diff --git a/src/__tests__/parallel-output-interleaving.test.ts b/src/__tests__/parallel-output-interleaving.test.ts
new file mode 100644
index 0000000..40d3844
--- /dev/null
+++ b/src/__tests__/parallel-output-interleaving.test.ts
@@ -0,0 +1,76 @@
+/**
+ * Reproduction tests for stdout/stderr interleaving around streamed text output.
+ */
+
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { ParallelLogger } from '../core/piece/index.js';
+import type { StreamEvent } from '../core/piece/index.js';
+
+describe('stream output interleaving', () => {
+  let output: string[];
+
+  beforeEach(() => {
+    vi.useRealTimers();
+    output = [];
+  });
+
+  it('reproduces sentence fragments when timed flush + worker-pool log interleave', async () => {
+    vi.useFakeTimers();
+
+    const logger = new ParallelLogger({
+      subMovementNames: ['testing-review'],
+      writeFn: (text) => output.push(`STDOUT:${text}`),
+      flushIntervalMs: 10,
+      minTimedFlushChars: 24,
+      maxTimedBufferMs: 200,
+    });
+
+    const handler = logger.createStreamHandler('testing-review', 0);
+
+    handler({
+      type: 'text',
+      data: { text: '[#429][reviewers][testing-review](2/30)(1) should include' },
+    } as StreamEvent);
+
+    await vi.advanceTimersByTimeAsync(20);
+
+    output.push('STDERR:[04:47:20.401] [DEBUG] [worker-pool] poll_tick\n');
+
+    handler({
+      type: 'text',
+      data: { text: ' ag' },
+    } as StreamEvent);
+
+    await vi.advanceTimersByTimeAsync(20);
+
+    output.push('STDERR:[04:47:20.401] [DEBUG] [worker-pool] no_new_tasks\n');
+
+    handler({
+      type: 'text',
+      data: { text: 'ent Error in throw message when provided' },
+    } as StreamEvent);
+
+    logger.flush();
+
+    expect(output.length).toBeGreaterThanOrEqual(2);
+
+    const rendered = output.join('');
+    expect(rendered).toContain('should');
+    expect(rendered).toContain('include');
+    expect(rendered).toContain('ent Error in throw message when provided');
+    expect(rendered).toContain('[worker-pool] poll_tick');
+    expect(rendered).toContain('no_new_tasks');
+
+    const stdoutText = output.filter((line) => line.startsWith('STDOUT:')).join('');
+    const stdoutChunks = output
+      .filter((line) => line.startsWith('STDOUT:'))
+      .map((line) => line.replace(/^STDOUT:/, '').replace(/^\u001b\[[0-9;]*m\[[^\]]+\]\u001b\[0m /, ''));
+
+    // この再現では、単独断片 " ag" がSTDOUTチャンクとして流れないことを確認する
+    expect(stdoutText).toContain('ent Error in throw message when provided');
+    expect(stdoutChunks).not.toContain(' ag');
+    expect(stdoutChunks.join('')).toContain('include agent Error in throw message when provided');
+
+    vi.useRealTimers();
+  });
+});
diff --git a/src/__tests__/stream-buffer.test.ts b/src/__tests__/stream-buffer.test.ts
index ca64966..f4459df 100644
--- a/src/__tests__/stream-buffer.test.ts
+++ b/src/__tests__/stream-buffer.test.ts
@@ -51,6 +51,31 @@ describe('LineTimeSliceBuffer', () => {
     expect(buffer.flushAll()).toEqual([{ key: 'a', text: 'buffer' }]);
   });
 
+  it.each([1, 10, 24, 30])(
+    '先頭の境界文字だけでは切らず、minTimedFlushChars=%s でも同一語を分割しない',
+    async (minTimedFlushChars) => {
+      vi.useFakeTimers();
+      const flushed: Array<{ key: string; text: string }> = [];
+      const buffer = new LineTimeSliceBuffer({
+        flushIntervalMs: 50,
+        minTimedFlushChars,
+        maxTimedBufferMs: 120,
+        onTimedFlush: (key, text) => flushed.push({ key, text }),
+      });
+
+      buffer.push('a', ' ag');
+      await vi.advanceTimersByTimeAsync(50);
+      expect(flushed).toHaveLength(0);
+
+      buffer.push('a', 'ent');
+      await vi.advanceTimersByTimeAsync(100);
+
+      expect(flushed).toHaveLength(1);
+      expect(flushed[0]).toEqual({ key: 'a', text: ' agent' });
+      expect(buffer.flushAll()).toEqual([]);
+    },
+  );
+
   it('境界がない文字列は maxTimedBufferMs 経過後に強制flushする', async () => {
     vi.useFakeTimers();
     const flushed: Array<{ key: string; text: string }> = [];
diff --git a/src/core/piece/engine/stream-buffer.ts b/src/core/piece/engine/stream-buffer.ts
index d40c36d..1f0d12c 100644
--- a/src/core/piece/engine/stream-buffer.ts
+++ b/src/core/piece/engine/stream-buffer.ts
@@ -100,9 +100,10 @@ export class LineTimeSliceBuffer {
       return undefined;
     }
 
-    const boundaryIndex = this.findBoundaryIndex(buffer);
-    const flushIndex = boundaryIndex > 0
-      ? boundaryIndex
+    const minBoundaryIndex = Math.max(this.minTimedFlushChars - 1, 1);
+    const boundaryIndex = this.findBoundaryIndex(buffer, minBoundaryIndex);
+    const flushIndex = boundaryIndex >= 0
+      ? boundaryIndex + 1
       : buffer.length;
 
     const flushText = buffer.slice(0, flushIndex);
@@ -118,15 +119,16 @@ export class LineTimeSliceBuffer {
     return flushText;
   }
 
-  private findBoundaryIndex(text: string): number {
+  private findBoundaryIndex(text: string, minIndex: number): number {
     let lastIndex = -1;
-    for (let i = 0; i < text.length; i += 1) {
+    const start = Math.min(Math.max(minIndex, 0), text.length - 1);
+    for (let i = start; i < text.length; i += 1) {
       const ch = text.charAt(i);
       if (this.isBoundary(ch)) {
         lastIndex = i;
       }
     }
-    return lastIndex + 1;
+    return lastIndex;
   }
 
   private isBoundary(ch: string): boolean {