🐛 BUG: vitest-plugin coverage shim doesn't handle __vitest_stopV8Coverage under vitest 5

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
83/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
typescript
Domain
devtools, testing

Research direction

Start in packages/vitest-plugin/src/worker/index.ts and trace how the runner Durable Object handles coverage commands. Compare the existing start and take handlers with the Vitest 5 stop command described in the report. Done means npx vitest run --coverage exits 0 for the five-file reproduction without unhandled errors.

Written by the indexing model from the issue text.

Description

bug package:vitest
What versions & operating system are you using?
System:
  OS: macOS 27.0.1
  CPU: (10) arm64 Apple M4
Binaries:
  Node: 26.8.2
  npm: 11.19.1
npmPackages:
  @cloudflare/vitest-plugin: 1.4.0
  @vitest/coverage-v8: 5.0.4
  vitest: 5.0.4
  wrangler: 4.149.0
Please provide a link to a minimal reproduction

Five small files, inline:

package.json

{
  "type": "module",
  "devDependencies": {
    "@cloudflare/vitest-plugin": "1.4.0",
    "@vitest/coverage-v8": "5.0.4",
    "vitest": "5.0.4",
    "wrangler": "^4.135.0"
  }
}

wrangler.jsonc

{ "name": "repro", "main": "src/index.ts", "compatibility_date": "2026-10-01", "compatibility_flags": ["nodejs_compat"] }

src/index.ts

export default {
  fetch() {
    return new Response("ok");
  },
};

test/index.test.ts

import { SELF } from "cloudflare:test";
import { expect, it } from "vitest";

it("responds", async () => {
  const res = await SELF.fetch("https://example.com");
  expect(await res.text()).toBe("ok");
});

vitest.config.ts (fsModuleCache is off only to avoid a separate vitest 5 bug; see https://github.com/cloudflare/workers-sdk/issues/16237)

import { cloudflareTest } from "@cloudflare/vitest-plugin";
import { defineConfig } from "vitest/config";

export default defineConfig({
  plugins: [cloudflareTest({ wrangler: { configPath: "./wrangler.jsonc" } })],
  test: { fsModuleCache: false, coverage: { provider: "v8" } },
});

Then npx vitest run --coverage.

Describe the Bug

Under vitest 5, @vitest/coverage-v8 drives coverage in the isolate through __vitest_browser_runner__.commands.triggerCommand. Its stopCoverage sends __vitest_stopV8Coverage at the end of every test file (unless isolate: false). The shim the plugin installs in the runner Durable Object handles __vitest_startV8Coverage, __vitest_takeV8Coverage and __vitest_writeCoverageFile. It has no branch for __vitest_stopV8Coverage, so the command reaches assert.strictEqual(command, "__vitest_writeCoverageFile") and throws. This is still the case on main in packages/vitest-plugin/src/worker/index.ts.

Expected: coverage runs cleanly and exits 0.

Actual: the test passes, but Vitest reports one unhandled error per test file and exits 1. The coverage report itself is unaffected: the numbers are identical with the fix below. The inspector session isn't closed at the end of the file; the next __vitest_startV8Coverage closes it.

Suggested fix (we're carrying it as a postinstall patch against 1.4.0's built output, and the error is gone):

if (command === "__vitest_stopV8Coverage") {
  await this.stopV8CoverageSession();
  return "";
}

placed before the __vitest_writeCoverageFile assertion.

Please provide any relevant error logs
AssertionError: Values are not strictly equal:

-   __vitest_stopV8Coverage
+   __vitest_writeCoverageFile

 ❯ assert.strictEqual node-internal:internal_assert:196:11
 ❯ Object.triggerCommand index.mjs:863:11
 ❯ triggerCommand node_modules/@vitest/coverage-v8/dist/browser.js:7:55
 ❯ Object.stopCoverage node_modules/@vitest/coverage-v8/dist/browser.js:24:9
 ❯ stopCoverageInsideWorker node_modules/vitest/dist/chunks/coverage.-bm5IxnW.js:22:58
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