diff --git a/core/sync/object_store.py b/core/sync/object_store.py
new file mode 100644
index 00000000..a41126b7
--- /dev/null
+++ b/core/sync/object_store.py
@@ -0,0 +1,47 @@
+"""Object-store adapter seam for edge/cloud sync.
+
+This module defines the transport boundary only.  It has no concrete S3
+implementation and imports no vendor SDK.  Hot-path CORE modules must not import
+this module; sync orchestration may depend on this protocol later.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Protocol, runtime_checkable
+
+
+class ObjectStoreError(RuntimeError):
+    """Base error for object-store adapter failures."""
+
+
+class ObjectNotFoundError(ObjectStoreError):
+    """Raised when a requested object key does not exist."""
+
+
+@dataclass(frozen=True, slots=True)
+class ObjectMetadata:
+    """Minimal metadata returned by object-store adapters."""
+
+    key: str
+    size_bytes: int
+    content_type: str | None = None
+    etag: str | None = None
+
+
+@runtime_checkable
+class ObjectStore(Protocol):
+    """Protocol implemented by concrete object-store adapters.
+
+    Implementations must map provider-specific exceptions into typed CORE sync
+    errors.  Implementations must not be used by active cognition hot paths.
+    """
+
+    def put_bytes(self, key: str, data: bytes, *, content_type: str) -> ObjectMetadata:
+        """Store bytes at a key and return metadata."""
+
+    def get_bytes(self, key: str) -> bytes:
+        """Return bytes for a key or raise ObjectNotFoundError."""
+
+    def exists(self, key: str) -> bool:
+        """Return True if a key exists."""