diff --git a/src/adapters/asl_pointer_fs/asl_pointer_fs.c b/src/adapters/asl_pointer_fs/asl_pointer_fs.c
index 1e1fcb4..2b3d1be 100644
--- a/src/adapters/asl_pointer_fs/asl_pointer_fs.c
+++ b/src/adapters/asl_pointer_fs/asl_pointer_fs.c
@@ -635,13 +635,13 @@ amduat_asl_pointer_error_t amduat_asl_pointer_cas(
     const amduat_reference_t *new_ref,
     bool *out_swapped) {
   char *head_path = NULL;
-  int fd = -1;
+  int lock_fd = -1;
   struct flock lock;
   bool exists = false;
   amduat_reference_t current_ref;
   amduat_reference_t prev_ref;
   bool has_prev = false;
-  bool created = false;
+  char *lock_path = NULL;
   amduat_asl_pointer_error_t err;
 
   if (out_swapped == NULL) {
@@ -663,8 +663,18 @@ amduat_asl_pointer_error_t amduat_asl_pointer_cas(
     return AMDUAT_ASL_POINTER_ERR_IO;
   }
 
-  fd = open(head_path, O_RDWR | O_CREAT, 0644);
-  if (fd < 0) {
+  lock_path = (char *)malloc(strlen(head_path) + sizeof(".lock"));
+  if (lock_path == NULL) {
+    free(head_path);
+    return AMDUAT_ASL_POINTER_ERR_IO;
+  }
+  snprintf(lock_path, strlen(head_path) + sizeof(".lock"), "%s.lock",
+           head_path);
+
+  /* Lock a stable sidecar file so lock ownership survives head-file renames. */
+  lock_fd = open(lock_path, O_RDWR | O_CREAT, 0644);
+  if (lock_fd < 0) {
+    free(lock_path);
     free(head_path);
     return AMDUAT_ASL_POINTER_ERR_IO;
   }
@@ -674,19 +684,13 @@ amduat_asl_pointer_error_t amduat_asl_pointer_cas(
   lock.l_whence = SEEK_SET;
   lock.l_start = 0;
   lock.l_len = 0;
-  if (fcntl(fd, F_SETLKW, &lock) != 0) {
-    close(fd);
+  if (fcntl(lock_fd, F_SETLKW, &lock) != 0) {
+    close(lock_fd);
+    free(lock_path);
     free(head_path);
     return AMDUAT_ASL_POINTER_ERR_IO;
   }
 
-  {
-    struct stat st;
-    if (fstat(fd, &st) == 0 && st.st_size == 0) {
-      created = true;
-    }
-  }
-
   err = amduat_asl_pointer_read_head(head_path, name, &exists, &current_ref,
                                      &prev_ref, &has_prev);
   if (err == AMDUAT_ASL_POINTER_ERR_NOT_FOUND) {
@@ -694,24 +698,21 @@ amduat_asl_pointer_error_t amduat_asl_pointer_cas(
     err = AMDUAT_ASL_POINTER_OK;
   }
   if (err != AMDUAT_ASL_POINTER_OK) {
-    close(fd);
+    close(lock_fd);
+    free(lock_path);
     free(head_path);
     return err;
   }
 
   if (expected_exists != exists) {
-    if (created) {
-      (void)unlink(head_path);
-    }
-    close(fd);
+    close(lock_fd);
+    free(lock_path);
     free(head_path);
     return AMDUAT_ASL_POINTER_OK;
   }
   if (expected_exists && !amduat_reference_eq(*expected_ref, current_ref)) {
-    if (created) {
-      (void)unlink(head_path);
-    }
-    close(fd);
+    close(lock_fd);
+    free(lock_path);
     free(head_path);
     return AMDUAT_ASL_POINTER_OK;
   }
@@ -729,7 +730,8 @@ amduat_asl_pointer_error_t amduat_asl_pointer_cas(
   if (has_prev) {
     amduat_reference_free(&prev_ref);
   }
-  close(fd);
+  close(lock_fd);
+  free(lock_path);
   free(head_path);
   return err;
 }