diff --git a/python/packages/server/src/officeconvert_server/storage.py b/python/packages/server/src/officeconvert_server/storage.py
index 20f1d94..940092c 100644
--- a/python/packages/server/src/officeconvert_server/storage.py
+++ b/python/packages/server/src/officeconvert_server/storage.py
@@ -39,9 +39,19 @@ class S3Store:
         )
 
     def ensure_bucket(self, bucket_name: str) -> None:
-        """Create a bucket if it does not already exist."""
-        if not self._client.bucket_exists(bucket_name):
+        """Create a bucket if it does not already exist.
+
+        Uses CreateBucket only, not HeadBucket. Some S3-compatible stores
+        (including SeaweedFS) mishandle or over-restrict HeadBucket; the MinIO
+        client's bucket_exists() maps non-NoSuchBucket errors to failures.
+        Idempotent create covers the same contract with fewer round trips.
+        """
+        try:
             self._client.make_bucket(bucket_name)
+        except S3Error as exc:
+            if exc.code in ("BucketAlreadyOwnedByYou", "BucketAlreadyExists"):
+                return
+            raise
 
     def presigned_put_url(self, bucket_name: str, object_key: str, *, ttl_seconds: int) -> str:
         """Generate a presigned PUT URL for a single object upload."""