diff --git a/integration-tests/sdk-typescript/message-event-pairing.test.ts b/integration-tests/sdk-typescript/message-event-pairing.test.ts
index 32b81b21b..b439ec276 100644
--- a/integration-tests/sdk-typescript/message-event-pairing.test.ts
+++ b/integration-tests/sdk-typescript/message-event-pairing.test.ts
@@ -351,7 +351,7 @@ describe('Message Start/Stop Event Pairing (E2E)', () => {
       expect(event.type).toBe('message_stop');
     });
 
-    it('should have message_start and message_stop paired by message_id', async () => {
+    it('should have message_start and message_stop paired by count', async () => {
       const startEvents: SDKPartialAssistantMessage[] = [];
       const stopEvents: SDKPartialAssistantMessage[] = [];
 
@@ -379,22 +379,19 @@ describe('Message Start/Stop Event Pairing (E2E)', () => {
         await q.close();
       }
 
-      // Verify message_start and message_stop are paired (same count)
+      // Verify message_start and message_stop appear in pairs (same count)
       expect(startEvents.length).toBeGreaterThan(0);
       expect(stopEvents.length).toBe(startEvents.length);
 
-      // Verify each message_start has a corresponding message_stop with the same message_id
-      const startMessageIds = new Set(
-        startEvents.map((e) => (e.event as { message_id?: string }).message_id),
-      );
-      const stopMessageIds = new Set(
-        stopEvents.map((e) => (e.event as { message_id?: string }).message_id),
-      );
-
-      // Each message_stop should have the same message_id as a message_start
-      startMessageIds.forEach((messageId) => {
-        expect(stopMessageIds.has(messageId)).toBe(true);
-      });
+      // Verify message_start carries the message id via its nested message.id field
+      for (const e of startEvents) {
+        const event = e.event as {
+          type: 'message_start';
+          message: { id: string };
+        };
+        expect(typeof event.message.id).toBe('string');
+        expect(event.message.id.length).toBeGreaterThan(0);
+      }
     });
   });
 
@@ -437,4 +434,437 @@ describe('Message Start/Stop Event Pairing (E2E)', () => {
       }
     });
   });
+
+  describe('Content Block Event Pairing', () => {
+    it('should emit paired content_block_start and content_block_stop for each content block', async () => {
+      const contentBlockStartEvents: SDKPartialAssistantMessage[] = [];
+      const contentBlockStopEvents: SDKPartialAssistantMessage[] = [];
+
+      const q = query({
+        prompt: 'Say hello',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (isSDKPartialAssistantMessage(message)) {
+            if (message.event.type === 'content_block_start') {
+              contentBlockStartEvents.push(message);
+            } else if (message.event.type === 'content_block_stop') {
+              contentBlockStopEvents.push(message);
+            }
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      // Verify content_block_start and content_block_stop are paired
+      expect(contentBlockStartEvents.length).toBeGreaterThan(0);
+      expect(contentBlockStopEvents.length).toBe(
+        contentBlockStartEvents.length,
+      );
+    });
+
+    it('should emit content_block_start before content_block_stop', async () => {
+      const events: Array<{ type: string; index: number; timestamp: number }> =
+        [];
+
+      const q = query({
+        prompt: 'Say hello world',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (isSDKPartialAssistantMessage(message)) {
+            if (
+              message.event.type === 'content_block_start' ||
+              message.event.type === 'content_block_stop'
+            ) {
+              events.push({
+                type: message.event.type,
+                index: message.event.index,
+                timestamp: Date.now(),
+              });
+            }
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      // Verify events exist
+      expect(events.length).toBeGreaterThanOrEqual(2);
+
+      // Group events by index
+      const eventsByIndex = new Map<number, typeof events>();
+      for (const event of events) {
+        if (!eventsByIndex.has(event.index)) {
+          eventsByIndex.set(event.index, []);
+        }
+        eventsByIndex.get(event.index)!.push(event);
+      }
+
+      // For each index, verify content_block_start comes before content_block_stop
+      eventsByIndex.forEach((indexEvents) => {
+        const startIndex = indexEvents.findIndex(
+          (e) => e.type === 'content_block_start',
+        );
+        const stopIndex = indexEvents.findIndex(
+          (e) => e.type === 'content_block_stop',
+        );
+        expect(startIndex).toBeGreaterThanOrEqual(0);
+        expect(stopIndex).toBeGreaterThanOrEqual(0);
+        expect(startIndex).toBeLessThan(stopIndex);
+      });
+    });
+
+    it('should have correct content_block_start event structure', async () => {
+      const contentBlockStartEvents: SDKPartialAssistantMessage[] = [];
+
+      const q = query({
+        prompt: 'Say hello',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (
+            isSDKPartialAssistantMessage(message) &&
+            message.event.type === 'content_block_start'
+          ) {
+            contentBlockStartEvents.push(message);
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      expect(contentBlockStartEvents.length).toBeGreaterThan(0);
+
+      // Verify each content_block_start has correct structure
+      for (const message of contentBlockStartEvents) {
+        const event = message.event as {
+          type: 'content_block_start';
+          index: number;
+          content_block: unknown;
+        };
+        expect(event.type).toBe('content_block_start');
+        expect(event).toHaveProperty('index');
+        expect(typeof event.index).toBe('number');
+        expect(event.index).toBeGreaterThanOrEqual(0);
+        expect(event).toHaveProperty('content_block');
+        expect(event.content_block).toBeDefined();
+      }
+    });
+
+    it('should have correct content_block_stop event structure', async () => {
+      const contentBlockStopEvents: SDKPartialAssistantMessage[] = [];
+
+      const q = query({
+        prompt: 'Say hello',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (
+            isSDKPartialAssistantMessage(message) &&
+            message.event.type === 'content_block_stop'
+          ) {
+            contentBlockStopEvents.push(message);
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      expect(contentBlockStopEvents.length).toBeGreaterThan(0);
+
+      // Verify each content_block_stop has correct structure
+      for (const message of contentBlockStopEvents) {
+        const event = message.event as {
+          type: 'content_block_stop';
+          index: number;
+        };
+        expect(event.type).toBe('content_block_stop');
+        expect(event).toHaveProperty('index');
+        expect(typeof event.index).toBe('number');
+        expect(event.index).toBeGreaterThanOrEqual(0);
+      }
+    });
+
+    it('should have matching index for paired content_block_start and content_block_stop', async () => {
+      const startEvents: SDKPartialAssistantMessage[] = [];
+      const stopEvents: SDKPartialAssistantMessage[] = [];
+
+      const q = query({
+        prompt: 'Say hello world',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (isSDKPartialAssistantMessage(message)) {
+            if (message.event.type === 'content_block_start') {
+              startEvents.push(message);
+            } else if (message.event.type === 'content_block_stop') {
+              stopEvents.push(message);
+            }
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      // Verify events exist and are paired
+      expect(startEvents.length).toBeGreaterThan(0);
+      expect(stopEvents.length).toBe(startEvents.length);
+
+      // Extract indices from start and stop events
+      const startIndices = startEvents.map(
+        (e) => (e.event as { index: number }).index,
+      );
+      const stopIndices = stopEvents.map(
+        (e) => (e.event as { index: number }).index,
+      );
+
+      // Verify each start index has a matching stop index
+      expect(new Set(stopIndices)).toEqual(new Set(startIndices));
+
+      // Verify each index appears the same number of times in both start and stop events
+      const startIndexCounts = new Map<number, number>();
+      const stopIndexCounts = new Map<number, number>();
+
+      for (const idx of startIndices) {
+        startIndexCounts.set(idx, (startIndexCounts.get(idx) || 0) + 1);
+      }
+      for (const idx of stopIndices) {
+        stopIndexCounts.set(idx, (stopIndexCounts.get(idx) || 0) + 1);
+      }
+
+      startIndexCounts.forEach((count, idx) => {
+        expect(stopIndexCounts.get(idx)).toBe(count);
+      });
+    });
+
+    it('should follow correct event flow: content_block_start -> content_block_delta -> content_block_stop', async () => {
+      const events: Array<{
+        type: string;
+        index: number;
+        position: number;
+      }> = [];
+
+      const q = query({
+        prompt: 'Write a short story about a cat',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      let pos = 0;
+      try {
+        for await (const message of q) {
+          if (isSDKPartialAssistantMessage(message)) {
+            const eventType = message.event.type;
+            if (
+              eventType === 'content_block_start' ||
+              eventType === 'content_block_delta' ||
+              eventType === 'content_block_stop'
+            ) {
+              events.push({
+                type: eventType,
+                index: (message.event as { index: number }).index,
+                position: pos++,
+              });
+            }
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      expect(events.length).toBeGreaterThanOrEqual(2);
+
+      // Pair content_block_start/stop sequentially (not by index, since
+      // block-type transitions reset the blocks array and reuse index 0).
+      // Each start is matched with the next stop that follows it.
+      const starts = events.filter((e) => e.type === 'content_block_start');
+      const stops = events.filter((e) => e.type === 'content_block_stop');
+      expect(starts.length).toBe(stops.length);
+
+      for (let i = 0; i < starts.length; i++) {
+        const start = starts[i];
+        const stop = stops[i];
+
+        // start must come before the paired stop
+        expect(start.position).toBeLessThan(stop.position);
+
+        // All deltas between this pair must sit between start and stop
+        const deltas = events.filter(
+          (e) =>
+            e.type === 'content_block_delta' &&
+            e.position > start.position &&
+            e.position < stop.position,
+        );
+        for (const delta of deltas) {
+          expect(delta.position).toBeGreaterThan(start.position);
+          expect(delta.position).toBeLessThan(stop.position);
+        }
+      }
+    });
+
+    it('should have content_block_start after message_start and before message_stop', async () => {
+      const events: Array<{
+        type: string;
+        timestamp: number;
+      }> = [];
+
+      const q = query({
+        prompt: 'Say hello',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (isSDKPartialAssistantMessage(message)) {
+            const eventType = message.event.type;
+            if (
+              eventType === 'message_start' ||
+              eventType === 'message_stop' ||
+              eventType === 'content_block_start'
+            ) {
+              events.push({
+                type: eventType,
+                timestamp: Date.now(),
+              });
+            }
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      // Verify message_start exists
+      const messageStartIndex = events.findIndex(
+        (e) => e.type === 'message_start',
+      );
+      expect(messageStartIndex).toBeGreaterThanOrEqual(0);
+
+      // Verify message_stop exists
+      const messageStopIndex = events.findIndex(
+        (e) => e.type === 'message_stop',
+      );
+      expect(messageStopIndex).toBeGreaterThanOrEqual(0);
+
+      // Verify content_block_start exists
+      const firstContentBlockStartIndex = events.findIndex(
+        (e) => e.type === 'content_block_start',
+      );
+      expect(firstContentBlockStartIndex).toBeGreaterThanOrEqual(0);
+
+      // content_block_start should be after message_start
+      expect(firstContentBlockStartIndex).toBeGreaterThan(messageStartIndex);
+
+      // content_block_start should be before message_stop
+      expect(firstContentBlockStartIndex).toBeLessThan(messageStopIndex);
+    });
+
+    it('should have content_block_stop after message_start and before message_stop', async () => {
+      const events: Array<{
+        type: string;
+        timestamp: number;
+      }> = [];
+
+      const q = query({
+        prompt: 'Say hello',
+        options: {
+          ...SHARED_TEST_OPTIONS,
+          includePartialMessages: true,
+          cwd: testDir,
+          debug: false,
+        },
+      });
+
+      try {
+        for await (const message of q) {
+          if (isSDKPartialAssistantMessage(message)) {
+            const eventType = message.event.type;
+            if (
+              eventType === 'message_start' ||
+              eventType === 'message_stop' ||
+              eventType === 'content_block_stop'
+            ) {
+              events.push({
+                type: eventType,
+                timestamp: Date.now(),
+              });
+            }
+          }
+        }
+      } finally {
+        await q.close();
+      }
+
+      // Verify message_start exists
+      const messageStartIndex = events.findIndex(
+        (e) => e.type === 'message_start',
+      );
+      expect(messageStartIndex).toBeGreaterThanOrEqual(0);
+
+      // Verify message_stop exists
+      const messageStopIndex = events.findIndex(
+        (e) => e.type === 'message_stop',
+      );
+      expect(messageStopIndex).toBeGreaterThanOrEqual(0);
+
+      // Verify content_block_stop exists (use reverse find for ES compatibility)
+      const lastContentBlockStopIndex =
+        events
+          .map((e, i) => ({ ...e, originalIndex: i }))
+          .reverse()
+          .find((e) => e.type === 'content_block_stop')?.originalIndex ?? -1;
+      expect(lastContentBlockStopIndex).toBeGreaterThanOrEqual(0);
+
+      // content_block_stop should be after message_start
+      expect(lastContentBlockStopIndex).toBeGreaterThan(messageStartIndex);
+
+      // content_block_stop should be before message_stop
+      expect(lastContentBlockStopIndex).toBeLessThan(messageStopIndex);
+    });
+  });
 });
diff --git a/package.json b/package.json
index a49760350..c1dfa2448 100644
--- a/package.json
+++ b/package.json
@@ -36,8 +36,8 @@
     "test:integration:sandbox:none": "cross-env QWEN_SANDBOX=false vitest run --root ./integration-tests",
     "test:integration:sandbox:docker": "cross-env QWEN_SANDBOX=docker npm run build:sandbox && QWEN_SANDBOX=docker vitest run --root ./integration-tests",
     "test:integration:sandbox:podman": "cross-env QWEN_SANDBOX=podman vitest run --root ./integration-tests",
-    "test:integration:sdk:sandbox:none": "cross-env QWEN_SANDBOX=false vitest run --root ./integration-tests sdk-typescript",
-    "test:integration:sdk:sandbox:docker": "cross-env QWEN_SANDBOX=docker npm run build:sandbox && QWEN_SANDBOX=docker vitest run --root ./integration-tests sdk-typescript",
+    "test:integration:sdk:sandbox:none": "cross-env QWEN_SANDBOX=false vitest run --root ./integration-tests --poolOptions.threads.maxThreads 2 sdk-typescript",
+    "test:integration:sdk:sandbox:docker": "cross-env QWEN_SANDBOX=docker npm run build:sandbox && QWEN_SANDBOX=docker vitest run --root ./integration-tests --poolOptions.threads.maxThreads 2 sdk-typescript",
     "test:integration:cli:sandbox:none": "cross-env QWEN_SANDBOX=false vitest run --root ./integration-tests --exclude '**/sdk-typescript/**'",
     "test:integration:cli:sandbox:docker": "cross-env QWEN_SANDBOX=docker npm run build:sandbox && QWEN_SANDBOX=docker vitest run --root ./integration-tests --exclude '**/sdk-typescript/**'",
     "test:terminal-bench": "cross-env VERBOSE=true KEEP_OUTPUT=true vitest run --config ./vitest.terminal-bench.config.ts --root ./integration-tests",
diff --git a/packages/cli/src/nonInteractive/io/BaseJsonOutputAdapter.ts b/packages/cli/src/nonInteractive/io/BaseJsonOutputAdapter.ts
index b0d6736a5..dc62f9ae2 100644
--- a/packages/cli/src/nonInteractive/io/BaseJsonOutputAdapter.ts
+++ b/packages/cli/src/nonInteractive/io/BaseJsonOutputAdapter.ts
@@ -282,12 +282,12 @@ export abstract class BaseJsonOutputAdapter {
       return;
     }
 
-    if (lastBlock.type === 'text') {
-      const index = state.blocks.length - 1;
-      this.onBlockClosed(state, index, actualParentToolUseId);
-      this.closeBlock(state, index);
-    } else if (lastBlock.type === 'thinking') {
-      const index = state.blocks.length - 1;
+    const index = state.blocks.length - 1;
+    if (!state.openBlocks.has(index)) {
+      return;
+    }
+
+    if (lastBlock.type === 'text' || lastBlock.type === 'thinking') {
       this.onBlockClosed(state, index, actualParentToolUseId);
       this.closeBlock(state, index);
     }
@@ -392,7 +392,9 @@ export abstract class BaseJsonOutputAdapter {
     }
 
     const message = this.buildMessage(parentToolUseId);
-    this.emitMessageImpl(message);
+    if (state.messageStarted) {
+      this.emitMessageImpl(message);
+    }
     return message;
   }
 
@@ -656,12 +658,7 @@ export abstract class BaseJsonOutputAdapter {
     parentToolUseId: string,
   ): CLIAssistantMessage {
     const state = this.getMessageState(parentToolUseId);
-    const message = this.finalizeAssistantMessageInternal(
-      state,
-      parentToolUseId,
-    );
-    this.updateLastAssistantMessage(message);
-    return message;
+    return this.finalizeAssistantMessageInternal(state, parentToolUseId);
   }
 
   /**
diff --git a/packages/cli/src/nonInteractive/io/JsonOutputAdapter.ts b/packages/cli/src/nonInteractive/io/JsonOutputAdapter.ts
index a76de53a8..68633675b 100644
--- a/packages/cli/src/nonInteractive/io/JsonOutputAdapter.ts
+++ b/packages/cli/src/nonInteractive/io/JsonOutputAdapter.ts
@@ -52,12 +52,10 @@ export class JsonOutputAdapter
   }
 
   finalizeAssistantMessage(): CLIAssistantMessage {
-    const message = this.finalizeAssistantMessageInternal(
+    return this.finalizeAssistantMessageInternal(
       this.mainAgentMessageState,
       null,
     );
-    this.updateLastAssistantMessage(message);
-    return message;
   }
 
   emitResult(options: ResultOptions): void {
diff --git a/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.test.ts b/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.test.ts
index 96977d5b0..64448c8a6 100644
--- a/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.test.ts
+++ b/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.test.ts
@@ -654,6 +654,24 @@ describe('StreamJsonOutputAdapter', () => {
         'Message not started',
       );
     });
+
+    it('should not emit empty assistant message when started but no content processed', () => {
+      stdoutWriteSpy.mockClear();
+      adapter.finalizeAssistantMessage();
+
+      const assistantCalls = stdoutWriteSpy.mock.calls.filter(
+        (call: unknown[]) => {
+          try {
+            const parsed = JSON.parse(call[0] as string);
+            return parsed.type === 'assistant';
+          } catch {
+            return false;
+          }
+        },
+      );
+
+      expect(assistantCalls).toHaveLength(0);
+    });
   });
 
   describe('emitResult', () => {
@@ -1007,56 +1025,68 @@ describe('StreamJsonOutputAdapter', () => {
     });
   });
 
-  describe('message_id in stream events', () => {
+  describe('content_block event identification', () => {
     beforeEach(() => {
       adapter = new StreamJsonOutputAdapter(mockConfig, true);
       adapter.startAssistantMessage();
     });
 
-    it('should include message_id in stream events after message starts', () => {
+    it('should not include message_id in content_block events', () => {
       adapter.processEvent({
         type: GeminiEventType.Content,
         value: 'Text',
       });
-      // Process another event to ensure messageStarted is true
       adapter.processEvent({
         type: GeminiEventType.Content,
         value: 'More',
       });
 
       const calls = stdoutWriteSpy.mock.calls;
-      // Find all delta events
-      const deltaCalls = calls.filter((call: unknown[]) => {
+      const contentBlockCalls = calls.filter((call: unknown[]) => {
         try {
           const parsed = JSON.parse(call[0] as string);
           return (
             parsed.type === 'stream_event' &&
-            parsed.event.type === 'content_block_delta'
+            (parsed.event.type === 'content_block_start' ||
+              parsed.event.type === 'content_block_delta' ||
+              parsed.event.type === 'content_block_stop')
           );
         } catch {
           return false;
         }
       });
 
-      expect(deltaCalls.length).toBeGreaterThan(0);
-      // The second delta event should have message_id (after messageStarted becomes true)
-      // message_id is added to the event object, so check parsed.event.message_id
-      if (deltaCalls.length > 1) {
-        const secondDelta = JSON.parse(
-          (deltaCalls[1] as unknown[])[0] as string,
-        );
-        // message_id is on the enriched event object
-        expect(
-          secondDelta.event.message_id || secondDelta.message_id,
-        ).toBeTruthy();
-      } else {
-        // If only one delta, check if message_id exists
-        const delta = JSON.parse((deltaCalls[0] as unknown[])[0] as string);
-        // message_id is added when messageStarted is true
-        // First event may or may not have it, but subsequent ones should
-        expect(delta.event.message_id || delta.message_id).toBeTruthy();
+      expect(contentBlockCalls.length).toBeGreaterThan(0);
+      for (const call of contentBlockCalls) {
+        const parsed = JSON.parse((call as unknown[])[0] as string);
+        expect(parsed.event.message_id).toBeUndefined();
       }
     });
+
+    it('should identify content_block events by session_id and index', () => {
+      adapter.processEvent({
+        type: GeminiEventType.Content,
+        value: 'Text',
+      });
+
+      const calls = stdoutWriteSpy.mock.calls;
+      const blockStartCall = calls.find((call: unknown[]) => {
+        try {
+          const parsed = JSON.parse(call[0] as string);
+          return (
+            parsed.type === 'stream_event' &&
+            parsed.event.type === 'content_block_start'
+          );
+        } catch {
+          return false;
+        }
+      });
+
+      expect(blockStartCall).toBeDefined();
+      const parsed = JSON.parse((blockStartCall as unknown[])[0] as string);
+      expect(parsed.session_id).toBe('test-session-id');
+      expect(typeof parsed.event.index).toBe('number');
+    });
   });
 
   describe('multiple text blocks', () => {
diff --git a/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.ts b/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.ts
index 346c4b072..c67190e6a 100644
--- a/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.ts
+++ b/packages/cli/src/nonInteractive/io/StreamJsonOutputAdapter.ts
@@ -36,6 +36,8 @@ export class StreamJsonOutputAdapter
   extends BaseJsonOutputAdapter
   implements JsonOutputAdapterInterface
 {
+  private mainTurnMessageStartEmitted = false;
+
   constructor(
     config: Config,
     private readonly includePartialMessages: boolean,
@@ -68,47 +70,27 @@ export class StreamJsonOutputAdapter
     return this.includePartialMessages;
   }
 
+  override startAssistantMessage(): void {
+    this.mainTurnMessageStartEmitted = false;
+    super.startAssistantMessage();
+  }
+
   finalizeAssistantMessage(): CLIAssistantMessage {
-    return this.finalizeAssistantMessageInternal(
+    const message = this.finalizeAssistantMessageInternal(
       this.mainAgentMessageState,
       null,
     );
-  }
-
-  /**
-   * Overrides base class to emit message_stop event when message is finalized.
-   * This ensures message_start and message_stop are always paired.
-   */
-  protected override finalizeAssistantMessageInternal(
-    state: MessageState,
-    parentToolUseId: string | null,
-  ): CLIAssistantMessage {
-    if (state.finalized) {
-      return this.buildMessage(parentToolUseId);
+    if (this.mainTurnMessageStartEmitted && this.includePartialMessages) {
+      const partial: CLIPartialAssistantMessage = {
+        type: 'stream_event',
+        uuid: randomUUID(),
+        session_id: this.getSessionId(),
+        parent_tool_use_id: null,
+        event: { type: 'message_stop' },
+      };
+      this.emitMessageImpl(partial);
     }
-    state.finalized = true;
-
-    this.finalizePendingBlocks(state, parentToolUseId);
-    const orderedOpenBlocks = Array.from(state.openBlocks).sort(
-      (a, b) => a - b,
-    );
-    for (const index of orderedOpenBlocks) {
-      this.onBlockClosed(state, index, parentToolUseId);
-      this.closeBlock(state, index);
-    }
-
-    // Emit message_stop for main agent when message was started and partial messages are enabled
-    if (
-      state.messageStarted &&
-      this.includePartialMessages &&
-      parentToolUseId === null
-    ) {
-      this.emitStreamEventIfEnabled({ type: 'message_stop' }, null);
-    }
-
-    const message = this.buildMessage(parentToolUseId);
-    this.updateLastAssistantMessage(message);
-    this.emitMessageImpl(message);
+    this.mainTurnMessageStartEmitted = false;
     return message;
   }
 
@@ -267,14 +249,15 @@ export class StreamJsonOutputAdapter
 
   /**
    * Overrides base class hook to emit message_start event when message is started.
-   * Only emits for main agent, not for subagents.
+   * Only emits once per turn for the main agent (guarded by mainTurnMessageStartEmitted),
+   * so block-type transitions inside a single turn do not produce spurious message_start events.
    */
   protected override onEnsureMessageStarted(
     state: MessageState,
     parentToolUseId: string | null,
   ): void {
-    // Only emit message_start for main agent, not for subagents
-    if (parentToolUseId === null) {
+    if (parentToolUseId === null && !this.mainTurnMessageStartEmitted) {
+      this.mainTurnMessageStartEmitted = true;
       this.emitStreamEventIfEnabled(
         {
           type: 'message_start',
@@ -282,6 +265,7 @@ export class StreamJsonOutputAdapter
             id: state.messageId!,
             role: 'assistant',
             model: this.config.getModel(),
+            content: [],
           },
         },
         null,
@@ -329,19 +313,12 @@ export class StreamJsonOutputAdapter
       return;
     }
 
-    const state = this.getMessageState(parentToolUseId);
-    const enrichedEvent = state.messageStarted
-      ? ({ ...event, message_id: state.messageId } as StreamEvent & {
-          message_id: string;
-        })
-      : event;
-
     const partial: CLIPartialAssistantMessage = {
       type: 'stream_event',
       uuid: randomUUID(),
       session_id: this.getSessionId(),
       parent_tool_use_id: parentToolUseId,
-      event: enrichedEvent,
+      event,
     };
     this.emitMessageImpl(partial);
   }
diff --git a/packages/cli/src/nonInteractive/types.ts b/packages/cli/src/nonInteractive/types.ts
index 84c2d0ff7..69eaa1dcd 100644
--- a/packages/cli/src/nonInteractive/types.ts
+++ b/packages/cli/src/nonInteractive/types.ts
@@ -201,6 +201,7 @@ export interface MessageStartStreamEvent {
     id: string;
     role: 'assistant';
     model: string;
+    content: [];
   };
 }
 
diff --git a/packages/cli/src/nonInteractiveCli.ts b/packages/cli/src/nonInteractiveCli.ts
index e4c22cebb..bf29f8f0e 100644
--- a/packages/cli/src/nonInteractiveCli.ts
+++ b/packages/cli/src/nonInteractiveCli.ts
@@ -390,6 +390,16 @@ export async function runNonInteractive(
         }
       }
     } catch (error) {
+      // Ensure message_start / message_stop (and content_block events) are
+      // properly paired even when an error aborts the turn mid-stream.
+      // The call is safe when no message was started (throws → caught) or
+      // when already finalized (idempotent guard inside the adapter).
+      try {
+        adapter.finalizeAssistantMessage();
+      } catch {
+        // Expected when no message was started or already finalized
+      }
+
       // For JSON and STREAM_JSON modes, compute usage from metrics
       const message = error instanceof Error ? error.message : String(error);
       const metrics = uiTelemetryService.getMetrics();