Jest + Testcontainers MongoDB: Intermittent ECONNREFUSED errors running integration tests

Problem

Starting November 10th, my Jest integration tests began failing randomly with MongoDB connection errors. The tests use @testcontainers/mongodb v11.5.1 and mongodb v8.0.12.

Error:

MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017
    at Timeout._onTimeout (node_modules/mongodb/src/sdam/topology.ts:591:30)

The failures are intermittent - sometimes all tests pass, sometimes a fair amount (20-50) fail with this connection error. If I run a single test file, the error might pop up there too.

● Product Import Integration Tests › Product Updates › Variable Products › Variation Creation › should create variations with correct SKUs

    MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:20017

      at Timeout._onTimeout (node_modules/mongodb/src/sdam/topology.ts:591:30)

  ● Product Import Integration Tests › Product Updates › Variable Products › Variation Creation › should create variations with correct SKUs

    MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:20017

      at Timeout._onTimeout (node_modules/mongodb/src/sdam/topology.ts:591:30)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 52 passed, 53 total
Snapshots:   0 total
Time:        55.721 s, estimated 65 s

Setup

Jest Configuration:

const config: Config = {
    globalSetup: "<rootDir>/tests/globalSetup.ts",
    globalTeardown: "<rootDir>/tests/globalTeardown.ts",
    projects: [
        {
            displayName: "integration",
            testEnvironment: "node",
            testMatch: ["<rootDir>/tests/integration/**/*.integration.test.ts"],
            setupFilesAfterEnv: ["<rootDir>/tests/integration/integrationSetup.ts"],
            testTimeout: 15000,
        },
    ],
};

globalSetup.ts (starts one shared MongoDB container):

import { MongoDBContainer } from "@testcontainers/mongodb";
import type { StartedMongoDBContainer } from "@testcontainers/mongodb";

declare global {
  var __MONGO_CONTAINER__: StartedMongoDBContainer | undefined;
}

export default async function globalSetup() {
  const mongoContainer = await new MongoDBContainer("mongo:8.0.12").start();
  global.__MONGO_CONTAINER__ = mongoContainer;
  process.env.MONGO_CONTAINER_URI = mongoContainer.getConnectionString();
}

integrationSetup.ts (runs per worker, creates isolated databases):

beforeAll(async () => {
  const baseMongoUri = process.env.MONGO_CONTAINER_URI;
  if (!baseMongoUri) {
    throw new Error("MongoDB container URI not found");
  }
  
  // Each worker gets its own database
  const workerId = process.env.JEST_WORKER_ID || "1";
  const dbName = `test-db-${workerId}`;
  const mongoUri = `${baseMongoUri}/${dbName}`;
  process.env.MONGODB_URI = mongoUri;
  
  await mongoose.connect(mongoUri, { directConnection: true });
}, 60000);

afterAll(async () => {
  if (mongoose.connection.readyState === 1) {
    await mongoose.connection.db.dropDatabase();
    await mongoose.disconnect();
  }
});

afterEach(async () => {
  if (mongoose.connection.readyState === 1 && mongoose.connection.db) {
    const collections = await mongoose.connection.db.collections();
    for (const collection of collections) {
      await collection.deleteMany({});
    }
  }
});

Env Configuration env.ts

import zennv from "zennv";
import dotenv from "dotenv";
import z from "zod";

// Load appropriate .env file based on NODE_ENV
// zennv uses dotenv under the hood, so we can load it here
// to ensure the environment variables are available before zennv processes them.
// https://github.com/tomanagle/zennv

// Only load dotenv if it hasn't been loaded already
if (!process.env.DOTENV_LOADED) {
  if (process.env.NODE_ENV === "test") {
    dotenv.config({ path: ".env.test", override: false });
  } else {
    dotenv.config({ override: false });
  }
  process.env.DOTENV_LOADED = "true";
}

export const env = zennv({
  dotenv: false, // We've already loaded dotenv
  schema: z.object({
    MONGODB_URI: z.string().default("mongodb://localhost:27017"),
  }),
});

What I've Tried

I changed the MongoDB port in .env.test from 27017 to 20017 to verify which configuration was being used. The error then showed ECONNREFUSED 127.0.0.1:20017, confirming that in some cases, tests are using the .env.test value instead of the dynamically set MONGODB_URI from the testcontainer.

This suggests that either:

1. Some tests start before globalSetup completes, or
2. Some workers don't receive the updated process.env.MONGODB_URI

Question

How can I ensure that all Jest workers reliably receive the MongoDB URI from integrationSetup before any tests execute?

Is there a race condition in my setup where parallel workers might start running tests before the environment variable is propagated? Should I be using a different mechanism to share the db connection string across workers?

Environment

• Node.js v22.18.0
• Jest v30.1.2