feat(transformers): add support for discriminator/oneOf schemas

[Copilot]

Problem

The @hey-api/transformers plugin did not support discriminated unions (oneOf with discriminator). When using schemas with discriminator and oneOf, the plugin would generate a warning and fail to produce transformer functions:

❗️ Transformers warning: schema {...} is too complex and won't be currently processed.

This prevented users from combining discriminated unions with transformers for date-time and other transformable properties.

Example

Given an OpenAPI schema with a discriminated union:

{
  "Animal": {
    "oneOf": [
      { "$ref": "#/components/schemas/Dog" },
      { "$ref": "#/components/schemas/Cat" }
    ],
    "discriminator": {
      "propertyName": "type"
    }
  },
  "Dog": {
    "properties": {
      "type": { "type": "string", "enum": ["dog"] },
      "firstBark": { "type": "string", "format": "date-time" }
    }
  },
  "Cat": {
    "properties": {
      "type": { "type": "string", "enum": ["cat"] },
      "firstMeow": { "type": "string", "format": "date-time" }
    }
  }
}

Previously, no transformer would be generated. Now, the plugin correctly generates:

const dogSchemaResponseTransformer = (data: any) => {
    if (data.firstBark) {
        data.firstBark = new Date(data.firstBark);
    }
    return data;
};

const catSchemaResponseTransformer = (data: any) => {
    if (data.firstMeow) {
        data.firstMeow = new Date(data.firstMeow);
    }
    return data;
};

Solution

1. Added discriminated union support: Modified the transformers plugin to handle schemas with logicalOperator === 'or' by processing each variant and collecting all transformations.

2. Made transformations defensive: All property transformations now include existence checks to prevent creating Invalid Date objects when properties don't exist on certain union variants.

Changes

• Modified packages/openapi-ts/src/plugins/@hey-api/transformers/plugin.ts to:
  • Process oneOf schemas with discriminators
  • Add safety checks for all property transformations
• Added test case: specs/3.1.x/transformers-discriminator-one-of.json
• Updated test snapshots across all OpenAPI versions

Testing

• All 612 existing tests pass
• New test case validates discriminated union transformer generation
• Manual verification confirms correct behavior with the issue's example

Closes #2806

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Polymorphism/discriminator/oneOf in combination with transformers</issue_title>
<issue_description>### Description

We are trying to combine oneOf + discriminator with transformers, but it seems the transformer code doesn't support it?

Are there any plans to support this combination?

Reproducible example or configuration

openapi-ts --file openapi-ts.config.ts --input openapi.json

openapi-ts.config.ts:

import { defineConfig } from "@hey-api/openapi-ts";
export default defineConfig({
    input: "openapi.json",
    output: "src/openapi",
    plugins: [
        "@tanstack/react-query",
        {
            name: "@hey-api/typescript",
            enums: "typescript"
        },
        {
            name: "@hey-api/sdk",
            transformer: true
        },
        {
            bigInt: true,
            dates: true,
            name: "@hey-api/transformers"
        }
    ]
});

OpenAPI specification (optional)

{
    "openapi": "3.0.4",
    "paths": {
        "/api/pets/{id}": {
            "get": {
                "operationId": "getPeyById",
                "responses": {
                    "200": {
                        "description": "OK",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/Animal"
                                }
                            }
                        }
                    }
                }
            }
        }
    },
    "components": {
        "schemas": {
            "Animal": {
                "oneOf": [{
                        "$ref": "#/components/schemas/Dog"
                    }, {
                        "$ref": "#/components/schemas/Cat"
                    }
                ],
                "discriminator": {
                    "propertyName": "type",
                    "mapping": {
                        "dog": "#/components/schemas/Dog",
                        "cat": "#/components/schemas/Cat"
                    }
                }
            },
            "Dog": {
                "type": "object",
                "properties": {
                    "barkVolume": {
                        "type": "integer"
                    },
                    "firstBark": {
                        "type": "string",
                        "format": "date-time"
                    }
                }
            },
            "Cat": {
                "type": "object",
                "properties": {
                    "whiskerLength": {
                        "type": "integer"
                    },
                    "firstMeow": {
                        "type": "string",
                        "format": "date-time"
                    }
                }
            }
        }
    }
}

System information (optional)

@hey-api/openapi-ts: 0.86.0
node: 24.10.0</issue_description>

<agent_instructions>The @hey-api/transformers plugin should handle simple discriminator cases with oneOf</agent_instructions>

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

Fixes #2808

---

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Project	Deployment	Preview	Comments	Updated (UTC)
hey-api-docs	Ready	Preview	Comment	Oct 22, 2025 7:11am

[jeremyfiel]

when considering support for OAS 3.1/OAS3.2+), an alternative and recommended approach for discriminated unions is to use if, then conditional statements which were introduced in JSON Schema draft-07. These would only be a viable option for OAS 3.1 or later. They can be quite verbose, but are much easier to read and understand than the discriminator keyword.

#2764 here's an example