fix(desktop): settle high-overlap sealed replies

apps/desktop/src/app/session/hooks/use-message-stream/index.ts

@@ -75,6 +75,61 @@ let streamMessageSeq = 0
 
 const nextStreamMessageId = (prefix: string) => `${prefix}-${Date.now()}-${++streamMessageSeq}`
 
+/**
+ * A sealed stream can lose a few characters while the authoritative final
+ * remains the same reply. Limit the tolerated edit distance so a separate
+ * assistant segment cannot replace a merely similar interim.
+ */
+function hasHighTextOverlap(left: string, right: string): boolean {
+  const maxLength = Math.max(left.length, right.length)
+
+  if (maxLength < 160) {
+    return false
+  }
+
+  const maxEdits = Math.max(1, Math.min(32, Math.floor(maxLength * 0.02)))
+
+  if (Math.abs(left.length - right.length) > maxEdits) {
+    return false
+  }
+
+  const [shorter, longer] = left.length < right.length ? [left, right] : [right, left]
+
+  let previous = Array.from({ length: shorter.length + 1 }, (_, index) =>
+    index <= maxEdits ? index : Number.POSITIVE_INFINITY
+  )
+
+  let current = new Array<number>(shorter.length + 1).fill(Number.POSITIVE_INFINITY)
+
+  for (let longerIndex = 1; longerIndex <= longer.length; longerIndex += 1) {
+    const start = Math.max(1, longerIndex - maxEdits)
+    const end = Math.min(shorter.length, longerIndex + maxEdits)
+    current.fill(Number.POSITIVE_INFINITY, start, end + 1)
+    current[start - 1] = start === 1 ? longerIndex : Number.POSITIVE_INFINITY
+
+    let rowMinimum = Number.POSITIVE_INFINITY
+
+    for (let shorterIndex = start; shorterIndex <= end; shorterIndex += 1) {
+      current[shorterIndex] = Math.min(
+        previous[shorterIndex] + 1,
+        current[shorterIndex - 1] + 1,
+        previous[shorterIndex - 1] + Number(longer[longerIndex - 1] !== shorter[shorterIndex - 1])
+      )
+      rowMinimum = Math.min(rowMinimum, current[shorterIndex])
+    }
+
+    if (rowMinimum > maxEdits) {
+      return false
+    }
+
+    const nextPrevious = current
+    current = previous
+    previous = nextPrevious
+  }
+
+  return previous[shorter.length] <= maxEdits
+}
+
 export function useMessageStream({
   activeGatewayProfile = 'default',
   activeSessionIdRef,
@@ -796,15 +851,18 @@ export function useMessageStream({
             // tui_gateway `_load_interim_assistant_messages`). When the final
             // completion is the SAME turn's reply, settle it onto that interim
             // instead of appending a second bubble. Continuity, not exact
-            // equality: streaming can drop characters and the final may add a
-            // trailing delta, so treat prefix-either-way as the same message.
+            // equality: streaming can drop a small number of characters and
+            // the final may add a trailing delta, so accept high overlap.
             // (mergeFinalAssistantText, via completeMessage, does the real
             // text merge — replaces the interim's text with the full final.)
             const finalContinuesInterim = Boolean(
               existing.interim &&
               finalText &&
               existingText &&
-              (finalText === existingText || finalText.startsWith(existingText) || existingText.startsWith(finalText))
+              (finalText === existingText ||
+                finalText.startsWith(existingText) ||
+                existingText.startsWith(finalText) ||
+                hasHighTextOverlap(finalText, existingText))
             )
 
             // A bare `error` event (e.g. the agent build failing) already
@@ -865,8 +923,8 @@ export function useMessageStream({
               //   final after streaming, e.g. pseudonym restore) shares the
               //   same no-continuity shape, so it takes the same boundary gate.
               //
-              // • finalContinuesInterim (prefix-either-way continuity, same
-              //   text or one a prefix of the other) is safe to settle
+              // • finalContinuesInterim (prefix-either-way or high-overlap
+              //   continuity) is safe to settle
               //   flag-free within this user occurrence: a `message.start`
               //   reset between this turn's interim and completion must not
               //   force an append of a duplicate bubble (#74560). This also

apps/desktop/src/app/session/hooks/use-message-stream/interim-sealing.test.tsx

@@ -238,6 +238,29 @@ describe('useMessageStream interim text sealing', () => {
     expect(texts[0]).toBe('partial answer continued')
   })
 
+  it('settles a long final with a few dropped streamed characters onto its interim', async () => {
+    mountStream()
+    await start()
+
+    const final = 'This long response contains enough context to identify the same streamed reply. '.repeat(12).trimEnd()
+    const streamed = final.replace('streamed', 'stremed').replace('response', 'respose')
+
+    await interim(streamed)
+    await complete(final)
+
+    expect(assistantMessages()).toEqual([final])
+  })
+
+  it('keeps short near-matching assistant segments distinct', async () => {
+    mountStream()
+    await start()
+
+    await interim('Status: B')
+    await complete('Status: A')
+
+    expect(assistantMessages()).toEqual(['Status: B', 'Status: A'])
+  })
+
   it('settles final onto interim even after message.start reset the boundary flag (#74560)', async () => {
     mountStream()
     await start()