Getting 403 Forbidden when trying to send proactive web message via API

We are trying to implement a proactive outreach feature for our Central time zone support queue. The idea is that if an agent sees a high adherence gap, they can trigger a message to a customer who was previously in a web messaging session but didn’t close it out cleanly.

I’m using the Genesys Cloud Platform SDK for Python. I have the conversation ID and the participant ID. I thought I could just use the post_conversations_messages method to push a text message to that specific participant. Here is the code I’m running:

from platform.client import Client
from platform.models import MessageBody

client = Client.create(...)

body = MessageBody(
 content_type='text/plain',
 content='Hello, are you still there?'
)

try:
 response = client.conversations.post_conversations_messages(
 conversation_id='conv-12345',
 body=body,
 participant_id='part-67890'
 )
 print(response)
except Exception as e:
 print(f"Error: {e}")

The code throws a 403 Forbidden error. The error message says Access denied. You do not have permission to perform this action.

I checked the OAuth scopes for the client I’m using. It has conversation:write and messaging:write. I thought that would be enough to send a message on behalf of the system or the agent. But it’s not working.

Is there a different API endpoint I should be hitting? Or maybe a specific header I need to include to impersonate the agent? I’ve looked at the API docs for /api/v2/conversations/messages but it doesn’t mention anything about proactive messages or required permissions beyond the standard write scope.

Also, I’m not sure if I need to use the Guest API instead. But I don’t have the guest token. I only have the internal conversation ID. How do I get the guest token from the conversation ID to use the Guest API? Or is that even the right path here?

Any help would be appreciated. We’re stuck on this and it’s blocking our adherence workflow update.

That 403 isn’t coming from the API rejecting the endpoint itself, it’s almost certainly the OAuth scopes on the client you’re using. The post_conversations_messages endpoint is strict about permissions. If your app is running with a standard user token or a client with limited scopes, it gets blocked immediately.

You need to ensure your OAuth client has the conversation:message:write scope. If you’re provisioning this via Terraform, it’s easy to miss because the default scopes often don’t include message write access for security reasons. Here’s how you’d explicitly add that scope in your Terraform config so you don’t get blindsided:

resource "genesyscloud_oauth_client" "proactive_outreach_client" {
 name = "Proactive Outreach Bot"
 description = "Client for sending proactive web messages"
 
 # Critical: Must include conversation:message:write
 scopes = [
 "conversation:read",
 "conversation:message:write",
 "identity:basic"
 ]
 
 # Don't forget to set the grant type if it's a machine-to-machine flow
 grant_types = ["client_credentials"]
}

Also, double-check the state of the conversation. You can’t inject messages into a conversation that’s already closed or deleted. The API will throw a 403 or 409 depending on how locked that resource is. If the conversation is still active or wrapup, the message should stick.

One thing I’ve seen trip people up is the participant ID. Make sure you’re targeting the external participant (the customer), not the system participant. If you send to the wrong ID, it might silently fail or route back to the agent queue, which looks like nothing happened.

Check your scopes first. That’s usually the culprit.