Lambda@Edge "The Lambda function result failed validation" in CI

CloudFront validates the object a Lambda@Edge function returns against a strict schema for each event type. A wrong status type, a missing field, or an illegal header makes CloudFront reject the result and return an error to the viewer.

What this error means

CloudFront returns a 502 and CloudWatch logs show "The Lambda function result failed validation: The body is not a string", "The status code ... is invalid", or a similar shape error during integration tests in CI.

The Lambda function result failed validation: The body is not a string,
is not an object, or exceeds the maximum size.

Common causes

The returned object has the wrong shape

A viewer response must return an object with a string status, a headers map of the right structure, and a string body. A number status or a malformed headers entry fails validation.

A disallowed or oversized header or body

The function sets a read-only header, exceeds the allowed body size for the event, or omits required fields, which CloudFront rejects.

How to fix it

Return the exact required shape

1. Check the event type (viewer/origin, request/response) and its schema.
2. Return status as a string and headers as the documented key/value-array map.
3. Re-run the integration test against CloudFront to confirm it validates.

callback(null, {
  status: '302',
  statusDescription: 'Found',
  headers: { location: [{ key: 'Location', value: '/new' }] },
});

Avoid setting read-only headers

Do not write headers CloudFront marks read-only for that event, and keep the generated body within the size limit.

How to prevent it

• Match the returned object to the schema for the specific event type.
• Use string status codes and the documented headers structure.
• Test the function against CloudFront, not just with a local invoke.

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