Retell AI MCP Server

Instructions

Implementation Reference

• src/index.ts:1141-1142 (handler)

  Handler case in executeTool function that implements the tool logic by calling retellRequest to POST to /create-sms-chat endpoint with input args.

  case "retell_create_sms_chat":
    return retellRequest("/create-sms-chat", "POST", args);

• src/index.ts:222-247 (schema)

  Tool schema definition including name, description, and inputSchema with required parameters from_number, to_number, agent_id.

  {
    name: "retell_create_sms_chat",
    description: "Start an outbound SMS conversation using a specified chat agent.",
    inputSchema: {
      type: "object",
      properties: {
        from_number: {
          type: "string",
          description: "The sender's phone number in E.164 format"
        },
        to_number: {
          type: "string",
          description: "The recipient's phone number in E.164 format"
        },
        agent_id: {
          type: "string",
          description: "The chat agent ID to handle the conversation"
        },
        metadata: {
          type: "object",
          description: "Optional: Custom metadata for the SMS chat"
        }
      },
      required: ["from_number", "to_number", "agent_id"]
    }
  },

• src/index.ts:1283-1285 (registration)

  MCP server handler for listing tools, which returns the tools array containing the retell_create_sms_chat tool definition.

  server.setRequestHandler(ListToolsRequestSchema, async () => {
    return { tools };
  });

• src/index.ts:1288-1313 (registration)

  MCP server handler for calling tools, which invokes executeTool(name, args) dispatching to the specific handler case.

  server.setRequestHandler(CallToolRequestSchema, async (request) => {
    const { name, arguments: args } = request.params;
  
    try {
      const result = await executeTool(name, args as Record<string, unknown>);
      return {
        content: [
          {
            type: "text",
            text: JSON.stringify(result, null, 2),
          },
        ],
      };
    } catch (error) {
      const errorMessage = error instanceof Error ? error.message : String(error);
      return {
        content: [
          {
            type: "text",
            text: `Error: ${errorMessage}`,
          },
        ],
        isError: true,
      };
    }
  });

• src/index.ts:23-57 (helper)

  Helper function used by all tool handlers to make authenticated HTTP requests to the Retell AI API.

  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();
  }