diff --git a/src/main/apiServer/routes/__tests__/messages.test.ts b/src/main/apiServer/routes/__tests__/messages.test.ts
new file mode 100644
index 0000000000..b07b82e317
--- /dev/null
+++ b/src/main/apiServer/routes/__tests__/messages.test.ts
@@ -0,0 +1,393 @@
+import { describe, expect, it } from 'vitest'
+
+import { estimateTokenCount } from '../messages'
+
+describe('estimateTokenCount', () => {
+  describe('Text Content', () => {
+    it('should estimate tokens for simple string content', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: 'Hello, world!'
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include text tokens + role overhead (3)
+      expect(tokens).toBeGreaterThan(3)
+      expect(tokens).toBeLessThan(20)
+    })
+
+    it('should estimate tokens for multiple messages', () => {
+      const input = {
+        messages: [
+          { role: 'user' as const, content: 'First message' },
+          { role: 'assistant' as const, content: 'Second message' },
+          { role: 'user' as const, content: 'Third message' }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include text tokens + role overhead (3 per message = 9)
+      expect(tokens).toBeGreaterThan(9)
+    })
+
+    it('should estimate tokens for text content blocks', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              { type: 'text' as const, text: 'Hello' },
+              { type: 'text' as const, text: 'World' }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      expect(tokens).toBeGreaterThan(3)
+    })
+
+    it('should handle empty messages array', () => {
+      const input = {
+        messages: []
+      }
+      const tokens = estimateTokenCount(input)
+      expect(tokens).toBe(0)
+    })
+
+    it('should handle messages with empty content', () => {
+      const input = {
+        messages: [{ role: 'user' as const, content: '' }]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should only have role overhead (3)
+      expect(tokens).toBe(3)
+    })
+  })
+
+  describe('System Messages', () => {
+    it('should estimate tokens for string system message', () => {
+      const input = {
+        messages: [{ role: 'user' as const, content: 'Hello' }],
+        system: 'You are a helpful assistant.'
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include system tokens + message tokens + role overhead
+      expect(tokens).toBeGreaterThan(3)
+    })
+
+    it('should estimate tokens for system content blocks', () => {
+      const input = {
+        messages: [{ role: 'user' as const, content: 'Hello' }],
+        system: [
+          { type: 'text' as const, text: 'System instruction 1' },
+          { type: 'text' as const, text: 'System instruction 2' }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      expect(tokens).toBeGreaterThan(3)
+    })
+  })
+
+  describe('Image Content', () => {
+    it('should estimate tokens for base64 images', () => {
+      // Create a fake base64 string (400 characters = ~300 bytes when decoded)
+      const fakeBase64 = 'A'.repeat(400)
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              {
+                type: 'image' as const,
+                source: {
+                  type: 'base64' as const,
+                  media_type: 'image/png' as const,
+                  data: fakeBase64
+                }
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should estimate based on data size: 400 * 0.75 / 100 = 3 tokens + role overhead (3)
+      expect(tokens).toBeGreaterThan(3)
+      expect(tokens).toBeLessThan(10)
+    })
+
+    it('should estimate tokens for URL images', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              {
+                type: 'image' as const,
+                source: {
+                  type: 'url' as const,
+                  url: 'https://example.com/image.png'
+                }
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should use default estimate: 1000 + role overhead (3)
+      expect(tokens).toBe(1003)
+    })
+
+    it('should estimate tokens for mixed text and image content', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              { type: 'text' as const, text: 'What is in this image?' },
+              {
+                type: 'image' as const,
+                source: {
+                  type: 'url' as const,
+                  url: 'https://example.com/image.png'
+                }
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include text tokens + 1000 (image) + role overhead (3)
+      expect(tokens).toBeGreaterThan(1003)
+    })
+  })
+
+  describe('Tool Content', () => {
+    it('should estimate tokens for tool_use blocks', () => {
+      const input = {
+        messages: [
+          {
+            role: 'assistant' as const,
+            content: [
+              {
+                type: 'tool_use' as const,
+                id: 'tool_123',
+                name: 'get_weather',
+                input: { location: 'San Francisco', unit: 'celsius' }
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include: tool name tokens + input JSON tokens + 10 (overhead) + 3 (role)
+      expect(tokens).toBeGreaterThan(13)
+    })
+
+    it('should estimate tokens for tool_result blocks with string content', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              {
+                type: 'tool_result' as const,
+                tool_use_id: 'tool_123',
+                content: 'The weather in San Francisco is 18°C and sunny.'
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include: content tokens + 10 (overhead) + 3 (role)
+      expect(tokens).toBeGreaterThan(13)
+    })
+
+    it('should estimate tokens for tool_result blocks with array content', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              {
+                type: 'tool_result' as const,
+                tool_use_id: 'tool_123',
+                content: [
+                  { type: 'text' as const, text: 'Result 1' },
+                  { type: 'text' as const, text: 'Result 2' }
+                ]
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include: text tokens + 10 (overhead) + 3 (role)
+      expect(tokens).toBeGreaterThan(13)
+    })
+
+    it('should handle tool_use without input', () => {
+      const input = {
+        messages: [
+          {
+            role: 'assistant' as const,
+            content: [
+              {
+                type: 'tool_use' as const,
+                id: 'tool_123',
+                name: 'no_input_tool',
+                input: {}
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include: tool name tokens + 10 (overhead) + 3 (role)
+      expect(tokens).toBeGreaterThan(13)
+    })
+  })
+
+  describe('Complex Scenarios', () => {
+    it('should estimate tokens for multi-turn conversation with various content types', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              { type: 'text' as const, text: 'Analyze this image' },
+              {
+                type: 'image' as const,
+                source: {
+                  type: 'url' as const,
+                  url: 'https://example.com/chart.png'
+                }
+              }
+            ]
+          },
+          {
+            role: 'assistant' as const,
+            content: [
+              {
+                type: 'tool_use' as const,
+                id: 'tool_1',
+                name: 'analyze_image',
+                input: { url: 'https://example.com/chart.png' }
+              }
+            ]
+          },
+          {
+            role: 'user' as const,
+            content: [
+              {
+                type: 'tool_result' as const,
+                tool_use_id: 'tool_1',
+                content: 'The chart shows sales data for Q4 2024.'
+              }
+            ]
+          },
+          {
+            role: 'assistant' as const,
+            content: 'Based on the analysis, the sales trend is positive.'
+          }
+        ],
+        system: 'You are a data analyst assistant.'
+      }
+      const tokens = estimateTokenCount(input)
+      // Should include:
+      // - System message tokens
+      // - Message 1: text + image (1000) + 3
+      // - Message 2: tool_use + 10 + 3
+      // - Message 3: tool_result + 10 + 3
+      // - Message 4: text + 3
+      expect(tokens).toBeGreaterThan(1032) // At least 1000 (image) + 32 (overhead)
+    })
+
+    it('should handle very long text content', () => {
+      const longText = 'word '.repeat(1000) // ~5000 characters
+      const input = {
+        messages: [{ role: 'user' as const, content: longText }]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should estimate based on text length using tokenx
+      expect(tokens).toBeGreaterThan(1000)
+    })
+
+    it('should handle multiple images in single message', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [
+              {
+                type: 'image' as const,
+                source: { type: 'url' as const, url: 'https://example.com/1.png' }
+              },
+              {
+                type: 'image' as const,
+                source: { type: 'url' as const, url: 'https://example.com/2.png' }
+              },
+              {
+                type: 'image' as const,
+                source: { type: 'url' as const, url: 'https://example.com/3.png' }
+              }
+            ]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should estimate: 3 * 1000 (images) + 3 (role)
+      expect(tokens).toBe(3003)
+    })
+  })
+
+  describe('Edge Cases', () => {
+    it('should handle undefined system message', () => {
+      const input = {
+        messages: [{ role: 'user' as const, content: 'Hello' }],
+        system: undefined
+      }
+      const tokens = estimateTokenCount(input)
+      expect(tokens).toBeGreaterThan(0)
+    })
+
+    it('should handle empty system message', () => {
+      const input = {
+        messages: [{ role: 'user' as const, content: 'Hello' }],
+        system: ''
+      }
+      const tokens = estimateTokenCount(input)
+      expect(tokens).toBeGreaterThan(0)
+    })
+
+    it('should handle content blocks with missing text', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: [{ type: 'text' as const, text: undefined as any }]
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should only have role overhead
+      expect(tokens).toBe(3)
+    })
+
+    it('should handle empty content array', () => {
+      const input = {
+        messages: [
+          {
+            role: 'user' as const,
+            content: []
+          }
+        ]
+      }
+      const tokens = estimateTokenCount(input)
+      // Should only have role overhead
+      expect(tokens).toBe(3)
+    })
+  })
+})
diff --git a/src/main/apiServer/routes/messages.ts b/src/main/apiServer/routes/messages.ts
index 0b26db3960..2264cbb846 100644
--- a/src/main/apiServer/routes/messages.ts
+++ b/src/main/apiServer/routes/messages.ts
@@ -48,12 +48,12 @@ const providerRouter = express.Router({ mergeParams: true })
  * Estimate token count from messages
  * Uses tokenx library for accurate token estimation and supports images, tools
  */
-interface CountTokensInput {
+export interface CountTokensInput {
   messages: MessageCreateParams['messages']
   system?: MessageCreateParams['system']
 }
 
-function estimateTokenCount(input: CountTokensInput): number {
+export function estimateTokenCount(input: CountTokensInput): number {
   const { messages, system } = input
   let totalTokens = 0