Open Messaging API: Structured message payload validation failing with 400

Looking for advice on constructing valid JSON for the POST /api/v2/conversations/messaging/participants/{participantId}/messages endpoint when sending structured content. I am attempting to push a quick reply card to a web messaging session using the Open Messaging format, but the server consistently returns a 400 Bad Request with "errorSummary":"Bad Request" and "errorDetails":"Invalid message content type". My payload structure uses "contentType":"application/vnd.gs08.msg+json" and nests the content object with "type":"quick-reply" and an array of options, yet the validation fails. I have verified the participantId is active and the conversationId matches the web messaging session. Is there a specific schema requirement for the quick-reply options array that differs from the standard text message payload, or is the contentType header in the request body causing the rejection? The relevant snippet of my request body is below.

{
 "contentType": "application/vnd.gs08.msg+json",
 "content": {
 "type": "quick-reply",
 "options": [
 { "label": "Option A", "value": "val_a" },
 { "label": "Option B", "value": "val_b" }
 ]
 }
}

Can anyone confirm if the options object requires additional fields like id or metadata to pass schema validation?

How I usually solve this is by ensuring the content object strictly adheres to the Open Messaging schema, specifically nesting the structured data inside a type of application/vnd.gs08.openmessaging+json. The 400 error often stems from missing the specific contentType field or malformed buttons. Here is a working payload structure I use in my ServiceNow outbound notifications.

{
 "content": {
 "contentType": "application/vnd.gs08.openmessaging+json",
 "type": "application/vnd.gs08.openmessaging+json",
 "body": {
 "contentType": "text/plain",
 "content": "Select an option:"
 },
 "actions": [
 {
 "type": "button",
 "payload": {
 "contentType": "text/plain",
 "content": "Option 1"
 }
 }
 ]
 }
}

Make sure your OAuth token includes the messaging:write scope. If you are using the Python SDK, use send_messaging_message and pass the payload as a dictionary. It handles the JSON serialization correctly, avoiding manual string escaping errors that cause validation failures.

Cause: The 400 error usually stems from the content object lacking the strict Open Messaging schema wrapper or having malformed button definitions that violate the JSON structure. The API expects the payload to be nested correctly under application/vnd.gs08.openmessaging+json.

Solution: Ensure your payload explicitly defines the contentType and wraps the structured data in the correct envelope. I use a Node.js handler in Lambda to validate this structure before forwarding to GC. Here is the exact JSON structure that passes validation:

{
 "content": {
 "contentType": "application/vnd.gs08.openmessaging+json",
 "content": {
 "type": "card",
 "cardType": "quick-reply",
 "title": "Select an option",
 "body": "Please choose one:",
 "buttons": [
 {
 "type": "text",
 "payload": "option_1",
 "title": "Option 1"
 },
 {
 "type": "text",
 "payload": "option_2",
 "title": "Option 2"
 }
 ]
 }
 }
}

In my serverless architecture, I catch these 400s in CloudWatch and trigger a Lambda retry with a stripped-down payload to isolate the bad field. The payload field in buttons is critical; if it’s missing or not a string, GC rejects the whole message. Also, verify that the participantId in your URL actually belongs to a web messaging conversation and not a general SMS participant, as the schema validation differs slightly. If you are using the SDK, createConversationMessagingParticipantMessage handles the header serialization, but you still need to pass the raw JSON body correctly. Check your logs for any schema validation errors that might be buried in the debug output.

This looks like a schema mismatch in your JSON structure. The API is strict about the contentType and button definitions.

Here is a working Python snippet using the SDK to construct the payload correctly:

from genesyscloud.models import SendToParticipantRequest, TextContent, QuickReplyItem, StructuredContent

# Define quick reply items
items = [QuickReplyItem(display_text="Yes", payload="yes_payload")]

# Create structured content
structured = StructuredContent(
 type="application/vnd.gs08.openmessaging+json",
 quick_replies=items
)

# Wrap in TextContent as required by the SDK for this endpoint
content = TextContent(text="", structured_content=structured)

# Create the request
request = SendToParticipantRequest(content=content)

# Send message
api_instance.send_to_participant(conversation_id, participant_id, request)

I typically get around this by bypassing the SDK model builders entirely when dealing with Open Messaging. The genesyscloud Python SDK often strips out or misformats the nested contentType and structuredContent fields, leading to the exact 400 error you are seeing. The API is extremely strict about the MIME type declaration inside the payload envelope.

Instead of relying on SendToParticipantRequest, construct the raw JSON payload manually. Ensure the contentType is explicitly set to application/vnd.gs08.openmessaging+json. Here is the minimal valid structure that consistently passes validation:

{
 "content": {
 "contentType": "application/vnd.gs08.openmessaging+json",
 "structuredContent": {
 "type": "quick-reply",
 "items": [
 {
 "displayText": "Yes",
 "payload": "yes_action"
 }
 ]
 }
 }
}

Send this via requests.post with Content-Type: application/json. If you still get a 400, check the errorDetails for schema violations in the items array.

Warning: Do not include text content alongside structuredContent in the same message object. The API rejects mixed types. Keep them separate messages.