AsyncAPI channel · Retell AI
· Retell AI WebSocket APIs
custom_llm/{call_id}
Custom LLM WebSocket channel. Retell AI opens a connection to the
developer's WebSocket server and exchanges JSON messages identified
by `interaction_type` (Retell to LLM) or `response_type` (LLM to
Retell).
Provider:Retell AIAsyncAPI: v2.6.0Spec: Retell AI WebSocket APIsOperations: 2Messages: 13
Channel address
custom_llm/{call_id}
Parameters
call_id — Unique identifier of the call.
Operations
publish
llmToRetell
Messages sent from the LLM server to Retell.
subscribe
retellToLlm
Messages sent from Retell to the LLM server.
Messages
LlmConfig
LLM server-side configuration sent to Retell.
Content-Type: application/json
LlmUpdateAgent
Update agent-level runtime settings.
Content-Type: application/json
LlmPingPong
Heartbeat ping sent from the LLM server to Retell.
Content-Type: application/json
LlmResponse
Streamed agent response chunk.
Content-Type: application/json
LlmAgentInterrupt
Agent-initiated interruption.
Content-Type: application/json
LlmToolCallInvocation
LLM reports a tool / function call invocation.
Content-Type: application/json
LlmToolCallResult
LLM reports the result of a tool / function call.
Content-Type: application/json
LlmMetadata
Custom metadata sent by the LLM server to Retell.
Content-Type: application/json
RetellPingPong
Heartbeat ping sent by Retell to the LLM server.
Content-Type: application/json
RetellCallDetails
Initial call metadata sent by Retell when `call_details` is enabled
via the LLM-side config message.
Content-Type: application/json
RetellUpdateOnly
Transcript / turn-taking update that does not require an LLM response.
Content-Type: application/json
RetellResponseRequired
Retell expects the LLM server to produce a response.
Content-Type: application/json
RetellReminderRequired
Retell signals that a reminder response is required after silence.
Content-Type: application/json
About AsyncAPI
The AsyncAPI specification describes event-driven APIs the way OpenAPI describes request/response APIs. A channel is the named pipe — a webhook URL, a Kafka topic, a WebSocket route, an MQTT subject — that producers and consumers publish or subscribe to. Each channel carries one or more messages with structured payloads, and an operation declares whether a given party sends or receives on that channel.