Recover Codex sessions with missing rollouts

server/src/session/codex/translate.rs

@@ -42,12 +42,13 @@ impl Translator {
         if line.get("id").is_some() && line.get("method").is_none() {
             return true;
         }
-        if line.get("method").and_then(Value::as_str) == Some("thread/started")
-            && line
-                .pointer("/params/thread/parentThreadId")
-                .is_some_and(|parent| !parent.is_null())
-        {
-            return false;
+        let kind = line.get("method").and_then(Value::as_str);
+        let body = kind.and_then(|_| line.get("params")).unwrap_or(line);
+        if let Some(root) = started_thread_is_root(kind, body) {
+            // A clear or missing-rollout recovery replaces the root id inside this same
+            // app-server. The new root cannot match the id this translator still holds; its null
+            // parent is the authoritative distinction from a newly spawned subagent.
+            return root;
         }
         match (self.thread_id.as_deref(), notification_thread(line)) {
             (Some(parent), Some(thread)) => parent == thread,
@@ -80,10 +81,13 @@ impl Translator {
         self.remember_spawned_thread(kind, body);
 
         let thread = notification_thread(line);
+        let root_started = started_thread_is_root(kind, body) == Some(true);
         let child = thread.filter(|thread| {
-            self.thread_id
-                .as_deref()
-                .is_some_and(|parent| parent != *thread)
+            !root_started
+                && self
+                    .thread_id
+                    .as_deref()
+                    .is_some_and(|parent| parent != *thread)
         });
         if let Some(id) = child {
             self.ensure_child(id, "subagent", None);
@@ -467,6 +471,13 @@ fn subagent_title(path: &str) -> String {
         .replace('_', " ")
 }
 
+fn started_thread_is_root(kind: Option<&str>, body: &Value) -> Option<bool> {
+    (kind == Some("thread/started")).then(|| {
+        body.pointer("/thread/parentThreadId")
+            .is_none_or(Value::is_null)
+    })
+}
+
 fn start_item(item: &Value) -> Vec<Event> {
     if matches!(
         item.get("type").and_then(Value::as_str),
@@ -935,6 +946,25 @@ mod tests {
         assert!(translator.limited());
     }
 
+    #[test]
+    fn a_replacement_root_is_not_mistaken_for_a_subagent() {
+        let mut translator = Translator {
+            thread_id: Some("old-root".to_string()),
+            ..Translator::default()
+        };
+        let replacement = line(
+            r#"{"method":"thread/started","params":{"thread":{"id":"new-root","parentThreadId":null}}}"#,
+        );
+        assert!(translator.is_parent(&replacement));
+        assert!(translator.translate(&replacement).is_empty());
+        assert_eq!(translator.thread_id.as_deref(), Some("new-root"));
+
+        let child = line(
+            r#"{"method":"thread/started","params":{"thread":{"id":"child","parentThreadId":"new-root"}}}"#,
+        );
+        assert!(!translator.is_parent(&child));
+    }
+
     #[test]
     fn command_translation_only_hides_the_known_bash_wrapper() {
         let legacy = tool(&line(