### Answer To integrate third-party APIs with your Retell AI agent using Tools, you'll need to use the Custom LLM integration approach via the LLM WebSocket. [(1) ]() When using Custom LLM, your server receives tool call invocations from Retell AI and can execute API calls to third-party services. Here's how the integration works: Tool Call Invocation Flow: 1. Your agent triggers a tool call during conversation 2. You send a Tool Call Invocation Event to Retell AI with the tool details 3. Execute your third-party API call on your server 4. Send the result back to Retell AI using a Tool Call Result Event Tool Call Invocation Event format:

json
{
  "response_type": "tool_call_invocation",
  "tool_call_id": "string",
  "name": "string",
  "arguments": "boolean"
}

Tool Call Result Event format:

json
{
  "response_type": "tool_call_result",
  "tool_call_id": "string",
  "content": "string"
}

The

tool_call_id

is globally unique and links the invocation to its result. The

content

field contains the result from your third-party API, which can be a string or stringified JSON. This approach allows you to maintain full control over API authentication, error handling, and business logic while Retell AI manages the conversational flow.

​ To learn more, see the sources I considered: [[1] LLM WebSocket](https://docs.retellai.com/api-references/llm-websocket) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.