diff --git a/codex-rs/core/tests/suite/fork_thread.rs b/codex-rs/core/tests/suite/fork_thread.rs
index 2d1532d678..831bfed7dd 100644
--- a/codex-rs/core/tests/suite/fork_thread.rs
+++ b/codex-rs/core/tests/suite/fork_thread.rs
@@ -371,17 +371,17 @@ fn latest_multi_agent_version(
     std::fs::read_to_string(path)
         .unwrap_or_else(|err| panic!("failed to read rollout file {}: {err}", path.display()))
         .lines()
+        .rev()
         .filter(|line| !line.trim().is_empty())
         .map(|line| {
             serde_json::from_str::<RolloutLine>(line)
                 .unwrap_or_else(|err| panic!("failed to parse rollout line `{line}`: {err}"))
         })
-        .filter_map(|line| match line.item {
+        .find_map(|line| match line.item {
             RolloutItem::SessionMeta(meta_line) if meta_line.meta.id == thread_id => {
                 Some(meta_line.meta.multi_agent_version)
             }
             _ => None,
         })
-        .last()
         .flatten()
 }
diff --git a/codex-rs/thread-store/src/local/mod.rs b/codex-rs/thread-store/src/local/mod.rs
index ea5cf8613f..c7fd7705f5 100644
--- a/codex-rs/thread-store/src/local/mod.rs
+++ b/codex-rs/thread-store/src/local/mod.rs
@@ -21,6 +21,7 @@ use std::collections::hash_map::Entry;
 use std::path::PathBuf;
 use std::sync::Arc;
 use tokio::sync::Mutex;
+use tokio::sync::Semaphore;
 
 use crate::AppendThreadItemsParams;
 use crate::ArchiveThreadParams;
@@ -59,7 +60,7 @@ use codex_protocol::protocol::MultiAgentVersion;
 pub struct LocalThreadStore {
     pub(super) config: LocalThreadStoreConfig,
     live_recorders: Arc<Mutex<HashMap<ThreadId, RolloutRecorder>>>,
-    multi_agent_version_seed_lock: Arc<Mutex<()>>,
+    multi_agent_version_seed_semaphore: Arc<Semaphore>,
     state_db: Option<StateDbHandle>,
 }
 
@@ -99,7 +100,7 @@ impl LocalThreadStore {
         Self {
             config,
             live_recorders: Arc::new(Mutex::new(HashMap::new())),
-            multi_agent_version_seed_lock: Arc::new(Mutex::new(())),
+            multi_agent_version_seed_semaphore: Arc::new(Semaphore::new(1)),
             state_db,
         }
     }
diff --git a/codex-rs/thread-store/src/local/set_multi_agent_version.rs b/codex-rs/thread-store/src/local/set_multi_agent_version.rs
index d077d073c3..d8ea67ab1a 100644
--- a/codex-rs/thread-store/src/local/set_multi_agent_version.rs
+++ b/codex-rs/thread-store/src/local/set_multi_agent_version.rs
@@ -16,7 +16,13 @@ pub(super) async fn set_multi_agent_version_if_unset(
     store: &LocalThreadStore,
     params: SetMultiAgentVersionIfUnsetParams,
 ) -> ThreadStoreResult<MultiAgentVersion> {
-    let _guard = store.multi_agent_version_seed_lock.lock().await;
+    let _permit = store
+        .multi_agent_version_seed_semaphore
+        .acquire()
+        .await
+        .map_err(|err| ThreadStoreError::Internal {
+            message: format!("failed to lock thread multi-agent version: {err}"),
+        })?;
     let has_live_writer = live_writer::rollout_path(store, params.thread_id)
         .await
         .is_ok();