--- a/storage/browser/file_system/sandbox_file_system_backend_delegate.h	2026-06-06 00:24:32.918499945 +0000
+++ b/storage/browser/file_system/sandbox_file_system_backend_delegate.h	2026-06-06 00:24:55.138051980 +0000
@@ -20,6 +20,7 @@
 #include "base/files/file_path.h"
 #include "base/memory/scoped_refptr.h"
 #include "base/memory/weak_ptr.h"
+#include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
 #include "base/time/time.h"
 #include "storage/browser/file_system/file_system_backend.h"
@@ -289,6 +290,10 @@
 
   std::set<std::pair<url::Origin, FileSystemType>> sticky_dirty_origins_;
 
+  // Guards the observer maps below; AddFile*Observer() may be invoked on the
+  // UI thread (FileSystemAccessBucketPathWatcher::Initialize) while
+  // CreateFileSystemOperationContext() reads them on the IO thread.
+  mutable base::Lock observers_lock_;
   std::map<FileSystemType, UpdateObserverList> update_observers_;
   std::map<FileSystemType, ChangeObserverList> change_observers_;
   std::map<FileSystemType, AccessObserverList> access_observers_;
--- a/storage/browser/file_system/sandbox_file_system_backend_delegate.cc	2026-06-06 00:24:32.920043380 +0000
+++ b/storage/browser/file_system/sandbox_file_system_backend_delegate.cc	2026-06-06 00:25:26.814029120 +0000
@@ -274,15 +274,17 @@
     return nullptr;
   }
 
-  const UpdateObserverList* update_observers = GetUpdateObservers(url.type());
-  const ChangeObserverList* change_observers = GetChangeObservers(url.type());
-  DCHECK(update_observers);
-
   auto operation_context =
       std::make_unique<FileSystemOperationContext>(context);
-  operation_context->set_update_observers(*update_observers);
-  operation_context->set_change_observers(
-      change_observers ? *change_observers : ChangeObserverList());
+  {
+    base::AutoLock locked(observers_lock_);
+    const UpdateObserverList* update_observers = GetUpdateObservers(url.type());
+    const ChangeObserverList* change_observers = GetChangeObservers(url.type());
+    DCHECK(update_observers);
+    operation_context->set_update_observers(*update_observers);
+    operation_context->set_change_observers(
+        change_observers ? *change_observers : ChangeObserverList());
+  }
 
   return operation_context;
 }
@@ -430,6 +432,7 @@
 #if DCHECK_IS_ON()
   DCHECK(!is_filesystem_opened_ || io_thread_checker_.CalledOnValidThread());
 #endif
+  base::AutoLock locked(observers_lock_);
   update_observers_[type] =
       update_observers_[type].AddObserver(std::move(observer), task_runner);
 }
@@ -441,6 +444,7 @@
 #if DCHECK_IS_ON()
   DCHECK(!is_filesystem_opened_ || io_thread_checker_.CalledOnValidThread());
 #endif
+  base::AutoLock locked(observers_lock_);
   change_observers_[type] =
       change_observers_[type].AddObserver(std::move(observer), task_runner);
 }
@@ -452,6 +456,7 @@
 #if DCHECK_IS_ON()
   DCHECK(!is_filesystem_opened_ || io_thread_checker_.CalledOnValidThread());
 #endif
+  base::AutoLock locked(observers_lock_);
   access_observers_[type] =
       access_observers_[type].AddObserver(std::move(observer), task_runner);
 }