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convertTo 3.1 drops nullable on allOf schemas (follow-up to #172)

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
76/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, node.js
Domain
api

Research direction

Start at the openapiConvertVersion entry point and reproduce the issue with the provided 3.0.3 document, comparing the allOf and oneOf conversions. Done means nullable allOf schemas preserve null acceptance through an anyOf-style representation while the existing oneOf behavior remains correct.

Written by the indexing model from the issue text.

Description

Converting 3.0 → 3.1 with convertTo: '3.1' drops nullable: true when the schema has allOf and no type. The converted schema then rejects null, which the original allowed. #172 fixed this for anyOf and oneOf, but allOf is still affected.

The pattern is common in GitHub's REST API description. For example, label.archived_by is { nullable: true, allOf: [{ $ref: '#/components/schemas/simple-user' }] }.

Reproduction (openapi-format 1.33.7, Node 24):

import { openapiConvertVersion } from 'openapi-format';

const doc = {
  openapi: '3.0.3',
  info: { title: 't', version: '1' },
  paths: {},
  components: {
    schemas: {
      User: { type: 'object', properties: { login: { type: 'string' } } },
      Label: {
        type: 'object',
        properties: {
          archived_by: { nullable: true, allOf: [{ $ref: '#/components/schemas/User' }] },
          milestone: { nullable: true, oneOf: [{ type: 'string' }, { type: 'integer' }] },
        },
      },
    },
  },
};

const { data } = await openapiConvertVersion(doc, { convertTo: '3.1' });
console.log(JSON.stringify(data.components.schemas.Label.properties, null, 2));

Actual:

{
  "archived_by": { "allOf": [{ "$ref": "#/components/schemas/User" }] },
  "milestone": { "oneOf": [{ "type": "string" }, { "type": "integer" }, { "type": "null" }] }
}

milestone (oneOf) is converted correctly. archived_by (allOf) loses nullable entirely.

Expected, for example:

"archived_by": {
  "anyOf": [
    { "allOf": [{ "$ref": "#/components/schemas/User" }] },
    { "type": "null" }
  ]
}

Adding { "type": "null" } to the allOf list itself wouldn't work, since a value can't match both a User and null. The null has to sit alongside the allOf, for example in an anyOf.

Dominant language
JavaScript
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