Review requested:

• @nodejs/test_runner

@BridgeAR PTAL

Codecov Report

❌ Patch coverage is 81.48148% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.74%. Comparing base (f6464c5) to head (0fee869).
⚠️ Report is 59 commits behind head on main.

@@            Coverage Diff             @@
##             main   #61716      +/-   ##
==========================================
- Coverage   89.76%   89.74%   -0.03%     
==========================================
  Files         673      676       +3     
  Lines      203944   204718     +774     
  Branches    39191    39352     +161     
==========================================
+ Hits       183080   183720     +640     
- Misses      13194    13272      +78     
- Partials     7670     7726      +56     

Sharing this investigation in case it helps review context.

I did a deeper investigation to clarify why output differs across the three cases described in the original issue comment:
#50397 (comment)

I used a one-off custom reporter (not committed) to inspect the parent-side test:fail payload, and I ran this from a fresh main build.

Commands used

# Case 1 (node --test + node:test) with payload inspection
./out/Release/node --test --test-isolation=process --test-reporter=./tmp/debug-reporter.mjs ./tmp/repro-50397.mjs
./out/Release/node --test --test-isolation=none --test-reporter=./tmp/debug-reporter.mjs ./tmp/repro-50397.mjs

# Case 2 (node --test without node:test) with payload inspection
./out/Release/node --test --test-isolation=process --test-reporter=./tmp/debug-reporter.mjs ./tmp/repro-50397-no-node-test.mjs

User-facing output commands (by case)

# Case 2-A: node --test (without node:test), process isolation
./out/Release/node --test --test-isolation=process ./tmp/repro-50397-no-node-test.mjs

# Case 2-B: plain node (without node:test), non-test-runner reference
./out/Release/node ./tmp/repro-50397-no-node-test.mjs

# Case 3: plain node + node:test (single-process node:test path)
./out/Release/node ./tmp/repro-50397-with-node-test.mjs

Key observed payload values

• Case 1 (node --test + node:test, --test-isolation=process):

  • errorCode: "ERR_TEST_FAILURE"
  • causeCode: "ERR_ASSERTION"
  • causeActualCtor: "Array" (ExtendedArray identity lost across boundary)
  • causeExpectedCtor: "Array"
  • causeDiffValue: "simple"

• Case 1 (node --test + node:test, --test-isolation=none):

  • causeActualCtor: "ExtendedArray"
  • causeExpectedCtor: "Array"

• Case 2 (node --test without node:test, --test-isolation=process):

  • errorCode: "ERR_TEST_FAILURE"
  • causeType: "[object String]"
  • causeCode: null
  • i.e. parent does not receive an AssertionError object as cause in this path

Observed behavior by case

Case 1 (node --test + node:test)

• --test-isolation=process:

  • structured test:fail event path
  • details.error is ERR_TEST_FAILURE
  • details.error.cause is AssertionError, but cause.actual.constructor.name is Array (identity lost across boundary)

• --test-isolation=none:

  • same structured event path, but cause.actual.constructor.name remains ExtendedArray

• this is the path affected by child->parent serialization/deserialization (and where this PR applies constructor/prototype identity restoration metadata)

Case 2 (node --test without node:test, observed with --test-isolation=process)

• different path: parent mainly sees test process failure + stderr forwarding
• assertion rendering comes from child-side stderr text, not parent-side reconstructed assertion object

Case 3 (node + node:test)

• no --test-isolation boundary here; this is the single-process node:test bootstrap path
• output shape difference is explained by AssertionError inspect behavior (actual: [ExtendedArray]), not the same boundary issue as Case 1

So the regression is specifically that, under --test-isolation=process, the parent-side AssertionError.cause.actual is deserialized as a plain Array, losing the ExtendedArray identity; this PR transports minimal metadata to restore that identity for reporting.

Conclusion

This PR addresses Case 1 (structured node:test + --test boundary, specifically --test-isolation=process).

Case 2 and Case 3 are different mechanisms and should be considered separate follow-up scopes if needed.

// ./tmp/debug-reporter.mjs
export default async function* debugReporter(source) {
  for await (const event of source) {
    if (event.type !== 'test:fail') continue;

    const details = event.data?.details ?? {};
    const error = details.error;
    const cause = error?.cause;

    const summary = {
      eventType: event.type,
      testName: event.data?.name ?? null,
      detailsKeys: Object.keys(details).sort(),

      errorCode: error?.code ?? null,
      errorKeys: error ? Object.keys(error).sort() : [],

      causeType: cause ? Object.prototype.toString.call(cause) : null,
      causeCode: cause?.code ?? null,
      causeKeys: cause && typeof cause === 'object' ? Object.keys(cause).sort() : [],
      causeHasActual: !!cause && Object.prototype.hasOwnProperty.call(cause, 'actual'),
      causeHasExpected: !!cause && Object.prototype.hasOwnProperty.call(cause, 'expected'),
      causeActualCtor: cause?.actual?.constructor?.name ?? null,
      causeExpectedCtor: cause?.expected?.constructor?.name ?? null,
      causeDiffType: typeof cause?.diff,
      causeDiffValue: typeof cause?.diff === 'string' ? cause.diff : null,
    };

    console.log('@@DEBUG_TEST_FAIL_EVENT@@' + JSON.stringify(summary));
  }
}

// ./tmp/repro-50397.mjs (case 1)
import assert from 'node:assert/strict';
import test from 'node:test';

class ExtendedArray extends Array {}

test('deepStrictEqual should print prototype diff', () => {
  assert.deepStrictEqual(new ExtendedArray('hello'), ['hello']);
});

// ./tmp/repro-50397-no-node-test.mjs (case 2)
import assert from 'node:assert/strict';

class ExtendedArray extends Array {}

assert.deepStrictEqual(new ExtendedArray('hello'), ['hello']);

// ./tmp/repro-50397-with-node-test.mjs (case 3)
import test from 'node:test';
import assert from 'node:assert/strict';

class ExtendedArray extends Array {}

test('deepStrictEqual should print prototype diff', () => {
  assert.deepStrictEqual(new ExtendedArray('hello'), ['hello']);
});