What version of Bun is running?

1.4.3 (c6da4a4). 1.4.2 (744846f) and 1.4.4-canary.1+0c864a07c behave the same.

What platform is your computer?

macOS 26.5.1, arm64

What steps can reproduce the bug?

An empty package.json and two test files:

a.test.ts

import { expect, test } from "bun:test";

test("a fails", () => {
  console.log("A1 printed by a.test.ts before its failure");
  expect(1).toBe(2);
});

test("a passes after the failure", () => {
  console.log("A2 printed by a.test.ts after its failure");
});

b.test.ts

import { test } from "bun:test";

test("b passes", async () => {
  await Bun.sleep(20);
  console.log("B1 printed by b.test.ts");
});

Run it about ten times:

bun test --parallel=2 --no-isolate

What is the expected behavior?

Every line a file's tests print shows under that file's header, as it always does with --isolate:

a.test.ts:
A1 printed by a.test.ts before its failure
error: expect(received).toBe(expected)
...
(fail) a fails
A2 printed by a.test.ts after its failure

b.test.ts:
B1 printed by b.test.ts

What do you see instead?

In about half the runs, the line a.test.ts prints after its failure shows under b.test.ts's header:

a.test.ts:
A1 printed by a.test.ts before its failure
error: expect(received).toBe(expected)

Expected: 2
Received: 1

      at <anonymous> (a.test.ts:5:13)
(fail) a fails [0.14ms]

b.test.ts:
A2 printed by a.test.ts after its failure
B1 printed by b.test.ts

Measured over 10 runs each: 5 with --no-isolate on 1.4.3, 4 on the canary, and 0 with --isolate.

Additional information

A reader trusts the header to say which file printed a line, so a log explaining a failure gets read as another file's. In a suite of 122 files on 8 workers, such a line landed under an unrelated file's header.

Drafted by Claude Opus 5.5 and posted by Marcos Pereira.