fix(vertex_ai): strip the batch custom_id tag via excludedFields, not keyField

Probed live: instanceConfig.keyField does not remove the field from the
instances the container receives, so vLLM 400s every row; excludedFields
performs the strip and attaches the value to the output row.

litellm/llms/vertex_ai/batches/handler.py

@@ -396,12 +396,13 @@ class VertexAIBatchPrediction(VertexLLM):
             "outputConfig": vertex_batch_request["outputConfig"],
             "unmanagedContainerModel": unmanaged,
             "dedicatedResources": batch_resources,
-            # keyField strips the custom_id tag from each instance before it reaches the
-            # container (vLLM rejects unknown fields) and echoes it back as `key` in the output
-            # row; it only takes effect alongside an explicit instanceType.
+            # excludedFields strips the custom_id tag from each instance before it reaches the
+            # container (vLLM rejects unknown fields) and attaches it to the output row's
+            # instance echo; keyField does NOT strip (probed live: the container still received
+            # the tag and 400'd every row).
             "instanceConfig": {
                 "instanceType": "object",
-                "keyField": VERTEX_CUSTOM_ENDPOINT_KEY_FIELD,
+                "excludedFields": [VERTEX_CUSTOM_ENDPOINT_KEY_FIELD],
             },
         }
         return resolved

litellm/types/llms/vertex_ai.py

@@ -700,6 +700,7 @@ class UnmanagedContainerModel(TypedDict, total=False):
 class BatchInstanceConfig(TypedDict, total=False):
     instanceType: ReadOnly[str]
     keyField: ReadOnly[str]
+    excludedFields: ReadOnly[list[str]]
 
 
 class VertexAIBatchPredictionJob(TypedDict, total=False):

tests/test_litellm/llms/vertex_ai/batches/test_handler.py

@@ -342,7 +342,7 @@ def test_create_batch_sync_custom_endpoint_builds_unmanaged_container_job():
         "startingReplicaCount": 1,
         "maxReplicaCount": 2,
     }
-    assert sent["instanceConfig"] == {"instanceType": "object", "keyField": "litellm_custom_id"}
+    assert sent["instanceConfig"] == {"instanceType": "object", "excludedFields": ["litellm_custom_id"]}
 
 
 def test_create_batch_sync_custom_endpoint_without_container_spec_raises_400():