Vapi MCP Server

Instructions

Implementation Reference

• src/tools/call.ts:15-23 (registration)

  Registration of the 'list_calls' MCP tool. It takes no input parameters and returns a list of the 10 most recent Vapi calls, transformed using transformCallOutput.

  server.tool(
    'list_calls',
    'Lists all Vapi calls',
    {},
    createToolHandler(async () => {
      const calls = await vapiClient.calls.list({ limit: 10 });
      return calls.map(transformCallOutput);
    })
  );

• src/tools/call.ts:19-22 (handler)

  The core handler logic for the list_calls tool: fetches the list of recent calls from Vapi API and maps each to the output format.

  createToolHandler(async () => {
    const calls = await vapiClient.calls.list({ limit: 10 });
    return calls.map(transformCallOutput);
  })

• src/schemas/index.ts:290-301 (schema)

  Zod schema defining the structure of individual call outputs returned by the list_calls tool.

  export const CallOutputSchema = BaseResponseSchema.extend({
    status: z.string(),
    endedReason: z.string().optional(),
    assistantId: z.string().optional(),
    phoneNumberId: z.string().optional(),
    customer: z
      .object({
        number: z.string(),
      })
      .optional(),
    scheduledAt: z.string().optional(),
  });

• src/tools/utils.ts:30-41 (helper)

  Utility function that wraps the raw handler with try-catch error handling and standardizes the ToolResponse format.

  export function createToolHandler<T>(
    handler: (params: T) => Promise<any>
  ): (params: T) => Promise<ToolResponse> {
    return async (params: T) => {
      try {
        const result = await handler(params);
        return createSuccessResponse(result);
      } catch (error) {
        return createErrorResponse(error);
      }
    };
  }

• src/transformers/index.ts:211-229 (helper)

  Helper function that transforms raw Vapi.Call objects to the standardized CallOutputSchema format used in list_calls response.

  export function transformCallOutput(
    call: Vapi.Call
  ): z.infer<typeof CallOutputSchema> {
    return {
      id: call.id,
      createdAt: call.createdAt,
      updatedAt: call.updatedAt,
      status: call.status || '',
      endedReason: call.endedReason,
      assistantId: call.assistantId,
      phoneNumberId: call.phoneNumberId,
      customer: call.customer
        ? {
            number: call.customer.number || '',
          }
        : undefined,
      scheduledAt: call.schedulePlan?.earliestAt,
    };
  }