diff --git a/backend/alembic/versions/033_remove_threejs_renderer.py b/backend/alembic/versions/033_remove_threejs_renderer.py
new file mode 100644
index 0000000..24ca4a0
--- /dev/null
+++ b/backend/alembic/versions/033_remove_threejs_renderer.py
@@ -0,0 +1,37 @@
+"""Remove threejs-renderer: reset thumbnail_renderer setting to 'blender'.
+
+CadFiles that were rendered with three.js will be re-rendered on the next
+"Regenerate All Thumbnails" admin action.
+
+Revision ID: 033
+Revises: 032
+Create Date: 2026-03-06
+"""
+from alembic import op
+
+revision = '033'
+down_revision = '032'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Switch any existing 'threejs' renderer setting to 'blender'
+    op.execute("""
+        UPDATE system_settings
+        SET value = 'blender'
+        WHERE key = 'thumbnail_renderer' AND value = 'threejs'
+    """)
+
+    # Remove obsolete threejs_render_size setting (no longer used)
+    op.execute("""
+        DELETE FROM system_settings WHERE key = 'threejs_render_size'
+    """)
+
+
+def downgrade():
+    op.execute("""
+        INSERT INTO system_settings (key, value, updated_at)
+        VALUES ('threejs_render_size', '1024', NOW())
+        ON CONFLICT (key) DO NOTHING
+    """)