diff --git a/strix/runtime/backends.py b/strix/runtime/backends.py
index 51abb7e6..d77bb8f8 100644
--- a/strix/runtime/backends.py
+++ b/strix/runtime/backends.py
@@ -81,16 +81,17 @@ def register_backend(
     backend: SandboxBackend,
     *,
     supports_bind_mounts: bool = False,
-    supports_mount_free: bool = True,
+    supports_mount_free: bool | None = None,
 ) -> None:
     """Register a custom backend under ``name``.
 
     Intended for downstream users who ship their own runtime — register
     before any ``session_manager.create_or_reuse`` call. Re-registering
-    an existing name overwrites the prior entry. ``supports_bind_mounts``
-    defaults to False: a remote runtime cannot see the caller's filesystem, so
-    it is handed local sources as manifest entries to upload instead.
+    an existing name overwrites the prior entry. Remote backends without bind
+    mounts default to mount-free support, while bind-mount backends must explicitly opt in.
     """
+    if supports_mount_free is None:
+        supports_mount_free = not supports_bind_mounts
     _BACKENDS[name] = backend
     if supports_bind_mounts:
         _BIND_MOUNT_BACKENDS.add(name)