fix(desktop): keep pre-compression transcript reachable after in-place /compress (#105256)

apps/desktop/src/app/session/hooks/use-prompt-actions/index.test.tsx

@@ -776,6 +776,56 @@ describe('usePromptActions /compress', () => {
     expect(renderedText).toContain('sure, here is the summary')
   })
 
+  it('keeps pre-compression messages reachable after the transcript rewrite (#105256)', async () => {
+    const seeds: Record<string, unknown>[] = []
+
+    const requestGateway = vi.fn(async (method: string, _params?: Record<string, unknown>, _timeoutMs?: number) => {
+      if (method === 'session.compress') {
+        return {
+          removed: 2,
+          summary: { headline: 'Compressed: 4 → 2 messages' },
+          messages: [
+            { role: 'user', content: 'summarized context' },
+            { role: 'assistant', content: 'sure, here is the summary' }
+          ]
+        } as never
+      }
+
+      return {} as never
+    })
+
+    let handle: HarnessHandle | null = null
+    await actRender(
+      <Harness
+        onReady={h => (handle = h)}
+        onSeedState={s => seeds.push(s)}
+        refreshSessions={async () => undefined}
+        requestGateway={requestGateway}
+        seedMessages={[
+          { id: 'pre-1', role: 'user', parts: [textPart('pre-compression user turn')], timestamp: 0 },
+          { id: 'pre-2', role: 'assistant', parts: [textPart('pre-compression assistant reply')], timestamp: 1 }
+        ]}
+      />
+    )
+
+    await handle!.submitText('/compress')
+
+    // The transcript was replaced with the post-compress history.
+    const finalMessages = seeds[seeds.length - 1]?.messages as Array<{ parts?: Array<{ text?: string }> }>
+
+    const renderedText = (finalMessages ?? [])
+      .flatMap(message => (message.parts ?? []).map(part => part.text ?? ''))
+      .join('\n')
+
+    expect(renderedText).toContain('summarized context')
+    expect(renderedText).toContain('sure, here is the summary')
+    // Active-session scrollback: the pre-compression segment stays in the
+    // transcript so scrolling up still reaches it.
+    expect(renderedText).toContain('pre-compression user turn')
+    expect(renderedText).toContain('pre-compression assistant reply')
+    expect(renderedText.indexOf('pre-compression user turn')).toBeLessThan(renderedText.indexOf('summarized context'))
+  })
+
   it('uses the compute-host response transcript and success output', async () => {
     const seeds: Record<string, unknown>[] = []
 

apps/desktop/src/app/session/hooks/use-prompt-actions/slash.ts

@@ -2,6 +2,7 @@ import { skillInvocationText } from '@hermes/shared'
 import { parseCommandDispatch, parseSlashCommand } from '@hermes/shared'
 import { type MutableRefObject, useCallback, useRef } from 'react'
 
+import { mergeOlderTranscriptPage } from '@/app/chat/transcript-backfill'
 import { prepareDefaultNewSession } from '@/app/session/new-session-route'
 import { invalidateContextBreakdown } from '@/app/shell/hooks/use-context-breakdown'
 import { getProfiles } from '@/hermes'
@@ -763,10 +764,18 @@ export function useSlashCommand(deps: SlashCommandDeps) {
             // toChatMessages handles it directly. updateSessionState only
             // publishes for the active runtime, guarding against a late result
             // clobbering the foreground after a session switch.
+            // The pre-compression segment stays reachable in the active
+            // session (#105256): the rewrite shares no anchor with the live
+            // transcript, so grafting would drop everything before the
+            // summary. Prepend it instead (deduped by row/message id), keeping
+            // the post-compress history authoritative for overlapping rows.
             if (Array.isArray(result?.messages)) {
               updateSessionState(
                 sessionId,
-                state => ({ ...state, messages: toChatMessages(result.messages!) }),
+                state => ({
+                  ...state,
+                  messages: mergeOlderTranscriptPage(toChatMessages(result.messages!), state.messages)
+                }),
                 storedSessionId
               )
             }