openapi-32-discriminator-default-mapping.md

title	OpenAPI 3.2: Use discriminator defaultMapping for more accurate union types
parentIssue	1673
labels	enhancement openapi-3.2
assignees
milestone

OpenAPI 3.2 adds a defaultMapping field to the Discriminator Object. This field specifies which schema to use when propertyName is absent or the discriminator value is not found in the mapping. It improves the accuracy of polymorphic schema representations.

Current state

SchemaTypeCoder.writeGroup() in src/typescript-generator/schema-type-coder.ts generates union types based on allOf/anyOf/oneOf but does not inspect the discriminator.defaultMapping field. As a result, the default variant of a discriminated union may be missing from the generated TypeScript type.

Proposed changes

• In SchemaTypeCoder.writeGroup(), read the discriminator.defaultMapping field when present
• Include the default variant schema in the emitted union type so the generated TypeScript accurately represents all possible values

Acceptance criteria

• A discriminated schema with defaultMapping generates a union type that includes the default variant
• The generated TypeScript for such a schema passes tsc --noEmit
• A discriminated schema without defaultMapping continues to generate the same types as before
• A unit test covers a discriminator with defaultMapping and confirms the correct union type is emitted