Add debug logging for JSON parse errors in ACP handler

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));
+            }
           }
         }
       }

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', () => {