens).mockClear();
+    // 'Hi': 2 * 0.25 = 0.5
+    // '你好': 2 * 1.3 = 2.6
+    // Total: 3.1 -> 3
+    const request = 'Hi你好';
+
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+
+    expect(count).toBe(3);
+    expect(mockContentGenerator.countTokens).not.toHaveBeenCalled();
+  });
+
+  it('should handle empty text', async () => {
+    const request = '';
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+    expect(count).toBe(0);
+  });
+
+  it('should fallback to local estimation when countTokens API fails', async () => {
+    vi.mocked(mockContentGenerator.countTokens).mockRejectedValue(
+      new Error('API error'),
+    );
+    const request = [
+      { text: 'Hello' },
+      { inlineData: { mimeType: 'image/png', data: 'data' } },
+    ];
+
+    const count = await calculateRequestTokenCount(
+      request,
+      mockContentGenerator,
+      model,
+    );
+
+    // Should fallback to estimation:
+    // 'Hello': 5 chars * 0.25 = 1.25
+    // inlineData: JSON.stringify length / 4
+    expect(count).toBeGreaterThan(0);
+    expect(mockContentGenerator.countTokens).toHaveBeenCalled();
+  });
+});
diff --git a/packages/core/src/utils/tokenCalculation.ts b/packages/core/src/utils/tokenCalculation.ts
new file mode 100644
index 0000000000..0359cb3e7c
--- /dev/null
+++ b/packages/core/src/utils/tokenCalculation.ts
@@ -0,0 +1,79 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { PartListUnion, Part } from '@google/genai';
+import type { ContentGenerator } from '../core/contentGenerator.js';
+
+// Token estimation constants
+// ASCII characters (0-127) are roughly 4 chars per token
+const ASCII_TOKENS_PER_CHAR = 0.25;
+// Non-ASCII characters (including CJK) are often 1-2 tokens per char.
+// We use 1.3 as a conservative estimate to avoid underestimation.
+const NON_ASCII_TOKENS_PER_CHAR = 1.3;
+
+/**
+ * Estimates token count for parts synchronously using a heuristic.
+ * - Text: character-based heuristic (ASCII vs CJK).
+ * - Non-text (Tools, etc): JSON string length / 4.
+ */
+export function estimateTokenCountSync(parts: Part[]): number {
+  let totalTokens = 0;
+  for (const part of parts) {
+    if (typeof part.text === 'string') {
+      for (const char of part.text) {
+        if (char.codePointAt(0)! <= 127) {
+          totalTokens += ASCII_TOKENS_PER_CHAR;
+        } else {
+          totalTokens += NON_ASCII_TOKENS_PER_CHAR;
+        }
+      }
+    } else {
+      // For non-text parts (functionCall, functionResponse, executableCode, etc.),
+      // we fallback to the JSON string length heuristic.
+      // Note: This is an approximation.
+      totalTokens += JSON.stringify(part).length / 4;
+    }
+  }
+  return Math.floor(totalTokens);
+}
+
+/**
+ * Calculates the token count of the request.
+ * If the request contains only text or tools, it estimates the token count locally.
+ * If the request contains media (images, files), it uses the countTokens API.
+ */
+export async function calculateRequestTokenCount(
+  request: PartListUnion,
+  contentGenerator: ContentGenerator,
+  model: string,
+): Promise<number> {
+  const parts: Part[] = Array.isArray(request)
+    ? request.map((p) => (typeof p === 'string' ? { text: p } : p))
+    : typeof request === 'string'
+      ? [{ text: request }]
+      : [request];
+
+  // Use countTokens API only for heavy media parts that are hard to estimate.
+  const hasMedia = parts.some((p) => {
+    const isMedia = 'inlineData' in p || 'fileData' in p;
+    return isMedia;
+  });
+
+  if (hasMedia) {
+    try {
+      const response = await contentGenerator.countTokens({
+        model,
+        contents: [{ role: 'user', parts }],
+      });
+      return response.totalTokens ?? 0;
+    } catch {
+      // Fallback to local estimation if the API call fails
+      return estimateTokenCountSync(parts);
+    }
+  }
+
+  return estimateTokenCountSync(parts);
+}