fix(desktop): prevent stale optimistic tails after compression (#69682)

apps/desktop/src/app/session/hooks/use-session-actions/utils.test.ts

@@ -327,6 +327,42 @@ describe('preserveLocalPendingTurnMessages', () => {
 
     expect(preserveLocalPendingTurnMessages(next, previous)).toBe(next)
   })
+
+  it('drops stale optimistic history after compression and keeps only the live tail', () => {
+    const compressedAuthority = [
+      msg('stored-user', 'user', 'first turn that survived compression'),
+      msg('stored-assistant', 'assistant', 'latest authoritative reply')
+    ]
+
+    const pollutedWarmCache = [
+      msg('user-old-1', 'user', 'compressed-away prompt one'),
+      msg('assistant-old-1', 'assistant', 'compressed-away reply one'),
+      msg('user-old-2', 'user', 'compressed-away prompt two'),
+      msg('assistant-old-2', 'assistant', 'compressed-away reply two'),
+      msg('user-inflight', 'user', 'the one genuinely in-flight prompt')
+    ]
+
+    expect(preserveLocalPendingTurnMessages(compressedAuthority, pollutedWarmCache).map(message => message.id)).toEqual([
+      'stored-user',
+      'stored-assistant',
+      'user-inflight'
+    ])
+  })
+
+  it('drops the live tail once the latest authoritative user has persisted it after compression', () => {
+    const compressedAuthority = [
+      msg('stored-user', 'user', 'the one genuinely in-flight prompt'),
+      msg('stored-assistant', 'assistant', 'its authoritative reply')
+    ]
+
+    const pollutedWarmCache = [
+      msg('user-old-1', 'user', 'compressed-away prompt one'),
+      msg('assistant-old-1', 'assistant', 'compressed-away reply one'),
+      msg('user-inflight', 'user', 'the one genuinely in-flight prompt')
+    ]
+
+    expect(preserveLocalPendingTurnMessages(compressedAuthority, pollutedWarmCache)).toBe(compressedAuthority)
+  })
 })
 
 describe('appendLiveSessionProjection', () => {

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

@@ -234,9 +234,12 @@ export function reconcileResumeMessages(nextMessages: ChatMessage[], previousMes
  * dropping either makes an accepted turn appear to vanish during transport
  * churn.
  *
- * Authoritative rows use different ids, so match by role ordinal. A matching
- * user row is considered committed only when its visible text also matches;
- * any authoritative assistant at the same ordinal supersedes the local stream.
+ * A lagging projection can be behind by one live turn, never a whole local
+ * history window. Preserve only the newest optimistic user row: compression
+ * rewrites past context, so older `user-*` rows in a warm cache are stale
+ * history, not in-flight work. The latest authoritative user confirms whether
+ * that tail has persisted; any authoritative assistant at the same ordinal
+ * supersedes the local stream.
  */
 export function preserveLocalPendingTurnMessages(
   nextMessages: ChatMessage[],
@@ -257,6 +260,10 @@ export function preserveLocalPendingTurnMessages(
 
   const nextIds = new Set(nextMessages.map(message => message.id))
   const previousRoleCounts = new Map<ChatMessage['role'], number>()
+  const newestOptimisticUser = [...previousMessages]
+    .reverse()
+    .find(message => message.role === 'user' && message.id.startsWith('user-'))
+  const latestAuthoritativeUser = [...nextMessages].reverse().find(message => message.role === 'user')
   const preserved: ChatMessage[] = []
 
   for (const message of previousMessages) {
@@ -272,6 +279,14 @@ export function preserveLocalPendingTurnMessages(
       continue
     }
 
+    if (isOptimisticUser && message !== newestOptimisticUser) {
+      continue
+    }
+
+    if (isOptimisticUser && latestAuthoritativeUser && chatMessageText(latestAuthoritativeUser).trim() === chatMessageText(message).trim()) {
+      continue
+    }
+
     const authoritative = nextByRoleOrdinal.get(`${message.role}:${ordinal}`)
 
     if (authoritative) {