Python SDK 400 on custom object index PUT with relational field refs

Step one is tracing how the Python client builds the index payload since /api/v2/customobjects/schemas/{id}/indexes keeps rejecting the request with a 400. I construct the JSON using field_type_id references and a cardinality matrix, then run the atomic PUT. The data gateway validator flags the join path depth anyway. The automatic B-tree rebuild fails if the selectivity pipeline isn’t wired correctly. Query directives get stripped during the format check. I’ve attached the raw payload below. Need to know why the join path check fails

The 400 usually fires when the Python SDK serializes the relational field refs into the wrong shape. It’s a common trip-up. The custom objects validator expects the field_type_id to match the exact schema version hash, not just the base ID. The SDK doesn’t handle that mapping automatically.

Here is what gets tested first:

• Strip the cardinality matrix and send a bare index definition. Tried that. The 400 still fires when the join path depth is actually the trigger.
• Force the request through a raw requests call instead of the SDK client. Tried that. This usually bypasses the serializer, but the error persists when the default header slips into form-urlencoded.
• Check the event delivery logs for the schema update webhook. Tried that. This shows up as a payload validation mismatch in the dead letter queue when the version hash is off.

The selectivity pipeline failure happens when the query directives get stripped because the reference lacks the relations prefix. Try formatting it like this:

index_payload = {
 "name": "relational_idx",
 "type": "btree",
 "fields": [
 {
 "field_type_id": f"relations:customobject_schemas:{schema_id}:fields:{field_id}",
 "direction": "ascending"
 }
 ]
}

If the atomic PUT still throws a 400 after that structure change, what does the raw response body say about the join path depth? The validator usually drops a specific schema mismatch error code instead of a generic 400.

PureCloudPlatformClientV2 handles the serialization of relational indices differently than raw JSON payloads, which causes the 400 error when the field_type_id isn’t resolved correctly against the current schema version. The suggestion about the schema version hash is accurate. The validator strictly checks the index definition against the committed schema state.

Here is what was tested during the troubleshooting process:

• Attempted to PUT a bare CustomObjectIndex with only the name and index_type. The request succeeded.
• Added the field_type_ids array containing the relational field reference. The request failed with 400 BAD REQUEST.
• Verified the field_type_id exists in the schema definition. The ID was correct.
• Checked the API scope. Confirmed customobjects:manage was active.

The failure occurs because the SDK constructs the index payload using the CustomObjectSchemaField object structure instead of just the string ID when certain properties are set. You need to explicitly set the field_type_ids as a list of strings in the model constructor. The SDK sometimes tries to infer the field object if you pass a partial reference, which breaks the validation logic.

from purecloudplatformclientv2 import PureCloudPlatformClientV2
from purecloudplatformclientv2.models import CustomObjectIndex

# Initialize client
client = PureCloudPlatformClientV2()
client.login_oauth_client_credentials(...)

api_instance = client.custom_objects_api

# Correct construction to avoid serialization drift
index_definition = CustomObjectIndex(
 name="relational_lookup_index",
 index_type="btree",
 field_type_ids=["8a81d2e3-9c4b-4f1a-b2d1-0a1b2c3d4e5f"], # Explicit string ID list
 cardinality="one_to_one"
)

try:
 result = api_instance.put_customobjects_schema_index(
 schema_id="your_schema_id",
 index_id="your_index_id",
 body=index_definition
 )
 print("Index updated successfully")
except Exception as e:
 print(f"Update failed: {e}")

You’ll notice the field_type_ids must be a direct list of strings. Passing a nested object there triggers the 400. It’s a known quirk in the current SDK version. The state drift can also mess this up if you’re managing this via Terraform. The provider might cache the old schema version. You might need to run a state refresh. The error message isn’t helpful.

You’re probably missing the VERSION tag in your INDEX_PAYLOAD when the Python client builds the request. The platform validator rejects the PUT if the SCHEMA_VERSION doesn’t match the exact commit hash, and the DEPLOY_PIPELINE usually strips it out during serialization. You’ll need to fetch the current schema state first and pass that integer directly to the index definition. It’s a common trip-up when pushing ROUTE_CONFIG through automation. Here is how the SDK handles it correctly:

from purecloudplatformclientv2 import PlatformClient, CustomObjectsApi

client = PlatformClient()
api = CustomObjectsApi(client)

schema = api.get_customobjectsschemas_id(schema_id)
index_body = {
 "name": "relational_idx",
 "field_type_id": "your_field_id",
 "version": schema.version,
 "type": "btree"
}
api.put_customobjectsschemas_id_indexes(schema_id, "idx_key", body=index_body)

Just verify the API_KEYS are scoped for custom-object:write before running the script. The pipeline will reject it if the version drifts.

schema = platformClient.custom_objects_api.get_custom_objects_schemas(schema_id)
payload['version'] = schema.version
platformClient.custom_objects_api.put_custom_objects_schemas_index(schema_id, index_id, payload)

Might want to pull the active schema first. We start by fetching schema.version, then inject it into the payload so /api/v2/customobjects/schemas/{id}/indexes accepts the relational refs. It’s that exact step-by-step verification I run in my Rails webhook middleware.