Fix discriminator values in writable type variants

[Copilot]

Writable type variants generated from schemas with discriminators were using schema names ('DogPayloadWritable') instead of preserving the original discriminator values ('dog') from the OpenAPI spec, causing backend deserialization failures.

Changes

Fixed discriminator values in writable type variants:

• Creates writable variants of parent schemas that have discriminators (e.g., AnimalPayloadWritable)
• Updates discriminator mappings in parent variants to point to writable child refs
• Updates $refs in writable children to point to the appropriate parent variant based on context (read/write)
• Updates discriminator mappings in oneOf/anyOf contexts to point to appropriate read/write variant refs

Test coverage:

• Added specs/3.1.x/discriminator-one-of-read-write.yaml demonstrating the issue with oneOf + discriminators + readOnly properties
• Added snapshot test in packages/openapi-ts-tests/main/test/3.1.x.test.ts to validate the fix

Solution

When schemas with discriminators are split into read/write variants, parent schemas (like AnimalPayload) that define discriminator mappings are now also cloned into writable variants (like AnimalPayloadWritable). The discriminator mappings in these parent variants are updated to point to the appropriate child variants:

• AnimalPayload (read) maps: dog -> DogPayload, cat -> CatPayload
• AnimalPayloadWritable (write) maps: dog -> DogPayloadWritable, cat -> CatPayloadWritable

Writable child schemas reference the writable parent variant via allOf, ensuring they inherit the correct discriminator mappings.

Result

All discriminator values now correctly preserve the original OpenAPI spec values:

export type DogPayloadWritable = Omit<AnimalPayloadWritable, 'typeDiscriminator'> & {
  breed: string;
  canFetch: boolean;
  typeDiscriminator: 'dog';  // ✓ Correct - preserves spec value
};

export type CatPayloadWritable = Omit<AnimalPayloadWritable, 'typeDiscriminator'> & {
  breed: string;
  livesRemaining: number;
  typeDiscriminator: 'cat';  // ✓ Correct - preserves spec value
};

export type CreatePetRequestWritable = {
  name: string;
  animal: ({
    typeDiscriminator: 'dog';  // ✓ Correct in oneOf contexts
  } & DogPayloadWritable) | ({
    typeDiscriminator: 'cat';  // ✓ Correct in oneOf contexts
  } & CatPayloadWritable);
};

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Write schemas generated for oneOf types override type discriminator values</issue_title>
<issue_description>### Description

When @hey-api/openapi-ts generates TypeScript types from an OpenAPI spec containing polymorphic types with readOnly properties, it creates "Writable" variants with incorrect typeDiscriminator values.

The generated types use a PascalCase discriminator derived from the type name (e.g., 'DogPayloadWritable') instead of preserving the discriminator value from the OpenAPI spec (e.g., 'dog').

This causes deserialization failures in the backend that still expects the original type discriminators it has defined.

Interestingly enough, this issue wasn't present in @hey-api/openapi-ts 0.84.4, which I suppose might have been due to writable types not being generated properly with polymorphic types so this bug never surfaced itself because of that.

Reproducible example or configuration

Environment

• @hey-api/openapi-ts: v0.90.10
• Backend: ASP.NET Core with System.Text.Json
• OpenAPI generator: Swashbuckle

Reproduction

.NET models

[JsonPolymorphic(TypeDiscriminatorPropertyName = "typeDiscriminator")]
[JsonDerivedType(typeof(DogPayload), "dog")]
[JsonDerivedType(typeof(CatPayload), "cat")]
public abstract record AnimalPayload;

public record DogPayload : AnimalPayload
{
    public required string Breed { get; init; }
    public required bool CanFetch { get; init; }

    /// Computed property - marked as readOnly in OpenAPI schema.
    /// This triggers hey-api to generate a Writable variant.
    public string DisplayBreed => $"{Breed} (Dog)";
}

public record CatPayload : AnimalPayload
{
    public required string Breed { get; init; }
    public required int LivesRemaining { get; init; }

    /// Computed property - marked as readOnly in OpenAPI schema.
    /// This triggers hey-api to generate a Writable variant.
    public string DisplayBreed => $"{Breed} (Cat)";
}

public record CreatePetRequest
{
    public required string Name { get; init; }
    public required AnimalPayload Animal { get; init; }
}

Generated TS Read types (correct)

export type DogPayload = Omit<AnimalPayload, 'typeDiscriminator'> & {
  breed: string
  canFetch: boolean
  readonly displayBreed: string
  typeDiscriminator: 'dog'  // Correct - matches OpenAPI spec
}

export type CatPayload = Omit<AnimalPayload, 'typeDiscriminator'> & {
  breed: string
  livesRemaining: number
  readonly displayBreed: string
  typeDiscriminator: 'cat'  // Correct - matches OpenAPI spec
}

Generated TS Writable types (incorrect):

export type DogPayloadWritable = Omit<AnimalPayload, 'typeDiscriminator'> & {
  breed: string
  canFetch: boolean
  typeDiscriminator: 'DogPayloadWritable'  // should be 'dog'
}

export type CatPayloadWritable = Omit<AnimalPayload, 'typeDiscriminator'> & {
  breed: string
  livesRemaining: number
  typeDiscriminator: 'CatPayloadWritable'  // should be 'cat'
}

export type CreatePetRequestWritable = {
  name: string
  animal: DogPayloadWritable | CatPayloadWritable  // Uses wrong discriminators
}

OpenAPI specification (optional)

OpenAPI Schema (Backend)

components:
  schemas:
    AnimalPayload:
      type: object
      required: [typeDiscriminator]
      properties:
        typeDiscriminator:
          type: string
      discriminator:
        propertyName: typeDiscriminator
        mapping:
          dog: '#/components/schemas/DogPayload'
          cat: '#/components/schemas/CatPayload'

    DogPayload:
      allOf:
        - $ref: '#/components/schemas/AnimalPayload'
        - type: object
          required: [breed, canFetch, displayBreed]
          properties:
            breed:
              type: string
            canFetch:
              type: boolean
            displayBreed:
              type: string
              readOnly: true  # <-- This triggers Writable variant generation

    CatPayload:
      allOf:
        - $ref: '#/components/schemas/AnimalPayload'
        - type: object
          required: [breed, livesRemaining, displayBreed]
          properties:
            breed:
              type: string
            livesRemaining:
              type: integer
            displayBreed:
              type: string
              readOnly: true  # <-- This triggers Writable variant generation

    CreatePetRequest:
      type: object
      required: [name, animal]
      properties:
        name:
          type: string
        animal:
          oneOf:
            - $ref: '#/components/schemas/DogPayload'
            - $ref: '#/components/schemas/CatPayload'

System information (optional)

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Write schemas generated for oneOf types override type discriminator values #3293

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commit: 7e6945d

[mrlubos]

@copilot you need to add an actual snapshot test using the spec you added in this pull request so we can verify the fix works

[Copilot]

@copilot you need to add an actual snapshot test using the spec you added in this pull request so we can verify the fix works

Added snapshot test for the new spec in commit 0b6a4fd. The test validates the current behavior where discriminator values in oneOf contexts are correct, while documenting the remaining issue with allOf inheritance.

[mrlubos]

@copilot have a look at the "Generated TS Writable types (incorrect):" section in the original issue and make sure you actually address the problems. So far, you've only confirmed we can reproduce the issue

[Copilot]

@copilot have a look at the "Generated TS Writable types (incorrect):" section in the original issue and make sure you actually address the problems. So far, you've only confirmed we can reproduce the...

Fixed in commit 8126ad1. Writable types now correctly preserve discriminator values from the OpenAPI spec (e.g., 'dog' and 'cat') instead of using schema names (e.g., 'DogPayloadWritable'). The solution creates writable variants of parent schemas with updated discriminator mappings and updates child schemas to reference the appropriate parent variant.