fix(native-chat): show Codex compaction markers in transcripts (#14765)

Codex paginated rollouts persist an item_completed ContextCompaction item
with no content, and legacy rollouts persist an event_msg context_compacted.
The Codex decoder dropped both, so the native Chat timeline never showed
that context was compacted. Emit a system row with the shared compaction
copy and presentation so it renders as the compaction divider.

Fixes #14188

Co-authored-by: Neil <neil@stably.ai>

src/main/native-chat/transcript-line-decoders-codex.ts

@@ -3,6 +3,7 @@ import {
   type NativeChatBlock,
   type NativeChatMessage
 } from '../../shared/native-chat-types'
+import { NATIVE_CHAT_NOTICE_COPY } from '../../shared/native-chat-notice'
 import {
   asRecord,
   extractString,
@@ -132,6 +133,10 @@ function codexEventMessage(
       source: 'transcript'
     }
   }
+  // Legacy rollouts persist this event; paginated rollouts persist a ContextCompaction item instead.
+  if (payload.type === 'context_compacted') {
+    return codexCompactionMarker(id, timestamp)
+  }
   if (payload.type === 'item_completed') {
     return codexCompletedTurnItem(payload, id, timestamp)
   }
@@ -160,6 +165,9 @@ function codexCompletedTurnItem(
     return null
   }
   const id = extractString(item.id) ?? fallbackId
+  if (item.type === 'ContextCompaction') {
+    return codexCompactionMarker(id, timestamp)
+  }
   const blocks = codexTurnItemBlocks(item.content)
   if (blocks.length === 0) {
     return null
@@ -173,6 +181,18 @@ function codexCompletedTurnItem(
   return null
 }
 
+function codexCompactionMarker(id: string, timestamp: number | null): NativeChatMessage {
+  return {
+    id,
+    role: 'system',
+    blocks: [
+      { type: 'text', text: NATIVE_CHAT_NOTICE_COPY.compaction, presentation: 'compaction' }
+    ],
+    timestamp,
+    source: 'transcript'
+  }
+}
+
 function codexTurnItemBlocks(content: unknown): NativeChatBlock[] {
   if (!Array.isArray(content)) {
     return []

src/main/native-chat/transcript-reader-codex-history-mode.test.ts

@@ -134,6 +134,7 @@ describe('Codex transcript history modes', () => {
         },
         '2026-08-09T10:00:01.000Z'
       ),
+      completedItem({ type: 'ContextCompaction', id: 'compaction-1' }, '2026-08-09T10:00:01.500Z'),
       {
         type: 'response_item',
         payload: {
@@ -172,6 +173,12 @@ describe('Codex transcript history modes', () => {
             { type: 'image-ref', path: '/tmp/reference.png' }
           ]
         },
+        {
+          id: 'compaction-1',
+          role: 'system',
+          timestamp: Date.parse('2026-08-09T10:00:01.500Z'),
+          blocks: [{ type: 'text', text: 'Context compacted', presentation: 'compaction' }]
+        },
         {
           id: 'assistant-1',
           role: 'assistant',
@@ -179,7 +186,6 @@ describe('Codex transcript history modes', () => {
         }
       ]
     })
-    expect('messages' in result && result.messages).toHaveLength(2)
     expect(tail).toMatchObject({ messages: 'messages' in result ? result.messages : [] })
   })
 
@@ -194,6 +200,8 @@ describe('Codex transcript history modes', () => {
         }
       },
       { type: 'event_msg', payload: { type: 'user_message', message: 'Prompt' } },
+      { type: 'compacted', payload: { message: 'summary', replacement_history: [] } },
+      { type: 'event_msg', payload: { type: 'context_compacted' } },
       { type: 'event_msg', payload: { type: 'agent_message', message: 'Response' } },
       {
         type: 'response_item',
@@ -210,6 +218,10 @@ describe('Codex transcript history modes', () => {
     expect(result).toMatchObject({
       messages: [
         { role: 'user', blocks: [{ type: 'text', text: 'Prompt' }] },
+        {
+          role: 'system',
+          blocks: [{ type: 'text', text: 'Context compacted', presentation: 'compaction' }]
+        },
         { role: 'assistant', blocks: [{ type: 'text', text: 'Response' }] }
       ]
     })