Short answer: you’re referencing a dynamic variable {{ application_id }} somewhere in your agent’s tool configuration, so the platform requires you to supply it at conversation start. Because your initiation webhook doesn’t return it, validation fails immediately.
What to do
Return application_id in dynamic_variables from your conversation initiation webhook (or when starting the WebSocket session). Example response shape:
{
"type": "conversation_initiation_client_data",
"dynamic_variables": {
"application_id": "your-value"
}
}
The initiation webhook must include all dynamic variables defined for the agent. Otherwise the conversation will fail before any messages are exchanged. (elevenlabs.io)
Or, remove the {{ application_id }} placeholder from your tool configuration (headers, path/query/body params, URL, etc.). If a variable isn’t referenced anywhere, it isn’t required at start. Dynamic variables can be used inside tool parameters and headers; if you keep the reference, you must provide it at runtime. (elevenlabs.io)
Double‑check naming and casing. Dynamic variable names are case‑sensitive and must be present in the dynamic_variables object you send at initiation. (elevenlabs.io)
Why this happens
ElevenLabs treats dynamic variables as required at initiation time if they are referenced by the agent (prompts, first message, or tools). For Twilio/SIP/WhatsApp flows this is enforced via the initiation webhook; missing variables cause the session to terminate early with “Missing required dynamic variables …”. (elevenlabs.io)
Optional notes
If you’re starting sessions from the browser “Talk‑to” page, you can pass dynamic variables via URL params (e.g., var_application_id=...). (elevenlabs.io)
Relevant docs
Dynamic variables overview, usage in tools, troubleshooting (case‑sensitivity, inclusion at initiation). (elevenlabs.io)
Twilio/telephony initiation webhook: dynamic_variables must contain all variables defined for the agent; example payload. (elevenlabs.io)
Help Center: common failure reason “Missing required dynamic variables …”. (help.elevenlabs.io)
Server tools docs (where dynamic variables can appear in tool
if i configure an assignment on a tool like application_id = response.application_id, could I then use application_id as a dynamic variable in the request body of another tool? would this satisfy supplying it at conversation start?
icy orbit
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