diff --git a/agents/rag/setup_corpus.py b/agents/rag/setup_corpus.py
index 7142201..d64f130 100644
--- a/agents/rag/setup_corpus.py
+++ b/agents/rag/setup_corpus.py
@@ -3,12 +3,9 @@
 setup_corpus.py — Create a Vertex AI RAG Engine corpus and import documents.
 Project: propane-will-491900-m5
 
-Explicitly uses Serverless (RagManagedDb) to avoid Spanner allowlist
-restrictions on new projects in us-central1.
-
-Tested against vertexai SDK 1.153.1:
-  RagVectorDbConfig(vector_db=RagManagedDb())
-  RagManagedDb is a no-arg marker class — selecting it is sufficient for serverless.
+Serverless mode is controlled via the project-level ragEngineConfig PATCH
+(ensure_serverless_engine_config). Passing backend_config to create_corpus
+triggers a SDK 1.153.1 bug and must be omitted.
 """
 
 import os
@@ -31,7 +28,8 @@ GCS_SOURCE = os.environ.get(
 def ensure_serverless_engine_config() -> None:
     """
     Pre-flight: set project-level RAG Engine Config to basic (serverless) tier.
-    Idempotent — safe to re-run.
+    This is what actually controls the backend — backend_config in create_corpus
+    is intentionally omitted due to SDK bug in 1.153.1.
     """
     print("Ensuring RAG Engine Config is set to serverless (basic) tier...")
     endpoint = (
@@ -71,21 +69,17 @@ def ensure_serverless_engine_config() -> None:
 
 
 def get_or_create_corpus() -> RagCorpus:
-    """Return existing corpus by display name, or create a new serverless one."""
+    """Return existing corpus by display name, or create a new one."""
     for c in rag.list_corpora():
         if c.display_name == CORPUS_DISPLAY_NAME:
             print(f"Corpus '{CORPUS_DISPLAY_NAME}' already exists: {c.name}")
             return c
 
-    print(f"Creating corpus '{CORPUS_DISPLAY_NAME}' in {LOCATION} (serverless)...")
-    corpus = rag.create_corpus(
-        display_name=CORPUS_DISPLAY_NAME,
-        # RagManagedDb() is a no-arg marker class in SDK 1.153.1
-        # Selecting it routes to serverless managed DB (basic tier)
-        backend_config=rag.RagVectorDbConfig(
-            vector_db=rag.RagManagedDb()
-        ),
-    )
+    print(f"Creating corpus '{CORPUS_DISPLAY_NAME}' in {LOCATION}...")
+    # NOTE: backend_config is intentionally omitted.
+    # SDK 1.153.1 has a bug where passing any backend_config crashes.
+    # Serverless mode is already guaranteed by ensure_serverless_engine_config().
+    corpus = rag.create_corpus(display_name=CORPUS_DISPLAY_NAME)
     print(f"✓ Corpus created: {corpus.name}")
     return corpus