diff --git a/package-lock.json b/package-lock.json
index 4bdf885..986e122 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1027,7 +1027,6 @@
       "resolved": "https://registry.npmjs.org/@octokit/core/-/core-7.0.6.tgz",
       "integrity": "sha512-DhGl4xMVFGVIyMwswXeyzdL4uXD5OGILGX5N8Y+f6W7LhC1Ze2poSNrkF/fedpVDHEEZ+PHFW0vL14I+mm8K3Q==",
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@octokit/auth-token": "^6.0.0",
         "@octokit/graphql": "^9.0.3",
@@ -1611,7 +1610,6 @@
       "integrity": "sha512-WJtwWJu7UdlvzEAUm484QNg5eAoq5QR08KDNx7g45Usrs2NtOPiX8ugDqmKdXkyL03rBqU5dYNYVQetEpBHq2g==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "undici-types": "~6.21.0"
       }
@@ -1661,7 +1659,6 @@
       "integrity": "sha512-nm3cvFN9SqZGXjmw5bZ6cGmvJSyJPn0wU9gHAZZHDnZl2wF9PhHv78Xf06E0MaNk4zLVHL8hb2/c32XvyJOLQg==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@typescript-eslint/scope-manager": "8.53.1",
         "@typescript-eslint/types": "8.53.1",
@@ -2063,7 +2060,6 @@
       "integrity": "sha512-NZyJarBfL7nWwIq+FDL6Zp/yHEhePMNnnJ0y3qfieCrmNvYct8uvtiV41UvlSe6apAfk0fY1FbWx+NwfmpvtTg==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "acorn": "bin/acorn"
       },
@@ -2560,7 +2556,6 @@
       "integrity": "sha512-LEyamqS7W5HB3ujJyvi0HQK/dtVINZvd5mAAp9eT5S/ujByGjiZLCzPcHVzuXbpJDJF/cxwHlfceVUDZ2lnSTw==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@eslint-community/eslint-utils": "^4.8.0",
         "@eslint-community/regexpp": "^4.12.1",
@@ -3849,7 +3844,6 @@
       "resolved": "https://registry.npmjs.org/picomatch/-/picomatch-4.0.3.tgz",
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -4002,7 +3996,6 @@
       "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
       "dev": true,
       "license": "Apache-2.0",
-      "peer": true,
       "bin": {
         "tsc": "bin/tsc",
         "tsserver": "bin/tsserver"
@@ -4208,7 +4201,6 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -4222,7 +4214,6 @@
       "integrity": "sha512-LUCP5ev3GURDysTWiP47wRRUpLKMOfPh+yKTx3kVIEiu5KOMeqzpnYNsKyOoVrULivR8tLcks4+lga33Whn90A==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@types/chai": "^5.2.2",
         "@vitest/expect": "3.2.4",
diff --git a/src/core/target-handlers/acp-handler.ts b/src/core/target-handlers/acp-handler.ts
index cfa023c..ab55244 100644
--- a/src/core/target-handlers/acp-handler.ts
+++ b/src/core/target-handlers/acp-handler.ts
@@ -121,8 +121,12 @@ export class ACPTargetHandler implements TargetHandler {
           try {
             const msg = JSON.parse(line) as ACPMessage;
             this.handleMessage(msg);
-          } catch {
-            // Ignore parse errors (partial messages)
+          } catch (err) {
+            // Log parse errors in debug mode
+            if (process.env['DEBUG'] || process.env['WORK_DEBUG']) {
+              console.warn('ACP message parse error:', err instanceof Error ? err.message : String(err));
+              console.warn('Problematic line:', line.substring(0, 200));
+            }
           }
         }
       }
diff --git a/tests/unit/core/target-handlers/acp-handler.test.ts b/tests/unit/core/target-handlers/acp-handler.test.ts
index 0c2806b..1bf244a 100644
--- a/tests/unit/core/target-handlers/acp-handler.test.ts
+++ b/tests/unit/core/target-handlers/acp-handler.test.ts
@@ -26,10 +26,17 @@ describe('ACPTargetHandler', () => {
   let mockProcess: any;
   const mockSpawn = spawn as any;
 
+  let savedDebug: string | undefined;
+  let savedWorkDebug: string | undefined;
+
   beforeEach(() => {
     vi.clearAllMocks();
     vi.useFakeTimers();
 
+    // Save environment variables for restoration
+    savedDebug = process.env['DEBUG'];
+    savedWorkDebug = process.env['WORK_DEBUG'];
+
     // Create a more realistic mock process
     mockProcess = {
       stdin: {
@@ -70,6 +77,18 @@ describe('ACPTargetHandler', () => {
 
   afterEach(() => {
     vi.useRealTimers();
+    
+    // Restore environment variables
+    if (savedDebug !== undefined) {
+      process.env['DEBUG'] = savedDebug;
+    } else {
+      delete process.env['DEBUG'];
+    }
+    if (savedWorkDebug !== undefined) {
+      process.env['WORK_DEBUG'] = savedWorkDebug;
+    } else {
+      delete process.env['WORK_DEBUG'];
+    }
   });
 
   describe('formatWorkItems', () => {
@@ -384,6 +403,92 @@ describe('ACPTargetHandler', () => {
       // Should not throw
       expect(true).toBe(true);
     });
+
+    it('should not log parse errors when DEBUG is not set', () => {
+      const consoleWarnSpy = vi.spyOn(console, 'warn');
+      delete process.env['DEBUG'];
+      delete process.env['WORK_DEBUG'];
+
+      (handler as any).ensureProcess(mockConfig);
+
+      // Send invalid JSON
+      mockProcess.stdout.emit('data', 'invalid json\n');
+
+      // Should not log anything
+      expect(consoleWarnSpy).not.toHaveBeenCalled();
+
+      consoleWarnSpy.mockRestore();
+    });
+
+    it('should log parse errors when DEBUG is set', () => {
+      const consoleWarnSpy = vi.spyOn(console, 'warn');
+      process.env['DEBUG'] = '1';
+
+      (handler as any).ensureProcess(mockConfig);
+
+      // Send invalid JSON
+      mockProcess.stdout.emit('data', 'invalid json\n');
+
+      // Should log error message first, then problematic line
+      expect(consoleWarnSpy).toHaveBeenCalledTimes(2);
+      expect(consoleWarnSpy).toHaveBeenNthCalledWith(
+        1,
+        'ACP message parse error:',
+        expect.stringContaining('JSON')
+      );
+      expect(consoleWarnSpy).toHaveBeenNthCalledWith(
+        2,
+        'Problematic line:',
+        'invalid json'
+      );
+
+      consoleWarnSpy.mockRestore();
+    });
+
+    it('should log parse errors when WORK_DEBUG is set', () => {
+      const consoleWarnSpy = vi.spyOn(console, 'warn');
+      process.env['WORK_DEBUG'] = '1';
+
+      (handler as any).ensureProcess(mockConfig);
+
+      // Send invalid JSON
+      mockProcess.stdout.emit('data', 'invalid json\n');
+
+      // Should log error message first, then problematic line
+      expect(consoleWarnSpy).toHaveBeenCalledTimes(2);
+      expect(consoleWarnSpy).toHaveBeenNthCalledWith(
+        1,
+        'ACP message parse error:',
+        expect.stringContaining('JSON')
+      );
+      expect(consoleWarnSpy).toHaveBeenNthCalledWith(
+        2,
+        'Problematic line:',
+        'invalid json'
+      );
+
+      consoleWarnSpy.mockRestore();
+    });
+
+    it('should truncate long problematic lines to 200 characters', () => {
+      const consoleWarnSpy = vi.spyOn(console, 'warn');
+      process.env['DEBUG'] = '1';
+
+      (handler as any).ensureProcess(mockConfig);
+
+      // Send invalid JSON that's longer than 200 characters
+      const longInvalidJson = 'x'.repeat(300) + '\n';
+      mockProcess.stdout.emit('data', longInvalidJson);
+
+      // Should log truncated line (200 chars max)
+      expect(consoleWarnSpy).toHaveBeenNthCalledWith(
+        2,
+        'Problematic line:',
+        'x'.repeat(200)
+      );
+
+      consoleWarnSpy.mockRestore();
+    });
   });
 
   describe('cleanup', () => {