fix(vitest): render non-Error causes from env setup so the actual diagnostic is not dropped

[spokodev]

What

When a custom test environment's setup() rejects with a non-Error value (a string, a plain object, or a wasm-bindgen JsValue), the actual diagnostic is silently dropped and the user only sees the generic, location-free [vitest-pool]: Failed to start <pool> worker for test files … from the pool wrapper.

Vitest 3.x rendered the same non-Error throw as Unknown Error: <value>, so the silent drop is a regression. After this PR the cause surfaces correctly for Error, plain-object, and primitive throws.

Closes #10557.

Why / root cause

The pool wrapper in packages/vitest/src/node/pools/pool.ts:127 already attaches the original failure as { cause: error } on the synthesised "Failed to start … worker" Error. The reporter in packages/vitest/src/node/printError.ts then descended into cause only when it satisfied a narrow shape check:

if (typeof e.cause === 'object' && e.cause && 'name' in e.cause) {
  (e.cause as any).name = `Caused by: ${(e.cause as any).name}`
  printErrorInner(e.cause, project, {...})
}

That filter passes for new Error(...) (object + truthy + has name) but excludes:

• throw "string" — typeof "string" === "object" is false, so the branch is skipped entirely.
• throw { reason: '...' } — object + truthy, but no name property, so 'name' in cause is false.
• wasm-bindgen JsValue — object + truthy, but no name (and in some bindings, 'name' in cause throws on the proxy).

In all three cases the value reaches printError via the cause chain but never reaches printErrorInner, so the actual diagnostic is lost.

The regression most likely landed alongside the worker-start error wrapping in #9337 (which correctly fixed the indefinite-hang behaviour from #9271). The wrapper is well-formed; only the reporter's filter is too narrow.

The fix normalises the cause into a printable shape before calling printErrorInner — which already knows how to handle primitives via its isPrimitive branch:

• Primitive cause → wrap as { name: 'Caused by', message: String(cause), stack: String(cause) }.
• Object without name → spread + name: 'Caused by'.
• Object with name → preserve the existing behaviour of prefixing name with Caused by:.

printErrorInner then renders the message exactly as it would for a top-level non-Error throw, mirroring Vitest 3.x behaviour. Real-world impact: native/wasm environments (e.g. @stacks/clarinet-sdk's vitest-environment-clarinet) reject from their wasm setup() with a JsValue when contract compilation fails — the failure reason now surfaces instead of being lost behind "Failed to start forks worker".

Tests added

New e2e test at test/e2e/test/env-setup-non-error-cause.test.ts using runInlineTests from test/test-utils. Three cases, each booting Vitest against a fixture environment whose setup() throws a different shape:

1. non-Error string thrown from env setup surfaces in stderr — the bug case from A non-Error value thrown from a test environment's setup() is dropped from the "Failed to start worker" report (regression from 3.x) #10557. Without this PR the string never appears in stderr; with it, the the real reason setup failed (a non-Error value) substring shows up.
2. non-Error plain object thrown from env setup surfaces in stderr — boundary H-case (wasm JsValue analogue). Without the PR the object is dropped entirely; with it, the object's keys / serialised value appear in stderr.
3. Error thrown from env setup still renders with Caused by prefix — happy-path regression. Confirms the existing Caused by: Error: … rendering is unchanged for proper Error instances.

Verified locally:

pnpm -F vitest build
CI=true pnpm test env-setup-non-error-cause   # 3/3 passing after fix
CI=true pnpm test print-error                 # 2/2 passing (no regression on existing reporter tests)
CI=true pnpm test pool.test                   # 16/16 passing (no regression on pool tests)

Before the fix: tests 1 and 2 fail with Expected to contain '…the real reason…' / Expected to match /wasm-init-failed|reason/. Test 3 already passed on master.

Repro

Minimal repro from the issue (https://stackblitz.com/edit/vitest-dev-vitest-ldzxobzz):

// throwing-env.js
export default {
  name: 'throwing-env',
  viteEnvironment: 'ssr',
  async setup() {
    throw 'the real reason setup failed (a non-Error value)'
  },
}

// vitest.config.js
import { defineConfig } from 'vitest/config'
export default defineConfig({
  test: { environment: './throwing-env.js' },
})

pnpm test shows only the generic Failed to start forks worker message on master; with this PR the cause string appears under Caused by:.

Validation

• pnpm -F vitest build — green
• CI=true pnpm test env-setup-non-error-cause — 3/3
• CI=true pnpm test print-error — 2/2 (no regression)
• CI=true pnpm test pool.test — 16/16 (no regression)
• pnpm lint:fix — clean

AI usage disclosure

Drafted with Claude assistance. The root-cause analysis (the 'name' in cause narrow filter and its three failure modes), the fix design, the test surface choice (runInlineTests instead of unit-testing printError), and this PR text are mine. I verified the red baseline → fix → green baseline loop locally and have read every line of the diff. I will engage genuinely with any review feedback within the 3-day window described in AGENTS.md.

[github-actions]

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[github-actions]

Hello @spokodev. Your PR has been labeled maybe automated because it appears to have been fully generated by AI with no human involvement.

To keep your PR open, please follow these steps:

• Confirm that you are a real human. If you are an automated agent, disclose that
• Confirm you've read, reviewed and stand behind its content
• Confirm you've read the full issue along with all of its comments, as well as any linked issues and their comments
• Make sure it follows our contribution guidelines and uses the correct GitHub template
• Disclose any AI tools you used (e.g. Claude, Copilot, Codex)

Please, do not generate or format the response with AI. If you do not speak English, reply in your native language or use translation software like Google Translate or Deepl. If the response is generated, the PR will be closed automatically.

These measures help us reduce maintenance burden and keep the team's work efficient. See our AI contributions policy for more context.

[spokodev]

Hi, this is Yarik. Spoko-dev.
Of course, I'm human.
I've read everything carefully and agree with everything. The fix is at
printError.ts:249 where the 'name' in cause filter let Error instances
through and silently dropped strings / plain objects / wasm JsValues.
Of course, I also use Claude for writing drafts. But rest assured, I'm
human:) I'm writing with my hands on the keyboard, everything is fine,
don't worry:)

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