fix(session): prevent double auto-compaction from filterCompacted reorder (#27545)

packages/opencode/src/session/message-v2.ts

@@ -1067,6 +1067,33 @@ export const filterCompactedEffect = Effect.fnUntraced(function* (sessionID: Ses
   return filterCompacted(stream(sessionID))
 })
 
+// filterCompacted reorders messages for model consumption
+// ([compaction-user, summary, ...retained tail..., continue-user]), so array
+// position is not chronological. Derive each binding by max id (MessageID
+// is monotonic via MessageID.ascending) so a pre-compaction overflowing tail
+// assistant doesn't get mistaken for the most recent turn. tasks are
+// compaction/subtask parts attached to user messages newer than the latest
+// finished assistant — i.e. unprocessed work.
+export function latest(msgs: WithParts[]) {
+  let user: User | undefined
+  let assistant: Assistant | undefined
+  let finished: Assistant | undefined
+  for (const msg of msgs) {
+    const info = msg.info
+    if (info.role === "user" && (!user || info.id > user.id)) user = info
+    if (info.role === "assistant" && (!assistant || info.id > assistant.id)) assistant = info
+    if (info.role === "assistant" && info.finish && (!finished || info.id > finished.id)) finished = info
+  }
+  const tasks = msgs.flatMap((m) =>
+    finished && m.info.id <= finished.id
+      ? []
+      : m.parts.filter(
+          (p): p is CompactionPart | SubtaskPart => p.type === "compaction" || p.type === "subtask",
+        ),
+  )
+  return { user, assistant, finished, tasks }
+}
+
 export function fromError(
   e: unknown,
   ctx: { providerID: ProviderID; aborted?: boolean },