github-issue-159-takt-run-noro (#166)

* caffeinate に -d フラグを追加し、ディスプレイスリープ中の App Nap によるプロセス凍結を防止

* takt 対話モードの save_task を takt add と同じ worktree 設定フローに統一

takt 対話モードで Save Task を選択した際に worktree/branch/auto_pr の
設定プロンプトがスキップされ、takt run で clone なしに実行されて成果物が
消失するバグを修正。promptWorktreeSettings() を共通関数として抽出し、
saveTaskFromInteractive() と addTask() の両方から使用するようにした。

* Release v0.9.0

* takt: github-issue-159-takt-run-noro

src/__tests__/runAllTasks-concurrency.test.ts

@@ -212,13 +212,25 @@ describe('runAllTasks concurrency', () => {
         .mockReturnValueOnce([task1, task2, task3])
         .mockReturnValueOnce([]);
 
+      // In parallel mode, task start messages go through TaskPrefixWriter → process.stdout.write
+      const stdoutChunks: string[] = [];
+      const writeSpy = vi.spyOn(process.stdout, 'write').mockImplementation((chunk: unknown) => {
+        stdoutChunks.push(String(chunk));
+        return true;
+      });
+
       // When
       await runAllTasks('/project');
+      writeSpy.mockRestore();
 
-      // Then: Task names displayed
-      expect(mockInfo).toHaveBeenCalledWith('=== Task: task-1 ===');
-      expect(mockInfo).toHaveBeenCalledWith('=== Task: task-2 ===');
-      expect(mockInfo).toHaveBeenCalledWith('=== Task: task-3 ===');
+      // Then: Task names displayed with prefix in stdout
+      const allOutput = stdoutChunks.join('');
+      expect(allOutput).toContain('[task-1]');
+      expect(allOutput).toContain('=== Task: task-1 ===');
+      expect(allOutput).toContain('[task-2]');
+      expect(allOutput).toContain('=== Task: task-2 ===');
+      expect(allOutput).toContain('[task-3]');
+      expect(allOutput).toContain('=== Task: task-3 ===');
       expect(mockStatus).toHaveBeenCalledWith('Total', '3');
     });
 

src/__tests__/task-prefix-writer.test.ts

@@ -0,0 +1,184 @@
+/**
+ * Tests for TaskPrefixWriter
+ */
+
+import { describe, it, expect, beforeEach } from 'vitest';
+import { TaskPrefixWriter } from '../shared/ui/TaskPrefixWriter.js';
+
+describe('TaskPrefixWriter', () => {
+  let output: string[];
+  let writeFn: (text: string) => void;
+
+  beforeEach(() => {
+    output = [];
+    writeFn = (text: string) => output.push(text);
+  });
+
+  describe('constructor', () => {
+    it('should cycle colors for different colorIndex values', () => {
+      const writer0 = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+      const writer4 = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 4, writeFn });
+
+      writer0.writeLine('hello');
+      writer4.writeLine('hello');
+
+      // Both index 0 and 4 should use cyan (\x1b[36m)
+      expect(output[0]).toContain('\x1b[36m');
+      expect(output[1]).toContain('\x1b[36m');
+    });
+
+    it('should assign correct colors in order', () => {
+      const writers = [0, 1, 2, 3].map(
+        (i) => new TaskPrefixWriter({ taskName: `t${i}`, colorIndex: i, writeFn }),
+      );
+
+      writers.forEach((w) => w.writeLine('x'));
+
+      expect(output[0]).toContain('\x1b[36m'); // cyan
+      expect(output[1]).toContain('\x1b[33m'); // yellow
+      expect(output[2]).toContain('\x1b[35m'); // magenta
+      expect(output[3]).toContain('\x1b[32m'); // green
+    });
+  });
+
+  describe('writeLine', () => {
+    it('should output single line with prefix', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'my-task', colorIndex: 0, writeFn });
+
+      writer.writeLine('Hello World');
+
+      expect(output).toHaveLength(1);
+      expect(output[0]).toContain('[my-task]');
+      expect(output[0]).toContain('Hello World');
+      expect(output[0]).toMatch(/\n$/);
+    });
+
+    it('should output empty line as bare newline', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'my-task', colorIndex: 0, writeFn });
+
+      writer.writeLine('');
+
+      expect(output).toHaveLength(1);
+      expect(output[0]).toBe('\n');
+    });
+
+    it('should split multi-line text and prefix each non-empty line', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'my-task', colorIndex: 0, writeFn });
+
+      writer.writeLine('Line 1\nLine 2\n\nLine 4');
+
+      expect(output).toHaveLength(4);
+      expect(output[0]).toContain('Line 1');
+      expect(output[1]).toContain('Line 2');
+      expect(output[2]).toBe('\n'); // empty line
+      expect(output[3]).toContain('Line 4');
+    });
+
+    it('should strip ANSI codes from input text', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'my-task', colorIndex: 0, writeFn });
+
+      writer.writeLine('\x1b[31mRed Text\x1b[0m');
+
+      expect(output).toHaveLength(1);
+      expect(output[0]).toContain('Red Text');
+      expect(output[0]).not.toContain('\x1b[31m');
+    });
+  });
+
+  describe('writeChunk (line buffering)', () => {
+    it('should buffer partial line and output on newline', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('Hello');
+      expect(output).toHaveLength(0);
+
+      writer.writeChunk(' World\n');
+      expect(output).toHaveLength(1);
+      expect(output[0]).toContain('[task-a]');
+      expect(output[0]).toContain('Hello World');
+    });
+
+    it('should handle multiple lines in single chunk', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('Line 1\nLine 2\n');
+
+      expect(output).toHaveLength(2);
+      expect(output[0]).toContain('Line 1');
+      expect(output[1]).toContain('Line 2');
+    });
+
+    it('should output empty line without prefix', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('Hello\n\nWorld\n');
+
+      expect(output).toHaveLength(3);
+      expect(output[0]).toContain('Hello');
+      expect(output[1]).toBe('\n');
+      expect(output[2]).toContain('World');
+    });
+
+    it('should keep trailing partial in buffer', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('Complete\nPartial');
+
+      expect(output).toHaveLength(1);
+      expect(output[0]).toContain('Complete');
+
+      writer.flush();
+      expect(output).toHaveLength(2);
+      expect(output[1]).toContain('Partial');
+    });
+
+    it('should strip ANSI codes from streamed chunks', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('\x1b[31mHello');
+      writer.writeChunk(' World\x1b[0m\n');
+
+      expect(output).toHaveLength(1);
+      expect(output[0]).toContain('Hello World');
+      expect(output[0]).not.toContain('\x1b[31m');
+    });
+  });
+
+  describe('flush', () => {
+    it('should output remaining buffered content with prefix', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('partial content');
+      expect(output).toHaveLength(0);
+
+      writer.flush();
+
+      expect(output).toHaveLength(1);
+      expect(output[0]).toContain('[task-a]');
+      expect(output[0]).toContain('partial content');
+      expect(output[0]).toMatch(/\n$/);
+    });
+
+    it('should not output anything when buffer is empty', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('complete line\n');
+      output.length = 0;
+
+      writer.flush();
+      expect(output).toHaveLength(0);
+    });
+
+    it('should clear buffer after flush', () => {
+      const writer = new TaskPrefixWriter({ taskName: 'task-a', colorIndex: 0, writeFn });
+
+      writer.writeChunk('content');
+      writer.flush();
+      output.length = 0;
+
+      writer.flush();
+      expect(output).toHaveLength(0);
+    });
+  });
+
+});

src/__tests__/workerPool.test.ts

@@ -102,17 +102,26 @@ describe('runWithWorkerPool', () => {
     expect(result).toEqual({ success: 2, fail: 1 });
   });
 
-  it('should display task name for each task', async () => {
+  it('should display task name for each task via prefix writer in parallel mode', async () => {
     // Given
     const tasks = [createTask('alpha'), createTask('beta')];
     const runner = createMockTaskRunner([]);
+    const stdoutChunks: string[] = [];
+    const writeSpy = vi.spyOn(process.stdout, 'write').mockImplementation((chunk: unknown) => {
+      stdoutChunks.push(String(chunk));
+      return true;
+    });
 
     // When
     await runWithWorkerPool(runner as never, tasks, 2, '/cwd', 'default', undefined, TEST_POLL_INTERVAL_MS);
 
-    // Then
-    expect(mockInfo).toHaveBeenCalledWith('=== Task: alpha ===');
-    expect(mockInfo).toHaveBeenCalledWith('=== Task: beta ===');
+    // Then: Task names appear in prefixed stdout output
+    writeSpy.mockRestore();
+    const allOutput = stdoutChunks.join('');
+    expect(allOutput).toContain('[alpha]');
+    expect(allOutput).toContain('=== Task: alpha ===');
+    expect(allOutput).toContain('[beta]');
+    expect(allOutput).toContain('=== Task: beta ===');
   });
 
   it('should pass taskPrefix for parallel execution (concurrency > 1)', async () => {
@@ -129,6 +138,7 @@ describe('runWithWorkerPool', () => {
     expect(parallelOpts).toEqual({
       abortSignal: expect.any(AbortSignal),
       taskPrefix: 'my-task',
+      taskColorIndex: 0,
     });
   });
 
@@ -146,6 +156,7 @@ describe('runWithWorkerPool', () => {
     expect(parallelOpts).toEqual({
       abortSignal: undefined,
       taskPrefix: undefined,
+      taskColorIndex: undefined,
     });
   });
 

src/features/tasks/execute/parallelExecution.ts

@@ -13,6 +13,7 @@
 
 import type { TaskRunner, TaskInfo } from '../../../infra/task/index.js';
 import { info, blankLine } from '../../../shared/ui/index.js';
+import { TaskPrefixWriter } from '../../../shared/ui/TaskPrefixWriter.js';
 import { createLogger } from '../../../shared/utils/index.js';
 import { executeAndCompleteTask } from './taskExecution.js';
 import { installSigIntHandler } from './sigintHandler.js';
@@ -102,6 +103,7 @@ export async function runWithWorkerPool(
 
   const queue = [...initialTasks];
   const active = new Map<Promise<boolean>, TaskInfo>();
+  const colorCounter = { value: 0 };
 
   try {
     while (queue.length > 0 || active.size > 0) {
@@ -109,7 +111,7 @@ export async function runWithWorkerPool(
         break;
       }
 
-      fillSlots(queue, active, concurrency, taskRunner, cwd, pieceName, options, abortController);
+      fillSlots(queue, active, concurrency, taskRunner, cwd, pieceName, options, abortController, colorCounter);
 
       if (active.size === 0) {
         break;
@@ -171,17 +173,25 @@ function fillSlots(
   pieceName: string,
   options: TaskExecutionOptions | undefined,
   abortController: AbortController,
+  colorCounter: { value: number },
 ): void {
   while (active.size < concurrency && queue.length > 0) {
     const task = queue.shift()!;
     const isParallel = concurrency > 1;
+    const colorIndex = colorCounter.value++;
 
+    if (isParallel) {
+      const writer = new TaskPrefixWriter({ taskName: task.name, colorIndex });
+      writer.writeLine(`=== Task: ${task.name} ===`);
+    } else {
       blankLine();
       info(`=== Task: ${task.name} ===`);
+    }
 
     const promise = executeAndCompleteTask(task, taskRunner, cwd, pieceName, options, {
       abortSignal: isParallel ? abortController.signal : undefined,
       taskPrefix: isParallel ? task.name : undefined,
+      taskColorIndex: isParallel ? colorIndex : undefined,
     });
     active.set(promise, task);
   }

src/features/tasks/execute/pieceExecution.ts

@@ -21,15 +21,16 @@ import {
 } from '../../../infra/config/index.js';
 import { isQuietMode } from '../../../shared/context.js';
 import {
-  header,
-  info,
-  warn,
-  error,
-  success,
-  status,
-  blankLine,
+  header as rawHeader,
+  info as rawInfo,
+  warn as rawWarn,
+  error as rawError,
+  success as rawSuccess,
+  status as rawStatus,
+  blankLine as rawBlankLine,
   StreamDisplay,
 } from '../../../shared/ui/index.js';
+import { TaskPrefixWriter } from '../../../shared/ui/TaskPrefixWriter.js';
 import {
   generateSessionId,
   createSessionLog,
@@ -62,6 +63,80 @@ import { installSigIntHandler } from './sigintHandler.js';
 
 const log = createLogger('piece');
 
+/**
+ * Output facade — routes through TaskPrefixWriter when task prefix is active,
+ * or falls through to the raw module functions for single-task execution.
+ */
+interface OutputFns {
+  header: (title: string) => void;
+  info: (message: string) => void;
+  warn: (message: string) => void;
+  error: (message: string) => void;
+  success: (message: string) => void;
+  status: (label: string, value: string, color?: 'green' | 'yellow' | 'red') => void;
+  blankLine: () => void;
+  logLine: (text: string) => void;
+}
+
+function createOutputFns(prefixWriter: TaskPrefixWriter | undefined): OutputFns {
+  if (!prefixWriter) {
+    return {
+      header: rawHeader,
+      info: rawInfo,
+      warn: rawWarn,
+      error: rawError,
+      success: rawSuccess,
+      status: rawStatus,
+      blankLine: rawBlankLine,
+      logLine: (text: string) => console.log(text),
+    };
+  }
+  return {
+    header: (title: string) => prefixWriter.writeLine(`=== ${title} ===`),
+    info: (message: string) => prefixWriter.writeLine(`[INFO] ${message}`),
+    warn: (message: string) => prefixWriter.writeLine(`[WARN] ${message}`),
+    error: (message: string) => prefixWriter.writeLine(`[ERROR] ${message}`),
+    success: (message: string) => prefixWriter.writeLine(message),
+    status: (label: string, value: string) => prefixWriter.writeLine(`${label}: ${value}`),
+    blankLine: () => prefixWriter.writeLine(''),
+    logLine: (text: string) => prefixWriter.writeLine(text),
+  };
+}
+
+/**
+ * Create a stream handler that routes all stream events through TaskPrefixWriter.
+ * Text and tool_output are line-buffered; block events are output per-line with prefix.
+ */
+function createPrefixedStreamHandler(
+  writer: TaskPrefixWriter,
+): (event: Parameters<ReturnType<StreamDisplay['createHandler']>>[0]) => void {
+  return (event) => {
+    switch (event.type) {
+      case 'text':
+        writer.writeChunk(event.data.text);
+        break;
+      case 'tool_use':
+        writer.writeLine(`[tool] ${event.data.tool}`);
+        break;
+      case 'tool_result': {
+        const label = event.data.isError ? '✗' : '✓';
+        writer.writeLine(`  ${label} ${event.data.content}`);
+        break;
+      }
+      case 'tool_output':
+        writer.writeChunk(event.data.output);
+        break;
+      case 'thinking':
+        writer.writeChunk(event.data.thinking);
+        break;
+      case 'init':
+      case 'result':
+      case 'error':
+        break;
+    }
+  };
+}
+
 /**
  * Truncate string to maximum length
  */
@@ -107,10 +182,16 @@ export async function executePiece(
   // projectCwd is where .takt/ lives (project root, not the clone)
   const projectCwd = options.projectCwd;
 
+  // When taskPrefix is set (parallel execution), route all output through TaskPrefixWriter
+  const prefixWriter = options.taskPrefix
+    ? new TaskPrefixWriter({ taskName: options.taskPrefix, colorIndex: options.taskColorIndex ?? 0 })
+    : undefined;
+  const out = createOutputFns(prefixWriter);
+
   // Always continue from previous sessions (use /clear to reset)
   log.debug('Continuing session (use /clear to reset)');
 
-  header(`${headerPrefix} ${pieceConfig.name}`);
+  out.header(`${headerPrefix} ${pieceConfig.name}`);
 
   const pieceSessionId = generateSessionId();
   let sessionLog = createSessionLog(task, projectCwd, pieceConfig.name);
@@ -139,10 +220,12 @@ export async function executePiece(
   // Track current display for streaming
   const displayRef: { current: StreamDisplay | null } = { current: null };
 
-  // Create stream handler that delegates to UI display
-  const streamHandler = (
-    event: Parameters<ReturnType<StreamDisplay['createHandler']>>[0]
-  ): void => {
+  // Create stream handler — when prefixWriter is active, use it for line-buffered
+  // output to prevent mid-line interleaving between concurrent tasks.
+  // When not in parallel mode, delegate to StreamDisplay as before.
+  const streamHandler = prefixWriter
+    ? createPrefixedStreamHandler(prefixWriter)
+    : (event: Parameters<ReturnType<StreamDisplay['createHandler']>>[0]): void => {
         if (!displayRef.current) return;
         if (event.type === 'result') return;
         displayRef.current.createHandler()(event);
@@ -180,14 +263,14 @@ export async function executePiece(
       displayRef.current = null;
     }
 
-    blankLine();
-    warn(
+    out.blankLine();
+    out.warn(
       getLabel('piece.iterationLimit.maxReached', undefined, {
         currentIteration: String(request.currentIteration),
         maxIterations: String(request.maxIterations),
       })
     );
-    info(getLabel('piece.iterationLimit.currentMovement', undefined, { currentMovement: request.currentMovement }));
+    out.info(getLabel('piece.iterationLimit.currentMovement', undefined, { currentMovement: request.currentMovement }));
 
     if (shouldNotify) {
       playWarningSound();
@@ -218,7 +301,7 @@ export async function executePiece(
         return additionalIterations;
       }
 
-      warn(getLabel('piece.iterationLimit.invalidInput'));
+      out.warn(getLabel('piece.iterationLimit.invalidInput'));
     }
   };
 
@@ -228,8 +311,8 @@ export async function executePiece(
           displayRef.current.flush();
           displayRef.current = null;
         }
-        blankLine();
-        info(request.prompt.trim());
+        out.blankLine();
+        out.info(request.prompt.trim());
         const input = await promptInput(getLabel('piece.iterationLimit.userInputPrompt'));
         return input && input.trim() ? input.trim() : null;
       }
@@ -311,7 +394,7 @@ export async function executePiece(
   engine.on('movement:start', (step, iteration, instruction) => {
     log.debug('Movement starting', { step: step.name, persona: step.personaDisplayName, iteration });
     currentIteration = iteration;
-    info(`[${iteration}/${pieceConfig.maxIterations}] ${step.name} (${step.personaDisplayName})`);
+    out.info(`[${iteration}/${pieceConfig.maxIterations}] ${step.name} (${step.personaDisplayName})`);
 
     // Log prompt content for debugging
     if (instruction) {
@@ -322,15 +405,18 @@ export async function executePiece(
     const movementIndex = pieceConfig.movements.findIndex((m) => m.name === step.name);
     const totalMovements = pieceConfig.movements.length;
 
+    // In parallel mode, StreamDisplay is not used (prefixWriter handles output).
+    // In single mode, StreamDisplay renders stream events directly.
+    if (!prefixWriter) {
       const quiet = isQuietMode();
-    const prefix = options.taskPrefix;
-    const agentLabel = prefix ? `${prefix}:${step.personaDisplayName}` : step.personaDisplayName;
+      const agentLabel = step.personaDisplayName;
       displayRef.current = new StreamDisplay(agentLabel, quiet, {
         iteration,
         maxIterations: pieceConfig.maxIterations,
         movementIndex: movementIndex >= 0 ? movementIndex : 0,
         totalMovements,
       });
+    }
 
     // Write step_start record to NDJSON log
     const record: NdjsonStepStart = {
@@ -364,25 +450,26 @@ export async function executePiece(
       displayRef.current.flush();
       displayRef.current = null;
     }
-    blankLine();
+    prefixWriter?.flush();
+    out.blankLine();
 
     if (response.matchedRuleIndex != null && step.rules) {
       const rule = step.rules[response.matchedRuleIndex];
       if (rule) {
         const methodLabel = response.matchedRuleMethod ? ` (${response.matchedRuleMethod})` : '';
-        status('Status', `${rule.condition}${methodLabel}`);
+        out.status('Status', `${rule.condition}${methodLabel}`);
       } else {
-        status('Status', response.status);
+        out.status('Status', response.status);
       }
     } else {
-      status('Status', response.status);
+      out.status('Status', response.status);
     }
 
     if (response.error) {
-      error(`Error: ${response.error}`);
+      out.error(`Error: ${response.error}`);
     }
     if (response.sessionId) {
-      status('Session', response.sessionId);
+      out.status('Session', response.sessionId);
     }
 
     // Write step_complete record to NDJSON log
@@ -408,8 +495,8 @@ export async function executePiece(
 
   engine.on('movement:report', (_step, filePath, fileName) => {
     const content = readFileSync(filePath, 'utf-8');
-    console.log(`\n📄 Report: ${fileName}\n`);
-    console.log(content);
+    out.logLine(`\n📄 Report: ${fileName}\n`);
+    out.logLine(content);
   });
 
   engine.on('piece:complete', (state) => {
@@ -445,8 +532,8 @@ export async function executePiece(
       : '';
     const elapsedDisplay = elapsed ? `, ${elapsed}` : '';
 
-    success(`Piece completed (${state.iteration} iterations${elapsedDisplay})`);
-    info(`Session log: ${ndjsonLogPath}`);
+    out.success(`Piece completed (${state.iteration} iterations${elapsedDisplay})`);
+    out.info(`Session log: ${ndjsonLogPath}`);
     if (shouldNotify) {
       notifySuccess('TAKT', getLabel('piece.notifyComplete', undefined, { iteration: String(state.iteration) }));
     }
@@ -459,6 +546,7 @@ export async function executePiece(
       displayRef.current.flush();
       displayRef.current = null;
     }
+    prefixWriter?.flush();
     abortReason = reason;
     sessionLog = finalizeSessionLog(sessionLog, 'aborted');
 
@@ -492,8 +580,8 @@ export async function executePiece(
       : '';
     const elapsedDisplay = elapsed ? ` (${elapsed})` : '';
 
-    error(`Piece aborted after ${state.iteration} iterations${elapsedDisplay}: ${reason}`);
-    info(`Session log: ${ndjsonLogPath}`);
+    out.error(`Piece aborted after ${state.iteration} iterations${elapsedDisplay}: ${reason}`);
+    out.info(`Session log: ${ndjsonLogPath}`);
     if (shouldNotify) {
       notifyError('TAKT', getLabel('piece.notifyAbort', undefined, { reason }));
     }
@@ -536,6 +624,7 @@ export async function executePiece(
       reason: abortReason,
     };
   } finally {
+    prefixWriter?.flush();
     sigintCleanup?.();
     if (onAbortSignal && options.abortSignal) {
       options.abortSignal.removeEventListener('abort', onAbortSignal);

src/features/tasks/execute/taskExecution.ts

@@ -52,7 +52,7 @@ function resolveTaskIssue(issueNumber: number | undefined): ReturnType<typeof fe
  * Execute a single task with piece.
  */
 export async function executeTask(options: ExecuteTaskOptions): Promise<boolean> {
-  const { task, cwd, pieceIdentifier, projectCwd, agentOverrides, interactiveUserInput, interactiveMetadata, startMovement, retryNote, abortSignal, taskPrefix } = options;
+  const { task, cwd, pieceIdentifier, projectCwd, agentOverrides, interactiveUserInput, interactiveMetadata, startMovement, retryNote, abortSignal, taskPrefix, taskColorIndex } = options;
   const pieceConfig = loadPieceByIdentifier(pieceIdentifier, projectCwd);
 
   if (!pieceConfig) {
@@ -83,6 +83,7 @@ export async function executeTask(options: ExecuteTaskOptions): Promise<boolean>
     retryNote,
     abortSignal,
     taskPrefix,
+    taskColorIndex,
   });
   return result.success;
 }
@@ -101,7 +102,7 @@ export async function executeAndCompleteTask(
   cwd: string,
   pieceName: string,
   options?: TaskExecutionOptions,
-  parallelOptions?: { abortSignal?: AbortSignal; taskPrefix?: string },
+  parallelOptions?: { abortSignal?: AbortSignal; taskPrefix?: string; taskColorIndex?: number },
 ): Promise<boolean> {
   const startedAt = new Date().toISOString();
   const executionLog: string[] = [];
@@ -120,6 +121,7 @@ export async function executeAndCompleteTask(
       retryNote,
       abortSignal: parallelOptions?.abortSignal,
       taskPrefix: parallelOptions?.taskPrefix,
+      taskColorIndex: parallelOptions?.taskColorIndex,
     });
     const completedAt = new Date().toISOString();
 

src/features/tasks/execute/types.ts

@@ -42,6 +42,8 @@ export interface PieceExecutionOptions {
   abortSignal?: AbortSignal;
   /** Task name prefix for parallel execution output (e.g. "[task-name] output...") */
   taskPrefix?: string;
+  /** Color index for task prefix (cycled mod 4 across concurrent tasks) */
+  taskColorIndex?: number;
 }
 
 export interface TaskExecutionOptions {
@@ -72,6 +74,8 @@ export interface ExecuteTaskOptions {
   abortSignal?: AbortSignal;
   /** Task name prefix for parallel execution output (e.g. "[task-name] output...") */
   taskPrefix?: string;
+  /** Color index for task prefix (cycled mod 4 across concurrent tasks) */
+  taskColorIndex?: number;
 }
 
 export interface PipelineExecutionOptions {

src/shared/ui/TaskPrefixWriter.ts

@@ -0,0 +1,96 @@
+/**
+ * Line-buffered, prefixed writer for task-level parallel execution.
+ *
+ * When multiple tasks run concurrently (takt run --concurrency N), each task's
+ * output must be identifiable and line-aligned to prevent mid-line interleaving.
+ * This class wraps process.stdout.write with line buffering and a colored
+ * `[taskName]` prefix on every non-empty line.
+ *
+ * Design mirrors ParallelLogger (movement-level) but targets task-level output:
+ * - Regular log lines (info, header, status) get the prefix
+ * - Stream output gets line-buffered then prefixed
+ * - Empty lines are passed through without prefix
+ */
+
+import { stripAnsi } from '../utils/text.js';
+
+/** ANSI color codes for task prefixes (cycled by task index) */
+const TASK_COLORS = ['\x1b[36m', '\x1b[33m', '\x1b[35m', '\x1b[32m'] as const;
+const RESET = '\x1b[0m';
+
+export interface TaskPrefixWriterOptions {
+  /** Task name used in the prefix */
+  taskName: string;
+  /** Color index for the prefix (cycled mod 4) */
+  colorIndex: number;
+  /** Override process.stdout.write for testing */
+  writeFn?: (text: string) => void;
+}
+
+/**
+ * Prefixed line writer for a single parallel task.
+ *
+ * All output goes through `writeLine` (complete lines) or `writeChunk`
+ * (buffered partial lines). The prefix `[taskName]` is prepended to every
+ * non-empty output line.
+ */
+export class TaskPrefixWriter {
+  private readonly prefix: string;
+  private readonly writeFn: (text: string) => void;
+  private lineBuffer = '';
+
+  constructor(options: TaskPrefixWriterOptions) {
+    const color = TASK_COLORS[options.colorIndex % TASK_COLORS.length];
+    this.prefix = `${color}[${options.taskName}]${RESET} `;
+    this.writeFn = options.writeFn ?? ((text: string) => process.stdout.write(text));
+  }
+
+  /**
+   * Write a complete line with prefix.
+   * Multi-line text is split and each non-empty line gets the prefix.
+   */
+  writeLine(text: string): void {
+    const cleaned = stripAnsi(text);
+    const lines = cleaned.split('\n');
+
+    for (const line of lines) {
+      if (line === '') {
+        this.writeFn('\n');
+      } else {
+        this.writeFn(`${this.prefix}${line}\n`);
+      }
+    }
+  }
+
+  /**
+   * Write a chunk of streaming text with line buffering.
+   * Partial lines are buffered until a newline arrives, then output with prefix.
+   */
+  writeChunk(text: string): void {
+    const cleaned = stripAnsi(text);
+    const combined = this.lineBuffer + cleaned;
+    const parts = combined.split('\n');
+
+    const remainder = parts.pop() ?? '';
+    this.lineBuffer = remainder;
+
+    for (const line of parts) {
+      if (line === '') {
+        this.writeFn('\n');
+      } else {
+        this.writeFn(`${this.prefix}${line}\n`);
+      }
+    }
+  }
+
+  /**
+   * Flush any remaining buffered content.
+   */
+  flush(): void {
+    if (this.lineBuffer !== '') {
+      this.writeFn(`${this.prefix}${this.lineBuffer}\n`);
+      this.lineBuffer = '';
+    }
+  }
+
+}

src/shared/ui/index.ts

@@ -29,3 +29,5 @@ export {
 export { Spinner } from './Spinner.js';
 
 export { StreamDisplay, type ProgressInfo } from './StreamDisplay.js';
+
+export { TaskPrefixWriter } from './TaskPrefixWriter.js';