diff --git a/packages/core/src/evaluation/providers/copilot-cli.ts b/packages/core/src/evaluation/providers/copilot-cli.ts
index 31684af05..bc70b704d 100644
--- a/packages/core/src/evaluation/providers/copilot-cli.ts
+++ b/packages/core/src/evaluation/providers/copilot-cli.ts
@@ -423,6 +423,7 @@ export class CopilotCliProvider implements Provider {
           attempt: request.attempt,
           format: this.config.logFormat ?? 'summary',
           headerLabel: 'Copilot CLI (ACP)',
+          chunkExtractor: extractAcpChunk,
         },
         summarizeAcpEvent,
       );
@@ -501,6 +502,31 @@ Fix options:
   - In .agentv/targets.yaml: executable: \${{ COPILOT_EXE }}`;
 }
 
+/**
+ * Extracts bufferable text from ACP streaming events.
+ *
+ * Return values control CopilotStreamLogger buffering:
+ *   string    — accumulate this text into the pending buffer
+ *   null      — reset (discard) the pending buffer without emitting it
+ *   undefined — not a chunk event; process normally
+ *
+ * Copilot ACP sends agent_message_chunk events in two passes:
+ *   1. A streaming preview batch (before extended thinking)
+ *   2. agent_thought_chunk events (extended reasoning)
+ *   3. A final response batch (after extended thinking)
+ *
+ * Returning null for agent_thought_chunk discards the preview batch so that
+ * only the final post-thinking response is emitted as [assistant_message].
+ */
+function extractAcpChunk(eventType: string, data: unknown): string | null | undefined {
+  if (eventType === 'agent_thought_chunk') return null;
+  if (eventType !== 'agent_message_chunk') return undefined;
+  if (!data || typeof data !== 'object') return undefined;
+  const d = data as Record<string, unknown>;
+  const content = d.content as Record<string, unknown> | undefined;
+  return content?.type === 'text' && typeof content.text === 'string' ? content.text : undefined;
+}
+
 function summarizeAcpEvent(eventType: string, data: unknown): string | undefined {
   if (!data || typeof data !== 'object') {
     return eventType;
diff --git a/packages/core/src/evaluation/providers/copilot-sdk.ts b/packages/core/src/evaluation/providers/copilot-sdk.ts
index b18a10a65..ac903f8e1 100644
--- a/packages/core/src/evaluation/providers/copilot-sdk.ts
+++ b/packages/core/src/evaluation/providers/copilot-sdk.ts
@@ -378,6 +378,7 @@ export class CopilotSdkProvider implements Provider {
           attempt: request.attempt,
           format: this.config.logFormat ?? 'summary',
           headerLabel: 'Copilot SDK',
+          chunkExtractor: extractSdkChunk,
         },
         summarizeSdkEvent,
       );
@@ -426,6 +427,19 @@ function normalizeByokBaseUrl(baseUrl: string, type: string): string {
   return trimmed;
 }
 
+/**
+ * Extracts bufferable text from SDK assistant.message_delta events.
+ * Returning a string causes the logger to accumulate the text rather than
+ * emit a line per delta. A single [assistant_message] line is written once
+ * all deltas for a turn have arrived (on the next non-chunk event or close).
+ */
+function extractSdkChunk(eventType: string, data: unknown): string | undefined {
+  if (eventType !== 'assistant.message_delta') return undefined;
+  if (!data || typeof data !== 'object') return undefined;
+  const d = data as Record<string, unknown>;
+  return typeof d.deltaContent === 'string' ? d.deltaContent : undefined;
+}
+
 function summarizeSdkEvent(eventType: string, data: unknown): string | undefined {
   if (!data || typeof data !== 'object') {
     return eventType;
diff --git a/packages/core/src/evaluation/providers/copilot-utils.ts b/packages/core/src/evaluation/providers/copilot-utils.ts
index 6a184de79..43bb8ae55 100644
--- a/packages/core/src/evaluation/providers/copilot-utils.ts
+++ b/packages/core/src/evaluation/providers/copilot-utils.ts
@@ -253,6 +253,27 @@ export interface StreamLoggerOptions {
   readonly attempt?: number;
   readonly format: 'summary' | 'json';
   readonly headerLabel: string;
+  /**
+   * Optional extractor for streaming text chunk events.
+   *
+   * When provided, the return value controls how each event is handled:
+   *   - `string`    — buffer this text; flush as `[assistant_message]` on the next
+   *                   non-chunk event or `close()`.
+   *   - `null`      — discard (reset) the accumulated buffer without emitting it.
+   *                   Use this for events that signal a new streaming pass is starting,
+   *                   e.g. `agent_thought_chunk` in Copilot ACP, which arrives between
+   *                   a streaming preview batch and the final response batch.
+   *   - `undefined` — not a chunk event; process normally (flush buffer first, then
+   *                   call `summarize` and write the line).
+   *
+   * Example (Copilot CLI ACP):
+   *   chunkExtractor: (type, data) => {
+   *     if (type === 'agent_thought_chunk') return null;   // reset pre-thinking buffer
+   *     if (type !== 'agent_message_chunk') return undefined;
+   *     return (data as any)?.content?.text ?? undefined;
+   *   }
+   */
+  readonly chunkExtractor?: (eventType: string, data: unknown) => string | null | undefined;
 }
 
 export class CopilotStreamLogger {
@@ -261,15 +282,19 @@ export class CopilotStreamLogger {
   private readonly startedAt = Date.now();
   private readonly format: 'summary' | 'json';
   private readonly summarize: (eventType: string, data: unknown) => string | undefined;
+  private readonly chunkExtractor?: (eventType: string, data: unknown) => string | null | undefined;
+  private pendingText = '';
 
   private constructor(
     filePath: string,
     format: 'summary' | 'json',
     summarize: (eventType: string, data: unknown) => string | undefined,
+    chunkExtractor?: (eventType: string, data: unknown) => string | null | undefined,
   ) {
     this.filePath = filePath;
     this.format = format;
     this.summarize = summarize;
+    this.chunkExtractor = chunkExtractor;
     this.stream = createWriteStream(filePath, { flags: 'a' });
   }
 
@@ -277,7 +302,12 @@ export class CopilotStreamLogger {
     options: StreamLoggerOptions,
     summarize: (eventType: string, data: unknown) => string | undefined,
   ): Promise<CopilotStreamLogger> {
-    const logger = new CopilotStreamLogger(options.filePath, options.format, summarize);
+    const logger = new CopilotStreamLogger(
+      options.filePath,
+      options.format,
+      summarize,
+      options.chunkExtractor,
+    );
     const header = [
       `# ${options.headerLabel} stream log`,
       `# target: ${options.targetName}`,
@@ -293,18 +323,49 @@ export class CopilotStreamLogger {
   }
 
   handleEvent(eventType: string, data: unknown): void {
-    const elapsed = formatElapsed(this.startedAt);
     if (this.format === 'json') {
+      const elapsed = formatElapsed(this.startedAt);
       this.stream.write(`${JSON.stringify({ time: elapsed, event: eventType, data })}\n`);
-    } else {
-      const summary = this.summarize(eventType, data);
-      if (summary) {
-        this.stream.write(`[+${elapsed}] [${eventType}] ${summary}\n`);
+      return;
+    }
+
+    // In summary mode, buffer chunk events and emit a single consolidated line.
+    if (this.chunkExtractor) {
+      const chunkText = this.chunkExtractor(eventType, data);
+      if (chunkText === null) {
+        // Reset signal: discard the accumulated buffer without emitting.
+        // Used for events like agent_thought_chunk that arrive between a
+        // streaming preview batch and the final response batch in Copilot ACP —
+        // the preview text is stale; the real message follows after thinking.
+        this.pendingText = '';
+        return;
+      }
+      if (chunkText !== undefined) {
+        this.pendingText += chunkText;
+        return;
       }
+      // Non-chunk event: flush any accumulated text first.
+      this.flushPendingText();
+    }
+
+    const elapsed = formatElapsed(this.startedAt);
+    const summary = this.summarize(eventType, data);
+    if (summary) {
+      this.stream.write(`[+${elapsed}] [${eventType}] ${summary}\n`);
     }
   }
 
+  private flushPendingText(): void {
+    if (!this.pendingText) return;
+    const elapsed = formatElapsed(this.startedAt);
+    this.stream.write(`[+${elapsed}] [assistant_message] ${this.pendingText}\n`);
+    this.pendingText = '';
+  }
+
   async close(): Promise<void> {
+    if (this.format !== 'json') {
+      this.flushPendingText();
+    }
     await new Promise<void>((resolve, reject) => {
       this.stream.once('error', reject);
       this.stream.end(() => resolve());
diff --git a/packages/core/test/evaluation/providers/copilot-stream-logger.test.ts b/packages/core/test/evaluation/providers/copilot-stream-logger.test.ts
new file mode 100644
index 000000000..926e57d61
--- /dev/null
+++ b/packages/core/test/evaluation/providers/copilot-stream-logger.test.ts
@@ -0,0 +1,185 @@
+import { mkdtemp, readFile, rm } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import path from 'node:path';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+
+import { CopilotStreamLogger } from '../../../src/evaluation/providers/copilot-utils.js';
+
+const noopSummarize = (_type: string, _data: unknown): string | undefined => undefined;
+
+describe('CopilotStreamLogger', () => {
+  let tempDir: string;
+
+  beforeEach(async () => {
+    tempDir = await mkdtemp(path.join(tmpdir(), 'copilot-stream-logger-'));
+  });
+
+  afterEach(async () => {
+    await rm(tempDir, { recursive: true, force: true });
+  });
+
+  it('writes summary events as separate lines', async () => {
+    const filePath = path.join(tempDir, 'test.log');
+    const summarize = (type: string, _data: unknown) =>
+      type === 'tool_call' ? 'read_file' : undefined;
+
+    const logger = await CopilotStreamLogger.create(
+      { filePath, targetName: 'test', format: 'summary', headerLabel: 'Test' },
+      summarize,
+    );
+    logger.handleEvent('tool_call', {});
+    logger.handleEvent('tool_call', {});
+    await logger.close();
+
+    const content = await readFile(filePath, 'utf8');
+    const lines = content.split('\n').filter((l) => l.includes('[tool_call]'));
+    expect(lines).toHaveLength(2);
+    expect(lines[0]).toMatch(/\[tool_call\] read_file/);
+  });
+
+  it('buffers chunk events and flushes as single [assistant_message] line on non-chunk event', async () => {
+    const filePath = path.join(tempDir, 'test.log');
+    const summarize = (type: string, _data: unknown) =>
+      type === 'tool_call' ? 'read_file' : undefined;
+    const chunkExtractor = (type: string, data: unknown): string | null | undefined => {
+      if (type !== 'agent_message_chunk') return undefined;
+      const d = data as Record<string, unknown>;
+      const content = d?.content as Record<string, unknown> | undefined;
+      return content?.type === 'text' && typeof content.text === 'string'
+        ? content.text
+        : undefined;
+    };
+
+    const logger = await CopilotStreamLogger.create(
+      { filePath, targetName: 'test', format: 'summary', headerLabel: 'Test', chunkExtractor },
+      summarize,
+    );
+
+    // Three chunks — should NOT produce three log lines
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: 'Hello' } });
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: ' world' } });
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: '!' } });
+    // Non-chunk event triggers flush
+    logger.handleEvent('tool_call', {});
+    await logger.close();
+
+    const content = await readFile(filePath, 'utf8');
+    const lines = content.split('\n').filter((l) => l.trim());
+
+    // No raw chunk lines
+    expect(lines.some((l) => l.includes('[agent_message_chunk]'))).toBe(false);
+    // One consolidated assistant_message line with full text
+    const msgLine = lines.find((l) => l.includes('[assistant_message]'));
+    expect(msgLine).toBeDefined();
+    expect(msgLine).toMatch(/Hello world!/);
+    // tool_call still emitted
+    expect(lines.some((l) => l.includes('[tool_call]'))).toBe(true);
+  });
+
+  it('flushes remaining buffered text on close', async () => {
+    const filePath = path.join(tempDir, 'test.log');
+    const chunkExtractor = (type: string, data: unknown): string | null | undefined => {
+      if (type !== 'agent_message_chunk') return undefined;
+      const d = data as Record<string, unknown>;
+      const content = d?.content as Record<string, unknown> | undefined;
+      return content?.type === 'text' && typeof content.text === 'string'
+        ? content.text
+        : undefined;
+    };
+
+    const logger = await CopilotStreamLogger.create(
+      { filePath, targetName: 'test', format: 'summary', headerLabel: 'Test', chunkExtractor },
+      noopSummarize,
+    );
+
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: 'Final answer' } });
+    // close() without any subsequent non-chunk event
+    await logger.close();
+
+    const content = await readFile(filePath, 'utf8');
+    expect(content).toMatch(/\[assistant_message\] Final answer/);
+  });
+
+  it('does not buffer in json format (keeps per-event for full fidelity)', async () => {
+    const filePath = path.join(tempDir, 'test.log');
+    const chunkExtractor = (type: string, data: unknown): string | null | undefined => {
+      if (type !== 'agent_message_chunk') return undefined;
+      const d = data as Record<string, unknown>;
+      const content = d?.content as Record<string, unknown> | undefined;
+      return content?.type === 'text' && typeof content.text === 'string'
+        ? content.text
+        : undefined;
+    };
+
+    const logger = await CopilotStreamLogger.create(
+      { filePath, targetName: 'test', format: 'json', headerLabel: 'Test', chunkExtractor },
+      noopSummarize,
+    );
+
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: 'chunk1' } });
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: 'chunk2' } });
+    await logger.close();
+
+    const content = await readFile(filePath, 'utf8');
+    const jsonLines = content
+      .split('\n')
+      .filter((l) => l.trim().startsWith('{'))
+      .map((l) => JSON.parse(l));
+    // Both chunks emitted individually as JSON
+    expect(jsonLines.filter((e) => e.event === 'agent_message_chunk')).toHaveLength(2);
+  });
+
+  it('handles chunk events with no extractable text gracefully', async () => {
+    const filePath = path.join(tempDir, 'test.log');
+    const chunkExtractor = (type: string, _data: unknown): string | null | undefined =>
+      type === 'agent_message_chunk' ? undefined : undefined;
+
+    const logger = await CopilotStreamLogger.create(
+      { filePath, targetName: 'test', format: 'summary', headerLabel: 'Test', chunkExtractor },
+      noopSummarize,
+    );
+
+    // Chunks with no extractable text are silently skipped (chunkExtractor returns undefined
+    // meaning "not a chunk" — treated as non-chunk events, summarize returns undefined, no output)
+    logger.handleEvent('agent_message_chunk', { content: { type: 'image' } });
+    await logger.close();
+
+    const content = await readFile(filePath, 'utf8');
+    expect(content).not.toMatch(/\[assistant_message\]/);
+  });
+
+  it('null return from chunkExtractor resets buffer without emitting (handles pre-thinking streaming)', async () => {
+    const filePath = path.join(tempDir, 'test.log');
+    // Simulates Copilot ACP: chunks → thought_chunks (reset) → chunks (final)
+    const chunkExtractor = (type: string, data: unknown): string | null | undefined => {
+      if (type === 'agent_thought_chunk') return null;
+      if (type !== 'agent_message_chunk') return undefined;
+      const d = data as Record<string, unknown>;
+      const content = d?.content as Record<string, unknown> | undefined;
+      return content?.type === 'text' && typeof content.text === 'string'
+        ? content.text
+        : undefined;
+    };
+
+    const logger = await CopilotStreamLogger.create(
+      { filePath, targetName: 'test', format: 'summary', headerLabel: 'Test', chunkExtractor },
+      noopSummarize,
+    );
+
+    // First pass: streaming preview (should be discarded)
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: 'Hi' } });
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: ' there.' } });
+    // Extended thinking: resets the buffer
+    logger.handleEvent('agent_thought_chunk', {});
+    logger.handleEvent('agent_thought_chunk', {});
+    // Second pass: final response
+    logger.handleEvent('agent_message_chunk', { content: { type: 'text', text: 'Hi there.' } });
+    await logger.close();
+
+    const content = await readFile(filePath, 'utf8');
+    const msgLines = content.split('\n').filter((l) => l.includes('[assistant_message]'));
+    // Only one consolidated line — the final response, not the preview
+    expect(msgLines).toHaveLength(1);
+    expect(msgLines[0]).toMatch(/\[assistant_message\] Hi there\.$/);
+  });
+});