Short answer: there isn’t a documented switch that automatically injects ElevenLabs’ conversation_id into the body your custom LLM endpoint receives. The two supported, documented options are:
Put the conversation ID into the messages your server receives:
Add {{ system__conversation_id }} to your agent’s system prompt (or first message). Dynamic variables are resolved by ElevenLabs and the resulting text is sent in the OpenAI-compatible messages array delivered to your custom LLM server. (elevenlabs.io)
Pass your own correlation ID as a parameter (if you need a separate field):
ElevenLabs will forward user_id and any elevenlabs_extra_body/custom_llm_extra_body you provide at session start to your custom LLM in the request body (e.g., user_id and elevenlabs_extra_body fields). You can use these to carry a client-generated ID. The sample custom LLM server shows user_id being forwarded; extra body is included when “Custom LLM extra body” is enabled. (elevenlabs.io)
How to obtain the current conversation ID at runtime:
Read it from the conversation_initiation_metadata event (conversation_id field) or via conversation.getId() in the JS Client/React SDK after the session starts. (elevenlabs.io)
Note: The docs show setting custom_llm_extra_body only at conversation initiation; they do not document updating that payload after the conversation_id becomes available. If you specifically need the ElevenLabs-generated conversation_id at your server, the documented path is to include {{ system__conversation_id }} in the prompt so it arrives in the messages your endpoint receives. (elevenlabs.io)
ElevenLabs generates the conversation_id and exposes it to you (e.g., via the WebSocket initiation metadata and as the system__conversation_id dynamic variable). There is no documented mechanism to override or replace it. (elevenlabs.io)
The Custom LLM “extra body” (elevenlabs_extra_body / custom_llm_extra_body) is simply forwarded to your LLM in the OpenAI-style request; it does not affect ElevenLabs’ conversation_id. (elevenlabs.io)
What you can do instead
Pass your own identifier as user_id when starting the session (or inside conversation_initiation_client_data). This value is stored with the conversation and is retrievable later. (elevenlabs.io)
If you also want your LLM to see your external ID, include it in the Custom LLM extra body you send. (elevenlabs.io)
Relevant docs:
Agent WebSockets message schema (shows conversation_id and custom_llm_extra_body) (elevenlabs.io)
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