@mergifyio/vitest@0.3.0: _configureRunner resolves dist/runner.js (ships .mjs/.cjs) → ERR_MODULE_NOT_FOUND under Vitest 4.x in CI

[seamnniel]

Summary

@mergifyio/vitest@0.3.0 (latest) breaks every test run in CI once the quarantine/flaky runner activates. MergifyReporter._configureRunner resolves the runner as dist/runner.js, but the package only ships dist/runner.mjs and dist/runner.cjs — there is no runner.js. Under Vitest 4.x this throws ERR_MODULE_NOT_FOUND and aborts the entire suite.

Root cause

_configureRunner builds the runner path with a hardcoded "runner.js":

// dist/index.mjs:116
const mergifyRunner = resolve(
  typeof __dirname !== "undefined" ? __dirname : dirname(fileURLToPath(import.meta.url)),
  "runner.js"
);
if (!vitest.config.runner) vitest.config.runner = mergifyRunner;

// dist/index.cjs:120
const mergifyRunner = (0, node_path.resolve)(/* … */, "runner.js");

But the build emits runner.mjs / runner.cjs (and the package exports map exposes them via ./runner → import: dist/runner.mjs, require: dist/runner.cjs). So the resolved absolute path …/dist/runner.js does not exist.

$ ls node_modules/@mergifyio/vitest/dist
index.cjs  index.mjs  runner.cjs  runner.mjs  …   # no runner.js

Reproduction

• Vitest 4.1.8, @mergifyio/vitest@0.3.0, ESM vite.config.ts registering new MergifyReporter().
• In CI (isInCI() true) with MERGIFY_TOKEN set and a non-empty quarantine list (so _configureRunner runs), every test errors:

Error: Cannot find module '…/node_modules/.pnpm/@mergifyio+vitest@0.3.0…/dist/runner.js'
  imported from …/vitest/dist/module-evaluator.js
{ code: 'ERR_MODULE_NOT_FOUND', url: 'file://…/dist/runner.js' }

It passes locally only because the reporter is gated on isInCI().

Suggested fix

Resolve the runner via the package's own ./runner export (so the correct .mjs/.cjs is chosen per build), e.g.:

const mergifyRunner = fileURLToPath(import.meta.resolve("@mergifyio/vitest/runner"));

or emit the runner with the same extension the entry resolves (runner.mjs from index.mjs, runner.cjs from index.cjs).

Workaround (for others hitting this)

Pre-pin the runner in your Vitest config so the reporter's broken assignment is skipped:

import { fileURLToPath } from "node:url";
const mergifyRunner = fileURLToPath(import.meta.resolve("@mergifyio/vitest/runner"));

export default defineConfig({
  test: {
    runner: mergifyRunner,
    reporters: ["default", new MergifyReporter()],
  },
});

Thanks for the plugin! Happy to test a patch.

[QuanticPotatoes]

Confirming this bug and adding two data points from a production monorepo hit today (2026-08-05):

• 0.3.1 is equally affected — dist/ still ships only runner.cjs/runner.mjs, and _configureRunner still resolves "runner.js". Bumping does not help.
• Vitest 3.2.4 is affected too, not just 4.x — same ERR_MODULE_NOT_FOUND, reproduced with new MergifyReporter({ quarantineList: ['x'] }).

Impact for us: the first quarantine entry ever created on the repo flipped the latent path on and took down every vitest tier org-wide (0 tests collected, one collect error per file) until we shipped the workaround.

Note for others applying the workaround: a bare specifier works as test.runner too (runner: '@mergifyio/vitest/runner' — vitest resolves it through the exports map), and the reporter then logs Custom runner already configured (…), quarantine may not work — that message is a false alarm, the configured runner is MergifyRunner and quarantine still flows via provide().

I've opened #253 with a fix (extension-derived sibling resolution) plus a dist-mode regression test — the in-repo tests couldn't catch this because vite-node remaps the .js specifier onto src/runner.ts, which native node resolution in a consumer's worker does not.