500 “Internal Server Error” when using TCP communication between NestJS microservices (Turborepo / Monorepo)

I’m building a monorepo using Turborepo and NestJS, following a microservices architecture with TCP communication between services.

Here’s my folder structure:

   apps/
 ├── api-gateway/
 │   ├── src/
 │   ├── package.json
 │   ├── tsconfig.json
 │   └── ...
 ├── auth-service/
 │   ├── src/
 │   ├── package.json
 │   ├── tsconfig.json
 │   └── ...
 └── notification-service/

I’m trying to make the api-gateway communicate with the auth-service via TCP, but I keep getting the following error on Insomnia:

{
  "statusCode": 500,
  "message": "Internal server error"
}

api-gateway/app.module.ts

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';
import { ClientsModule, Transport } from '@nestjs/microservices';
@Module({
  imports: [
    ClientsModule.register([
      {
        name: 'AUTH_SERVICE',
        transport: Transport.TCP,
        options: {
          host: '127.0.0.1',
          port: 3001,
        },
      },
    ]),
  ],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

auth-service/main.ts

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { MicroserviceOptions, Transport } from '@nestjs/microservices';
import { Logger } from '@nestjs/common';

async function bootstrap() {
  const logger = new Logger('AuthService');
  console.log('ola');
  try {
    const app = await NestFactory.createMicroservice<MicroserviceOptions>(
      AppModule,
      {
        transport: Transport.TCP,
        options: {
          host: '127.0.0.1',
          port: 3001,
        },
      },
    );

    await app.listen();
    logger.log('Auth Service is listening on port 8877');
  } catch (error) {
    logger.error('Failed to start Auth Service', error);
    process.exit(1);
  }
}

bootstrap();

What I’ve checked so far:

• The auth-service starts successfully with no errors.
• The api-gateway runs fine.
• Both services are running on the same host (127.0.0.1).
• Port 3001 is free before running the services.

Still, whenever I call a route from the api-gateway that depends on the AUTH_SERVICE, I get a 500 Internal Server Error.