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The concurrentTestGlob option in bunfig.toml runs tests concurrently in files whose names match a glob pattern.

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Project Structure

Project Structure
my-project/
├── bunfig.toml
├── tests/
│   ├── unit/
│   │   ├── math.test.ts          # Sequential
│   │   └── utils.test.ts         # Sequential
│   └── integration/
│       ├── concurrent-api.test.ts     # Concurrent
│       └── concurrent-database.test.ts # Concurrent

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Configuration

Configure your bunfig.toml to run test files with the “concurrent-” prefix concurrently:
bunfig.toml
[test]
# Run all test files with "concurrent-" prefix concurrently
concurrentTestGlob = "**/concurrent-*.test.ts"

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Test Files

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Unit Test (Sequential)

Tests that share state or depend on ordering should stay sequential:
https://mintcdn.com/bun-1dd33a4e/JUhaF6Mf68z_zHyy/icons/typescript.svg?fit=max&auto=format&n=JUhaF6Mf68z_zHyy&q=85&s=7ac549adaea8d5487d8fbd58cc3ea35btests/unit/math.test.ts
import { test, expect } from "bun:test";

// These tests run sequentially by default
let sharedState = 0;

test("addition", () => {
  sharedState = 5 + 3;
  expect(sharedState).toBe(8);
});

test("uses previous state", () => {
  // This test depends on the previous test's state
  expect(sharedState).toBe(8);
});

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Integration Test (Concurrent)

Tests in files matching the glob pattern automatically run concurrently:
https://mintcdn.com/bun-1dd33a4e/JUhaF6Mf68z_zHyy/icons/typescript.svg?fit=max&auto=format&n=JUhaF6Mf68z_zHyy&q=85&s=7ac549adaea8d5487d8fbd58cc3ea35btests/integration/concurrent-api.test.ts
import { test, expect } from "bun:test";

// These tests automatically run concurrently due to filename matching the glob pattern.
// Using test() is equivalent to test.concurrent() when the file matches concurrentTestGlob.
// Each test is independent and can run in parallel.

test("fetch user data", async () => {
  const response = await fetch("/api/user/1");
  expect(response.ok).toBe(true);
});

// can also use test.concurrent() for explicitly marking it as concurrent
test.concurrent("fetch posts", async () => {
  const response = await fetch("/api/posts");
  expect(response.ok).toBe(true);
});

// can also use test.serial() for explicitly marking it as sequential
test.serial("fetch comments", async () => {
  const response = await fetch("/api/comments");
  expect(response.ok).toBe(true);
});

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Running Tests

terminal
# Run all tests - concurrent-*.test.ts files will run concurrently
bun test

# Override: Force ALL tests to run concurrently
# Note: This overrides bunfig.toml and runs all tests concurrently, regardless of glob
bun test --concurrent

# Run only unit tests (sequential)
bun test tests/unit

# Run only integration tests (concurrent due to glob pattern)
bun test tests/integration

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Benefits

  1. Gradual migration: rename files one at a time to move them to concurrent execution
  2. Clear organization: the filename tells you how a file’s tests run
  3. Performance: independent integration tests finish faster in parallel
  4. Safety: unit tests stay sequential where they need to

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Migration Strategy

To migrate existing tests to concurrent execution:
  1. Start with independent integration tests - these typically don’t share state
  2. Rename files to match the glob pattern: mv api.test.ts concurrent-api.test.ts
  3. Run bun test - check for race conditions and flaky or unexpected failures
  4. Continue migrating stable tests file by file

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Tips

  • Use descriptive prefixes: concurrent-, parallel-, async-
  • Keep related sequential tests together in the same directory
  • Document why certain tests must remain sequential with comments
  • Use test.concurrent() for fine-grained control in sequential files (in files matched by concurrentTestGlob, plain test() already runs concurrently)

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Multiple Patterns

concurrentTestGlob also accepts multiple patterns:
bunfig.toml
[test]
concurrentTestGlob = [
  "**/integration/*.test.ts",
  "**/e2e/*.test.ts",
  "**/concurrent-*.test.ts"
]
Tests in files matching any of these patterns run concurrently:
  • All tests in integration/ directories
  • All tests in e2e/ directories
  • All tests with concurrent- prefix anywhere in the project