OpenAPI repair

The nullable keyword is not supported by API Gateway

Decide explicitly how the API should handle null and omitted values.

Match the finding

These titles and messages come from this local checker. They are not AWS service error quotes.

Cause and scope

In OpenAPI 3.0, nullable: true permits null alongside a type declared on the same Schema Object, subject to the other constraints. API Gateway REST does not support nullable, so import can lose this part of the contract.

What to do

Keep the field until you have decided where nullability is enforced. If null is part of the contract, retain that behavior in application or other appropriate validation and verify the imported API accepts the intended requests.

  1. At the reported schema, identify its type and whether the containing object requires this property.
  2. Write tests for a normal value, an explicit null, and an omitted property, using the actual remaining schema constraints.
  3. Choose and test the validation layer that preserves the intended behavior. If the contract changes, update clients and documentation before removing the keyword.

Schema fragment: optional and nullable are separate choices

components:
  schemas:
    Pet:
      type: object
      required: [id]
      properties:
        id:
          type: string
        nickname:
          type: string
          nullable: true

For this fragment, nickname may be a string, null, or absent because it is not required. id must be present and a string. Keep tests for all three nickname cases; deleting nullable would remove the declared null case.

Avoid a misleading fix

Do not silently delete nullable, replace it with type: [string, null], or treat an omitted property as equivalent to a property whose value is null. Changing the accepted payloads needs a deliberate contract change.

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Related findings

Sources and scope

Scope: this local checker, API Gateway REST APIs, and OpenAPI 3.0. Guidance reviewed 4 October 2026. A passing check does not guarantee import or runtime behavior.