feat: implement MCP output compression mechanism

- Add compressionService.ts with AI-powered compression using OpenAI
- Add compression configuration to SystemConfig type
- Update SystemConfig database entity with compression field
- Integrate compression with mcpService tool call responses
- Add migration support for compression settings
- Add comprehensive unit tests (23 new tests)
- Support graceful fallback when compression is unavailable

Co-authored-by: samanhappy <2755122+samanhappy@users.noreply.github.com>

src/db/entities/SystemConfig.ts

@@ -33,6 +33,9 @@ export class SystemConfig {
   @Column({ type: 'boolean', nullable: true })
   enableSessionRebuild?: boolean;
 
+  @Column({ type: 'simple-json', nullable: true })
+  compression?: Record<string, any>;
+
   @CreateDateColumn({ name: 'created_at', type: 'timestamp' })
   createdAt: Date;
 

src/db/repositories/SystemConfigRepository.ts

@@ -32,6 +32,7 @@ export class SystemConfigRepository {
         oauth: {},
         oauthServer: {},
         enableSessionRebuild: false,
+        compression: {},
       });
       config = await this.repository.save(config);
     }

src/services/compressionService.ts

@@ -0,0 +1,266 @@
+import OpenAI from 'openai';
+import { getSmartRoutingConfig, SmartRoutingConfig } from '../utils/smartRouting.js';
+import { getSystemConfigDao } from '../dao/index.js';
+
+/**
+ * Compression configuration interface
+ */
+export interface CompressionConfig {
+  enabled: boolean;
+  model?: string;
+  maxInputTokens?: number;
+  targetReductionRatio?: number;
+}
+
+/**
+ * Default compression configuration
+ */
+const DEFAULT_COMPRESSION_CONFIG: CompressionConfig = {
+  enabled: false,
+  model: 'gpt-4o-mini',
+  maxInputTokens: 100000,
+  targetReductionRatio: 0.5,
+};
+
+/**
+ * Get compression configuration from system settings
+ */
+export async function getCompressionConfig(): Promise<CompressionConfig> {
+  try {
+    const systemConfigDao = getSystemConfigDao();
+    const systemConfig = await systemConfigDao.get();
+    const compressionSettings = systemConfig?.compression || {};
+
+    return {
+      enabled: compressionSettings.enabled ?? DEFAULT_COMPRESSION_CONFIG.enabled,
+      model: compressionSettings.model ?? DEFAULT_COMPRESSION_CONFIG.model,
+      maxInputTokens: compressionSettings.maxInputTokens ?? DEFAULT_COMPRESSION_CONFIG.maxInputTokens,
+      targetReductionRatio:
+        compressionSettings.targetReductionRatio ?? DEFAULT_COMPRESSION_CONFIG.targetReductionRatio,
+    };
+  } catch (error) {
+    console.warn('Failed to get compression config, using defaults:', error);
+    return DEFAULT_COMPRESSION_CONFIG;
+  }
+}
+
+/**
+ * Check if compression is available and enabled
+ */
+export async function isCompressionEnabled(): Promise<boolean> {
+  const config = await getCompressionConfig();
+  if (!config.enabled) {
+    return false;
+  }
+
+  // Check if we have OpenAI API key configured (via smart routing config)
+  const smartRoutingConfig = await getSmartRoutingConfig();
+  return !!smartRoutingConfig.openaiApiKey;
+}
+
+/**
+ * Get OpenAI client for compression
+ */
+async function getOpenAIClient(smartRoutingConfig: SmartRoutingConfig): Promise<OpenAI | null> {
+  if (!smartRoutingConfig.openaiApiKey) {
+    return null;
+  }
+
+  return new OpenAI({
+    apiKey: smartRoutingConfig.openaiApiKey,
+    baseURL: smartRoutingConfig.openaiApiBaseUrl || 'https://api.openai.com/v1',
+  });
+}
+
+/**
+ * Estimate token count for a string (rough approximation)
+ * Uses ~4 characters per token as a rough estimate
+ */
+export function estimateTokenCount(text: string): number {
+  return Math.ceil(text.length / 4);
+}
+
+/**
+ * Check if content should be compressed based on token count
+ */
+export function shouldCompress(content: string, maxInputTokens: number): boolean {
+  const estimatedTokens = estimateTokenCount(content);
+  // Only compress if content is larger than a reasonable threshold
+  const compressionThreshold = Math.min(maxInputTokens * 0.1, 1000);
+  return estimatedTokens > compressionThreshold;
+}
+
+/**
+ * Compress MCP tool output using AI
+ *
+ * @param content The MCP tool output content to compress
+ * @param context Optional context about the tool that generated this output
+ * @returns Compressed content or original content if compression fails/is disabled
+ */
+export async function compressOutput(
+  content: string,
+  context?: {
+    toolName?: string;
+    serverName?: string;
+  },
+): Promise<{ compressed: string; originalLength: number; compressedLength: number; wasCompressed: boolean }> {
+  const originalLength = content.length;
+
+  // Check if compression is enabled
+  const compressionConfig = await getCompressionConfig();
+  if (!compressionConfig.enabled) {
+    return {
+      compressed: content,
+      originalLength,
+      compressedLength: originalLength,
+      wasCompressed: false,
+    };
+  }
+
+  // Check if content should be compressed
+  if (!shouldCompress(content, compressionConfig.maxInputTokens || 100000)) {
+    return {
+      compressed: content,
+      originalLength,
+      compressedLength: originalLength,
+      wasCompressed: false,
+    };
+  }
+
+  try {
+    const smartRoutingConfig = await getSmartRoutingConfig();
+    const openai = await getOpenAIClient(smartRoutingConfig);
+
+    if (!openai) {
+      console.warn('Compression enabled but OpenAI API key not configured');
+      return {
+        compressed: content,
+        originalLength,
+        compressedLength: originalLength,
+        wasCompressed: false,
+      };
+    }
+
+    const targetRatio = compressionConfig.targetReductionRatio || 0.5;
+    const toolContext = context?.toolName ? `from tool "${context.toolName}"` : '';
+    const serverContext = context?.serverName ? `on server "${context.serverName}"` : '';
+
+    const systemPrompt = `You are a data compression assistant. Your task is to compress MCP (Model Context Protocol) tool outputs while preserving all essential information.
+
+Guidelines:
+- Remove redundant information, formatting, and verbose descriptions
+- Preserve all data values, identifiers, and critical information
+- Keep error messages and status information intact
+- Maintain structured data (JSON, arrays) in a compact but readable format
+- Target approximately ${Math.round(targetRatio * 100)}% reduction in size
+- If the content cannot be meaningfully compressed, return it as-is
+
+The output is ${toolContext} ${serverContext}.`;
+
+    const userPrompt = `Compress the following MCP tool output while preserving all essential information:
+
+${content}`;
+
+    const response = await openai.chat.completions.create({
+      model: compressionConfig.model || 'gpt-4o-mini',
+      messages: [
+        { role: 'system', content: systemPrompt },
+        { role: 'user', content: userPrompt },
+      ],
+      temperature: 0.1,
+      max_tokens: Math.ceil(estimateTokenCount(content) * targetRatio * 1.5),
+    });
+
+    const compressedContent = response.choices[0]?.message?.content;
+
+    if (!compressedContent) {
+      console.warn('Compression returned empty result, using original content');
+      return {
+        compressed: content,
+        originalLength,
+        compressedLength: originalLength,
+        wasCompressed: false,
+      };
+    }
+
+    const compressedLength = compressedContent.length;
+
+    // Only use compressed version if it's actually smaller
+    if (compressedLength >= originalLength) {
+      console.log('Compression did not reduce size, using original content');
+      return {
+        compressed: content,
+        originalLength,
+        compressedLength: originalLength,
+        wasCompressed: false,
+      };
+    }
+
+    const reductionPercent = ((originalLength - compressedLength) / originalLength * 100).toFixed(1);
+    console.log(`Compressed output: ${originalLength} -> ${compressedLength} chars (${reductionPercent}% reduction)`);
+
+    return {
+      compressed: compressedContent,
+      originalLength,
+      compressedLength,
+      wasCompressed: true,
+    };
+  } catch (error) {
+    console.error('Compression failed, using original content:', error);
+    return {
+      compressed: content,
+      originalLength,
+      compressedLength: originalLength,
+      wasCompressed: false,
+    };
+  }
+}
+
+/**
+ * Compress tool call result content
+ * This handles the MCP tool result format with content array
+ */
+export async function compressToolResult(
+  result: any,
+  context?: {
+    toolName?: string;
+    serverName?: string;
+  },
+): Promise<any> {
+  // Check if compression is enabled first
+  const compressionEnabled = await isCompressionEnabled();
+  if (!compressionEnabled) {
+    return result;
+  }
+
+  // Handle error results - don't compress error messages
+  if (result?.isError) {
+    return result;
+  }
+
+  // Handle content array format
+  if (!result?.content || !Array.isArray(result.content)) {
+    return result;
+  }
+
+  const compressedContent = await Promise.all(
+    result.content.map(async (item: any) => {
+      // Only compress text content
+      if (item?.type !== 'text' || !item?.text) {
+        return item;
+      }
+
+      const compressionResult = await compressOutput(item.text, context);
+
+      return {
+        ...item,
+        text: compressionResult.compressed,
+      };
+    }),
+  );
+
+  return {
+    ...result,
+    content: compressedContent,
+  };
+}

src/services/mcpService.ts

@@ -27,6 +27,7 @@ import { getDataService } from './services.js';
 import { getServerDao, getSystemConfigDao, ServerConfigWithName } from '../dao/index.js';
 import { initializeAllOAuthClients } from './oauthService.js';
 import { createOAuthProvider } from './mcpOAuthProvider.js';
+import { compressToolResult } from './compressionService.js';
 
 const servers: { [sessionId: string]: Server } = {};
 
@@ -1260,7 +1261,7 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
         const result = await openApiClient.callTool(cleanToolName, finalArgs, passthroughHeaders);
 
         console.log(`OpenAPI tool invocation result: ${JSON.stringify(result)}`);
-        return {
+        const openApiResult = {
           content: [
             {
               type: 'text',
@@ -1268,6 +1269,10 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
             },
           ],
         };
+        return compressToolResult(openApiResult, {
+          toolName: cleanToolName,
+          serverName: targetServerInfo.name,
+        });
       }
 
       // Call the tool on the target server (MCP servers)
@@ -1297,7 +1302,10 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
       );
 
       console.log(`Tool invocation result: ${JSON.stringify(result)}`);
-      return result;
+      return compressToolResult(result, {
+        toolName,
+        serverName: targetServerInfo.name,
+      });
     }
 
     // Regular tool handling
@@ -1356,7 +1364,7 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
       );
 
       console.log(`OpenAPI tool invocation result: ${JSON.stringify(result)}`);
-      return {
+      const openApiResult = {
         content: [
           {
             type: 'text',
@@ -1364,6 +1372,10 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
           },
         ],
       };
+      return compressToolResult(openApiResult, {
+        toolName: cleanToolName,
+        serverName: serverInfo.name,
+      });
     }
 
     // Handle MCP servers
@@ -1374,6 +1386,7 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
 
     const separator = getNameSeparator();
     const prefix = `${serverInfo.name}${separator}`;
+    const originalToolName = request.params.name;
     request.params.name = request.params.name.startsWith(prefix)
       ? request.params.name.substring(prefix.length)
       : request.params.name;
@@ -1383,7 +1396,10 @@ export const handleCallToolRequest = async (request: any, extra: any) => {
       serverInfo.options || {},
     );
     console.log(`Tool call result: ${JSON.stringify(result)}`);
-    return result;
+    return compressToolResult(result, {
+      toolName: originalToolName,
+      serverName: serverInfo.name,
+    });
   } catch (error) {
     console.error(`Error handling CallToolRequest: ${error}`);
     return {

src/types/index.ts

@@ -173,6 +173,12 @@ export interface SystemConfig {
   oauth?: OAuthProviderConfig; // OAuth provider configuration for upstream MCP servers
   oauthServer?: OAuthServerConfig; // OAuth authorization server configuration for MCPHub itself
   enableSessionRebuild?: boolean; // Controls whether server session rebuild is enabled
+  compression?: {
+    enabled?: boolean; // Enable/disable AI compression of MCP tool outputs
+    model?: string; // AI model to use for compression (default: 'gpt-4o-mini')
+    maxInputTokens?: number; // Maximum input tokens for compression (default: 100000)
+    targetReductionRatio?: number; // Target reduction ratio, 0.0-1.0 (default: 0.5)
+  };
 }
 
 export interface UserConfig {

src/utils/migration.ts

@@ -117,6 +117,7 @@ export async function migrateToDatabase(): Promise<boolean> {
         oauth: settings.systemConfig.oauth || {},
         oauthServer: settings.systemConfig.oauthServer || {},
         enableSessionRebuild: settings.systemConfig.enableSessionRebuild,
+        compression: settings.systemConfig.compression || {},
       };
       await systemConfigRepo.update(systemConfig);
       console.log('  - System configuration updated');

tests/services/compressionService.test.ts

@@ -0,0 +1,428 @@
+// Mock the DAO module before imports
+jest.mock('../../src/dao/index.js', () => ({
+  getSystemConfigDao: jest.fn(),
+}));
+
+// Mock smart routing config
+jest.mock('../../src/utils/smartRouting.js', () => ({
+  getSmartRoutingConfig: jest.fn(),
+}));
+
+// Mock OpenAI
+jest.mock('openai', () => {
+  return {
+    __esModule: true,
+    default: jest.fn().mockImplementation(() => ({
+      chat: {
+        completions: {
+          create: jest.fn(),
+        },
+      },
+    })),
+  };
+});
+
+import {
+  getCompressionConfig,
+  isCompressionEnabled,
+  estimateTokenCount,
+  shouldCompress,
+  compressOutput,
+  compressToolResult,
+} from '../../src/services/compressionService.js';
+import { getSystemConfigDao } from '../../src/dao/index.js';
+import { getSmartRoutingConfig } from '../../src/utils/smartRouting.js';
+import OpenAI from 'openai';
+
+describe('CompressionService', () => {
+  const mockSystemConfigDao = {
+    get: jest.fn(),
+    getSection: jest.fn(),
+    update: jest.fn(),
+    updateSection: jest.fn(),
+  };
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    (getSystemConfigDao as jest.Mock).mockReturnValue(mockSystemConfigDao);
+  });
+
+  describe('getCompressionConfig', () => {
+    it('should return default config when no config is set', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({});
+
+      const config = await getCompressionConfig();
+
+      expect(config).toEqual({
+        enabled: false,
+        model: 'gpt-4o-mini',
+        maxInputTokens: 100000,
+        targetReductionRatio: 0.5,
+      });
+    });
+
+    it('should return configured values when set', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: {
+          enabled: true,
+          model: 'gpt-4o',
+          maxInputTokens: 50000,
+          targetReductionRatio: 0.3,
+        },
+      });
+
+      const config = await getCompressionConfig();
+
+      expect(config).toEqual({
+        enabled: true,
+        model: 'gpt-4o',
+        maxInputTokens: 50000,
+        targetReductionRatio: 0.3,
+      });
+    });
+
+    it('should use defaults for missing values', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: {
+          enabled: true,
+        },
+      });
+
+      const config = await getCompressionConfig();
+
+      expect(config).toEqual({
+        enabled: true,
+        model: 'gpt-4o-mini',
+        maxInputTokens: 100000,
+        targetReductionRatio: 0.5,
+      });
+    });
+
+    it('should return defaults on error', async () => {
+      mockSystemConfigDao.get.mockRejectedValue(new Error('Test error'));
+
+      const config = await getCompressionConfig();
+
+      expect(config).toEqual({
+        enabled: false,
+        model: 'gpt-4o-mini',
+        maxInputTokens: 100000,
+        targetReductionRatio: 0.5,
+      });
+    });
+  });
+
+  describe('isCompressionEnabled', () => {
+    it('should return false when compression is disabled', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: false },
+      });
+
+      const enabled = await isCompressionEnabled();
+
+      expect(enabled).toBe(false);
+    });
+
+    it('should return false when enabled but no API key', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: '',
+      });
+
+      const enabled = await isCompressionEnabled();
+
+      expect(enabled).toBe(false);
+    });
+
+    it('should return true when enabled and API key is set', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+      });
+
+      const enabled = await isCompressionEnabled();
+
+      expect(enabled).toBe(true);
+    });
+  });
+
+  describe('estimateTokenCount', () => {
+    it('should estimate tokens for short text', () => {
+      const text = 'Hello world';
+      const tokens = estimateTokenCount(text);
+
+      // 11 characters / 4 = 2.75, ceil = 3
+      expect(tokens).toBe(3);
+    });
+
+    it('should estimate tokens for longer text', () => {
+      const text = 'This is a longer piece of text that should have more tokens';
+      const tokens = estimateTokenCount(text);
+
+      // 59 characters / 4 = 14.75, ceil = 15
+      expect(tokens).toBe(15);
+    });
+
+    it('should handle empty string', () => {
+      const tokens = estimateTokenCount('');
+
+      expect(tokens).toBe(0);
+    });
+  });
+
+  describe('shouldCompress', () => {
+    it('should return false for small content', () => {
+      const content = 'Small content';
+      const result = shouldCompress(content, 100000);
+
+      expect(result).toBe(false);
+    });
+
+    it('should return true for large content', () => {
+      // Create content larger than the threshold (1000 tokens = ~4000 chars)
+      const content = 'x'.repeat(5000);
+      const result = shouldCompress(content, 100000);
+
+      expect(result).toBe(true);
+    });
+
+    it('should use 10% of maxInputTokens as threshold', () => {
+      // With maxInputTokens = 1000, threshold = 100 tokens = ~400 chars
+      const smallContent = 'x'.repeat(300);
+      const largeContent = 'x'.repeat(500);
+
+      expect(shouldCompress(smallContent, 1000)).toBe(false);
+      expect(shouldCompress(largeContent, 1000)).toBe(true);
+    });
+  });
+
+  describe('compressOutput', () => {
+    it('should return original content when compression is disabled', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: false },
+      });
+
+      const content = 'Test content';
+      const result = await compressOutput(content);
+
+      expect(result).toEqual({
+        compressed: content,
+        originalLength: content.length,
+        compressedLength: content.length,
+        wasCompressed: false,
+      });
+    });
+
+    it('should return original content when content is too small', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true, maxInputTokens: 100000 },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+      });
+
+      const content = 'Small content';
+      const result = await compressOutput(content);
+
+      expect(result.wasCompressed).toBe(false);
+      expect(result.compressed).toBe(content);
+    });
+
+    it('should return original content when no API key is configured', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: '',
+      });
+
+      const content = 'x'.repeat(5000);
+      const result = await compressOutput(content);
+
+      expect(result.wasCompressed).toBe(false);
+      expect(result.compressed).toBe(content);
+    });
+
+    it('should compress content when enabled and content is large', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true, model: 'gpt-4o-mini', maxInputTokens: 100000 },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+        openaiApiBaseUrl: 'https://api.openai.com/v1',
+      });
+
+      const originalContent = 'x'.repeat(5000);
+      const compressedContent = 'y'.repeat(2000);
+
+      // Mock OpenAI response
+      const mockCreate = jest.fn().mockResolvedValue({
+        choices: [{ message: { content: compressedContent } }],
+      });
+
+      (OpenAI as unknown as jest.Mock).mockImplementation(() => ({
+        chat: {
+          completions: {
+            create: mockCreate,
+          },
+        },
+      }));
+
+      const result = await compressOutput(originalContent, {
+        toolName: 'test-tool',
+        serverName: 'test-server',
+      });
+
+      expect(result.wasCompressed).toBe(true);
+      expect(result.compressed).toBe(compressedContent);
+      expect(result.originalLength).toBe(originalContent.length);
+      expect(result.compressedLength).toBe(compressedContent.length);
+    });
+
+    it('should return original content when compressed is larger', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true, model: 'gpt-4o-mini', maxInputTokens: 100000 },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+        openaiApiBaseUrl: 'https://api.openai.com/v1',
+      });
+
+      const originalContent = 'x'.repeat(5000);
+      const largerContent = 'y'.repeat(6000);
+
+      const mockCreate = jest.fn().mockResolvedValue({
+        choices: [{ message: { content: largerContent } }],
+      });
+
+      (OpenAI as unknown as jest.Mock).mockImplementation(() => ({
+        chat: {
+          completions: {
+            create: mockCreate,
+          },
+        },
+      }));
+
+      const result = await compressOutput(originalContent);
+
+      expect(result.wasCompressed).toBe(false);
+      expect(result.compressed).toBe(originalContent);
+    });
+
+    it('should return original content on API error', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true, model: 'gpt-4o-mini', maxInputTokens: 100000 },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+        openaiApiBaseUrl: 'https://api.openai.com/v1',
+      });
+
+      const mockCreate = jest.fn().mockRejectedValue(new Error('API error'));
+
+      (OpenAI as unknown as jest.Mock).mockImplementation(() => ({
+        chat: {
+          completions: {
+            create: mockCreate,
+          },
+        },
+      }));
+
+      const content = 'x'.repeat(5000);
+      const result = await compressOutput(content);
+
+      expect(result.wasCompressed).toBe(false);
+      expect(result.compressed).toBe(content);
+    });
+  });
+
+  describe('compressToolResult', () => {
+    it('should return original result when compression is disabled', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: false },
+      });
+
+      const result = {
+        content: [{ type: 'text', text: 'Test output' }],
+      };
+
+      const compressed = await compressToolResult(result);
+
+      expect(compressed).toEqual(result);
+    });
+
+    it('should not compress error results', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+      });
+
+      const result = {
+        content: [{ type: 'text', text: 'Error message' }],
+        isError: true,
+      };
+
+      const compressed = await compressToolResult(result);
+
+      expect(compressed).toEqual(result);
+    });
+
+    it('should handle results without content array', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+      });
+
+      const result = { someOtherField: 'value' };
+
+      const compressed = await compressToolResult(result);
+
+      expect(compressed).toEqual(result);
+    });
+
+    it('should only compress text content items', async () => {
+      mockSystemConfigDao.get.mockResolvedValue({
+        compression: { enabled: true, maxInputTokens: 100000 },
+      });
+      (getSmartRoutingConfig as jest.Mock).mockResolvedValue({
+        openaiApiKey: 'test-api-key',
+        openaiApiBaseUrl: 'https://api.openai.com/v1',
+      });
+
+      const largeText = 'x'.repeat(5000);
+      const compressedText = 'y'.repeat(2000);
+
+      const mockCreate = jest.fn().mockResolvedValue({
+        choices: [{ message: { content: compressedText } }],
+      });
+
+      (OpenAI as unknown as jest.Mock).mockImplementation(() => ({
+        chat: {
+          completions: {
+            create: mockCreate,
+          },
+        },
+      }));
+
+      const result = {
+        content: [
+          { type: 'text', text: largeText },
+          { type: 'image', data: 'base64data' },
+        ],
+      };
+
+      const compressed = await compressToolResult(result);
+
+      expect(compressed.content[0].text).toBe(compressedText);
+      expect(compressed.content[1]).toEqual({ type: 'image', data: 'base64data' });
+    });
+  });
+});