vitest-dev/vitest · error · Error

${getAssertionName(assertion)} cannot be used with "not"

Error message

${getAssertionName(assertion)} cannot be used with "not"

What it means

`validateAssertion` runs before every snapshot matcher and rejects usage with chai's negation flag (`.not`). Snapshot negation (`expect(x).not.toMatchSnapshot()`) has no meaningful semantics — there is no 'does not match this snapshot' update path — so Vitest throws `<matcher> cannot be used with "not"` to prevent a confusing always-failing assertion.

Source

Thrown at packages/vitest/src/integrations/snapshot/chai.ts:78

    throw new Error('Assertion name is not set. This is a bug in Vitest. Please, open a new issue with reproduction.')
  }
  return name
}

function getTest(obj: Chai.Assertion) {
  const test = chai.util.flag(obj, 'vitest-test') as Test | undefined
  if (!test) {
    throw new Error(`'${getAssertionName(obj)}' cannot be used without test context`)
  }
  if (test.fails) {
    throw new TestSyntaxError(`'${getAssertionName(obj)}' cannot be used with 'test.fails'`)
  }
  return test
}

function validateAssertion(assertion: Chai.Assertion): void {
  if (chai.util.flag(assertion, 'negate')) {
    throw new Error(`${getAssertionName(assertion)} cannot be used with "not"`)
  }
}

export const SnapshotPlugin: ChaiPlugin = (chai, utils) => {
  for (const key of ['matchSnapshot', 'toMatchSnapshot']) {
    utils.addMethod(
      chai.Assertion.prototype,
      key,
      wrapAssertion(utils, key, function (
        this,
        propertiesOrHint?: object | string,
        hint?: string,
      ) {
        const result = toMatchSnapshotImpl({
          assertion: this,
          received: utils.flag(this, 'object'),
          ...normalizeArguments(propertiesOrHint, hint),
        })

View on GitHub (pinned to d568f8ce37)

Solutions

1. Drop `.not` from the snapshot matcher.
2. To assert divergence from a stored snapshot, compare the value explicitly (e.g. `expect(value).not.toEqual(storedValue)`) and delete the stale snapshot with `-u` if needed.
3. Use `toMatchFileSnapshot` with intentional content if you want to lock a specific non-equality.

Example fix

// before
expect(result).not.toMatchSnapshot()
// after
expect(result).toMatchSnapshot()

// lint/grep your tests for `.not.toMatch` and `.not.toThrowErrorMatching` and remove `.not`

When it happens

Trigger: Writing `expect(value).not.toMatchSnapshot()`, `expect(value).not.toMatchInlineSnapshot()`, `.not.toMatchFileSnapshot()`, `.not.toThrowErrorMatchingSnapshot()`, or `.not.toThrowErrorMatchingInlineSnapshot()`.

Common situations: Copy-pasting an existing assertion and adding `.not`, or attempting to assert that a value has diverged from a stored snapshot.

Related errors

• expect.poll() is not supported in combination with .${key}()
• '${getAssertionName(obj)}' cannot be used with 'test.fails'
• expected must be a function, received ${typeof expected}
• snapshot function didn't throw
• '${getAssertionName(obj)}' cannot be used without test conte

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/612b79d7d2ac2c0d.json. Report an issue: GitHub.