response.union undefined

Author: hammypantsCreated Aug 21, 2026Updated Aug 21, 2026

Version 2.7.2. go version 1.26.6. runtime 1.4.2.

A natural migration from 2.6 -> 2.7.2 breaks for us.

Going from 2.6 to 2.7.2 introduced this: https://github.com/oapi-codegen/oapi-codegen/issues/2403 for our spec. The claimed fix is to upgrade the runtime to 1.4.2 https://github.com/oapi-codegen/oapi-codegen/issues/2403#issuecomment-4756703428.

Bumping the runtime to 1.4.2 then introduces response.union undefined for responses that utilize oneOf. I see numerous issues and questions lingering around this, so I'll not link them here. I would have expected a version bump for this lib just for this.

Chasing this down it looks like the first version it breaks on is 2.7.0. Similar reasons to other issues already opened.

Reproducible spec:

package: openapi
generate:
  std-http-server: true
  strict-server: true
  models: true
  embedded-spec: true
output-options:
  skip-prune: true

openapi: 3.0.3
info:
  title: repro
  version: 1.0.0
paths:
  /things:
    get:
      operationId: listThings
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Thing'
        '400':
          $ref: '#/components/responses/BadRequest'
        '404':
          $ref: '#/components/responses/NotFound'
components:
  responses:
    # This breaks: a *shared* (components/responses) response whose inline schema is a oneOf. Referenced from an operation via $ref.
    BadRequest:
      description: Invalid request
      content:
        application/json:
          schema:
            oneOf:
              - $ref: '#/components/schemas/Error'
              - $ref: '#/components/schemas/ValidationError'
    # Doesn't break: same shape, but a plain $ref schema. Generates fine.
    NotFound:
      description: Resource not found
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Error'
  schemas:
    Thing:
      type: object
      required: [id]
      properties:
        id:
          type: string
    Error:
      type: object
      required: [error, message]
      properties:
        error:
          type: string
        message:
          type: string
    ValidationError:
      type: object
      required: [error, message]
      properties:
        error:
          type: string
        message:
          type: string
        fields:
          type: array
          items:
            type: string

Source: oapi-codegen/oapi-codegen

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