What causes "${getAssertionName(assertion)} cannot be used with "not"" in vitest-dev/vitest?

Drop the `.not` and update the snapshot to the expected value (or assert the specific difference with `toEqual`/`toMatchObject`).

vitest-dev/vitest · error · Error

cannot be used with "not

Error message

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

What it means

Thrown by `validateAssertion` when a snapshot assertion is chained with `.not` (the Chai negation flag is set). Snapshot matching is an equivalence check against a stored artifact; negating it ('this value does NOT match the stored snapshot') is ambiguous and not supported, so Vitest rejects the combination. The message names the offending assertion.

Solutions

Drop the `.not` and update the snapshot to the expected value (or assert the specific difference with `toEqual`/`toMatchObject`).
If you need to assert a value is NOT a specific shape, use `expect(value).not.toEqual(unwanted)` with an explicit expected rather than a snapshot.
For inline checks, compare against an explicit literal instead of an inline snapshot.

Example fix

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

// after
expect(result).toEqual({ status: 'ok' })

Defensive patterns

Strategy: validation

Validate before calling

// never chain .not before a snapshot matcher
// static check: reject 'not.toMatchSnapshot' / 'not.toMatchInlineSnapshot' in review

Prevention

Never use .not with snapshot matchers.
For 'is not equal to X' assertions, use toEqual with an explicit expected value.
Add an ESLint custom rule or code-review check to forbid .not.toMatchSnapshot.

When it happens

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

Common situations: Reaching for `.not` out of habit when asserting a value differs from a snapshot; misunderstanding that snapshots are write-once/compare rather than boolean matches.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/612b79d7d2ac2c0d. Report an issue: GitHub.

Appendix: 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),
        })

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