diff --git a/preprocess_schemas.py b/preprocess_schemas.py
index 9294143..6346ccf 100644
--- a/preprocess_schemas.py
+++ b/preprocess_schemas.py
@@ -532,6 +532,25 @@ def generate_variants(path, schema, ops, global_variant_requirements):
 # --- Global Normalization ---
 
 
+def metadata_union_members(ucp_schema):
+    """Return the ``$defs`` names that form the UcpMetadata root union.
+
+    The union spans the discovery profiles (platform/business) and every
+    response schema declared in ``ucp.json``. Deriving the list from
+    ``$defs`` keeps the generated ``UcpMetadata`` complete as the protocol
+    adds response types — a previous hardcoded list silently omitted
+    ``response_catalog_schema``, dropping catalog responses from every
+    model's ``ucp`` field.
+    """
+    defs = ucp_schema.get("$defs", {})
+    return [
+        name
+        for name in defs
+        if name in ("platform_schema", "business_schema")
+        or (name.startswith("response_") and name.endswith("_schema"))
+    ]
+
+
 def normalize_metadata_schemas(schemas, target_dir):
     """
     Ensures ucp.json has a root union and other files point to it generically.
@@ -542,14 +561,7 @@ def normalize_metadata_schemas(schemas, target_dir):
     if ucp_path in schemas:
         ucp = schemas[ucp_path]
         ucp["oneOf"] = [
-            {"$ref": f"#/$defs/{d}"}
-            for d in [
-                "platform_schema",
-                "business_schema",
-                "response_checkout_schema",
-                "response_order_schema",
-                "response_cart_schema",
-            ]
+            {"$ref": f"#/$defs/{name}"} for name in metadata_union_members(ucp)
         ]
 
     for p_abs, s in schemas.items():
diff --git a/src/ucp_sdk/models/schemas/ucp.py b/src/ucp_sdk/models/schemas/ucp.py
index 4e8b977..373f7a9 100644
--- a/src/ucp_sdk/models/schemas/ucp.py
+++ b/src/ucp_sdk/models/schemas/ucp.py
@@ -338,7 +338,8 @@ class ResponseCatalogSchema(Base):
         | BusinessSchema
         | ResponseCheckoutSchema
         | ResponseOrderSchema
-        | ResponseCartSchema,
+        | ResponseCartSchema
+        | ResponseCatalogSchema,
         Field(..., title="UCP Metadata"),
     ],
 )
diff --git a/src/ucp_sdk/models/schemas/ucp_create_request.py b/src/ucp_sdk/models/schemas/ucp_create_request.py
index 5f68cf8..830d878 100644
--- a/src/ucp_sdk/models/schemas/ucp_create_request.py
+++ b/src/ucp_sdk/models/schemas/ucp_create_request.py
@@ -338,7 +338,8 @@ class ResponseCatalogSchema(Base):
         | BusinessSchema
         | ResponseCheckoutSchema
         | ResponseOrderSchema
-        | ResponseCartSchema,
+        | ResponseCartSchema
+        | ResponseCatalogSchema,
         Field(..., title="UCP Metadata Create Request"),
     ],
 )
diff --git a/src/ucp_sdk/models/schemas/ucp_update_request.py b/src/ucp_sdk/models/schemas/ucp_update_request.py
index 57cf54c..423f664 100644
--- a/src/ucp_sdk/models/schemas/ucp_update_request.py
+++ b/src/ucp_sdk/models/schemas/ucp_update_request.py
@@ -338,7 +338,8 @@ class ResponseCatalogSchema(Base):
         | BusinessSchema
         | ResponseCheckoutSchema
         | ResponseOrderSchema
-        | ResponseCartSchema,
+        | ResponseCartSchema
+        | ResponseCatalogSchema,
         Field(..., title="UCP Metadata Update Request"),
     ],
 )
diff --git a/tests/test_codegen_pipeline.py b/tests/test_codegen_pipeline.py
index 4890720..31f44cb 100644
--- a/tests/test_codegen_pipeline.py
+++ b/tests/test_codegen_pipeline.py
@@ -401,7 +401,18 @@ def test_normalize_metadata_schemas_sets_root_union_and_ucp_refs(
             (target_dir / "checkout_create_request.json").resolve()
         )
         schemas = {
-            ucp_path: {"$defs": {}},
+            ucp_path: {
+                "$defs": {
+                    "version": {"type": "string"},
+                    "entity": {"type": "object"},
+                    "platform_schema": {"type": "object"},
+                    "business_schema": {"type": "object"},
+                    "response_checkout_schema": {"type": "object"},
+                    "response_order_schema": {"type": "object"},
+                    "response_cart_schema": {"type": "object"},
+                    "response_catalog_schema": {"type": "object"},
+                }
+            },
             checkout_path: {
                 "properties": {
                     "ucp": {"$ref": "ucp.json#/$defs/response_schema"}
@@ -424,6 +435,7 @@ def test_normalize_metadata_schemas_sets_root_union_and_ucp_refs(
                 {"$ref": "#/$defs/response_checkout_schema"},
                 {"$ref": "#/$defs/response_order_schema"},
                 {"$ref": "#/$defs/response_cart_schema"},
+                {"$ref": "#/$defs/response_catalog_schema"},
             ],
         )
         self.assertEqual(
@@ -546,6 +558,70 @@ def test_main_preprocesses_schema_tree_end_to_end(self) -> None:
         self.assertEqual(child_variant["required"], ["value"])
 
 
+class MetadataUnionTest(unittest.TestCase):
+    """The UcpMetadata root union is derived from ucp.json $defs."""
+
+    def test_includes_profiles_and_all_response_schemas(self) -> None:
+        """Profiles and every response_*_schema belong to the union."""
+        ucp = {
+            "$defs": {
+                "version": {"type": "string"},
+                "version_constraint": {"type": "object"},
+                "requires": {"type": "object"},
+                "entity": {"type": "object"},
+                "base": {"type": "object"},
+                "success": {"type": "object"},
+                "error": {"type": "object"},
+                "platform_schema": {"type": "object"},
+                "business_schema": {"type": "object"},
+                "response_checkout_schema": {"type": "object"},
+                "response_order_schema": {"type": "object"},
+                "response_cart_schema": {"type": "object"},
+                "response_catalog_schema": {"type": "object"},
+            }
+        }
+        self.assertEqual(
+            preprocess_schemas.metadata_union_members(ucp),
+            [
+                "platform_schema",
+                "business_schema",
+                "response_checkout_schema",
+                "response_order_schema",
+                "response_cart_schema",
+                "response_catalog_schema",
+            ],
+        )
+
+    def test_picks_up_new_response_types_automatically(self) -> None:
+        """A response schema added upstream is included without code changes."""
+        ucp = {
+            "$defs": {
+                "platform_schema": {"type": "object"},
+                "business_schema": {"type": "object"},
+                "response_invoice_schema": {"type": "object"},
+            }
+        }
+        self.assertEqual(
+            preprocess_schemas.metadata_union_members(ucp),
+            ["platform_schema", "business_schema", "response_invoice_schema"],
+        )
+
+    def test_excludes_non_schema_defs(self) -> None:
+        """Helper and shared defs never leak into the metadata union."""
+        ucp = {
+            "$defs": {
+                "entity": {"type": "object"},
+                "request_schema": {"type": "object"},
+                "base": {"type": "object"},
+            }
+        }
+        self.assertEqual(preprocess_schemas.metadata_union_members(ucp), [])
+
+    def test_empty_defs_yields_empty_union(self) -> None:
+        """No $defs means no union members."""
+        self.assertEqual(preprocess_schemas.metadata_union_members({}), [])
+
+
 @unittest.skipUnless(
     HAVE_SDK, "requires the installed package (pip install -e .)"
 )