fix(desktop): require structure-bearing row for live-tail same-turn carry

Structure-only same-turn carry used (live(previous) || live(message)), so a
new live text-only assistant at a compression-rewritten ordinal could inherit
reasoning/tool parts from an unrelated historical structured row.

Require the structure-bearing cached row itself to be live-tail (pending /
assistant-stream-* / interim). Add regressions for non-extending live dump
carry and the compression graft rejection.

Addresses salvage path on #76744 / #76444.

apps/desktop/src/app/session/hooks/use-session-actions/resume-structural-parts.test.ts

@@ -73,4 +73,70 @@ describe('reconcileResumeMessages — structural parts on a mid-turn switch', ()
 
     expect(assistant.parts.filter(p => p.type === 'tool-call')).toHaveLength(1)
   })
+
+  it('keeps live-tail structure when the flat dump is not a strict text extension', () => {
+    // Mid-turn sandwich path: cache holds reasoning/tools; resume returns a
+    // longer non-extending dump. Structure source must be live-tail.
+    const cached: ChatMessage[] = [
+      {
+        id: 'assistant-stream-1',
+        pending: true,
+        parts: [
+          { type: 'reasoning', text: 'thinking about tools' },
+          { type: 'tool-call', toolCallId: 'c1', toolName: 'terminal', args: {} },
+          { type: 'text', text: 'partial' }
+        ],
+        role: 'assistant'
+      }
+    ]
+
+    const authoritative: ChatMessage[] = [
+      {
+        id: 'assistant-stream-1',
+        pending: true,
+        parts: [{ type: 'text', text: 'thinking about tools\nRan terminal\npartial and more dump' }],
+        role: 'assistant'
+      }
+    ]
+
+    const [assistant] = reconcileResumeMessages(authoritative, cached)
+
+    expect(assistant.parts.some(part => part.type === 'reasoning')).toBe(true)
+    expect(assistant.parts.some(part => part.type === 'tool-call')).toBe(true)
+    expect(assistant.parts.filter(part => part.type === 'text').map(part => ('text' in part ? part.text : ''))).toEqual([
+      'partial'
+    ])
+  })
+
+  it('does not graft historical structure onto a live text-only row after compression rewrote ordinals', () => {
+    // Previous cache still has a completed structured assistant at ordinal 0.
+    // Resume after compression returns a new live text-only assistant at the
+    // same role ordinal for an unrelated turn — must not inherit foreign parts.
+    const cached: ChatMessage[] = [
+      {
+        id: 'old-assistant',
+        parts: [
+          { type: 'reasoning', text: 'old thinking' },
+          { type: 'tool-call', toolCallId: 'old-call', toolName: 'terminal', args: {} },
+          { type: 'text', text: 'old answer' }
+        ],
+        role: 'assistant'
+      }
+    ]
+
+    const authoritative: ChatMessage[] = [
+      {
+        id: 'assistant-stream-runtime-1',
+        pending: true,
+        parts: [{ type: 'text', text: 'brand new partial' }],
+        role: 'assistant'
+      }
+    ]
+
+    const [assistant] = reconcileResumeMessages(authoritative, cached)
+
+    expect(assistant.parts.some(part => part.type === 'reasoning')).toBe(false)
+    expect(assistant.parts.some(part => part.type === 'tool-call')).toBe(false)
+    expect(assistant.parts).toEqual([{ type: 'text', text: 'brand new partial' }])
+  })
 })

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

@@ -295,10 +295,12 @@ export function reconcileResumeMessages(nextMessages: ChatMessage[], previousMes
     // the strict equality path — they reconcile a SETTLED row, and a growing
     // row is by definition not settled.
     //
-    // Live-tail identity: only treat structure as same-turn evidence when the
-    // cached row is still the in-flight stream (pending / stream id), not a
-    // historical completed assistant that merely shares a role ordinal after
-    // compression rewrote history (#76444 review).
+    // Live-tail identity: structure-only same-turn carry is allowed only when
+    // the *structure-bearing cached row* is still the in-flight stream
+    // (pending / stream id / interim). Marking only the text-only next row
+    // live is not enough — after compression a new live assistant can share a
+    // role ordinal with an unrelated historical structured row and must not
+    // inherit its reasoning/tool parts (#76444 review / salvage).
     const isLiveTailRow = (row: ChatMessage): boolean =>
       Boolean(row.pending) ||
       row.id.startsWith('assistant-stream-') ||
@@ -311,7 +313,7 @@ export function reconcileResumeMessages(nextMessages: ChatMessage[], previousMes
         previous.role === 'assistant' &&
         hasStructuralParts(previous) &&
         !hasStructuralParts(message) &&
-        (isLiveTailRow(previous) || isLiveTailRow(message)))
+        isLiveTailRow(previous))
 
     if (sameTurn) {
       preserved = preserveStructuralParts(preserved, previous)