Retell AI MCP Server

Instructions

Implementation Reference

• src/index.ts:1125-1126 (handler)

  Handler case for retell_create_web_call tool. Makes a POST request to the Retell API's /v2/create-web-call endpoint using the generic retellRequest helper.

  case "retell_create_web_call":
    return retellRequest("/v2/create-web-call", "POST", args);

• src/index.ts:95-112 (schema)

  Input schema for retell_create_web_call defining required agent_id and optional metadata/llm variables.

  inputSchema: {
    type: "object",
    properties: {
      agent_id: {
        type: "string",
        description: "The agent ID to use for the web call"
      },
      metadata: {
        type: "object",
        description: "Optional: Custom metadata to attach to the call"
      },
      retell_llm_dynamic_variables: {
        type: "object",
        description: "Optional: Dynamic variables to pass to the LLM"
      }
    },
    required: ["agent_id"]
  }

• src/index.ts:92-113 (registration)

  Tool registration in the tools array: defines name, description, and inputSchema for MCP tool listing.

  {
    name: "retell_create_web_call",
    description: "Create a new web call session. Returns a call ID and access token for establishing a WebRTC connection.",
    inputSchema: {
      type: "object",
      properties: {
        agent_id: {
          type: "string",
          description: "The agent ID to use for the web call"
        },
        metadata: {
          type: "object",
          description: "Optional: Custom metadata to attach to the call"
        },
        retell_llm_dynamic_variables: {
          type: "object",
          description: "Optional: Dynamic variables to pass to the LLM"
        }
      },
      required: ["agent_id"]
    }
  },

• src/index.ts:23-57 (helper)

  Generic helper function used by all Retell tools to make authenticated API requests to Retell AI servers.

  async function retellRequest(
    endpoint: string,
    method: string = "GET",
    body?: Record<string, unknown>
  ): Promise<unknown> {
    const apiKey = getApiKey();
  
    const headers: Record<string, string> = {
      "Authorization": `Bearer ${apiKey}`,
      "Content-Type": "application/json",
    };
  
    const options: RequestInit = {
      method,
      headers,
    };
  
    if (body && method !== "GET") {
      options.body = JSON.stringify(body);
    }
  
    const response = await fetch(`${RETELL_API_BASE}${endpoint}`, options);
  
    if (!response.ok) {
      const errorText = await response.text();
      throw new Error(`Retell API error (${response.status}): ${errorText}`);
    }
  
    // Handle 204 No Content
    if (response.status === 204) {
      return { success: true };
    }
  
    return response.json();
  }

• src/index.ts:14-20 (helper)

  Helper to retrieve and validate the Retell API key from environment variable.

  function getApiKey(): string {
    const apiKey = process.env.RETELL_API_KEY;
    if (!apiKey) {
      throw new Error("RETELL_API_KEY environment variable is required");
    }
    return apiKey;
  }