type arrays (OpenAPI 3.1 / JSON Schema nullable syntax) generate `unknown` instead of the correct TS type

[Akallabet]

Title

type arrays (OpenAPI 3.1 / JSON Schema nullable syntax) generate unknown instead of the correct TS type

Summary

When an OpenAPI 3.1 schema expresses nullability using the JSON Schema 2020-12 array form — type: [string, "null"] — instead of the OpenAPI 3.0 nullable: true keyword, the generated TypeScript client types the field as unknown instead of string | null.

Reproduction

openapi.yaml (OpenAPI 3.1.0):

openapi: 3.1.0
...
components:
  schemas:
    Transaction:
      type: object
      properties:
        description:
          type: [string, "null"]
          maxLength: 255
      required: [id]

Generate a frontend TS client:

massimo openapi.yaml --frontend --language ts --name generated-client --folder src/generated-client

Actual generated type:

export type Transaction = { ...; 'description'?: unknown; ... }

Expected:

export type Transaction = { ...; 'description'?: string | null; ... }

As a workaround, rewriting the schema with the OpenAPI 3.0-style keyword produces the correct type:

description:
  type: string
  nullable: true

'description'?: string | null   // correct

Root cause (pointer, not a fix)

getType() in lib/get-type.js (massimo-cli) branches on typeDef.type via strict string checks — typeDef.type === 'array', typeDef.type === 'object', and a switch (type) inside JSONSchemaToTsType().
All of these assume type is a single string. When type is an array (valid per OpenAPI 3.1 / JSON Schema 2020-12), none of the checks match, so generation falls through the switch default and returns 'unknown'.

This likely affects not just plain string/null fields but any property whose type is expressed as an array — including ["object", "null"] and ["array", "null"] — since those are checked earlier in getType, before JSONSchemaToTsType is ever reached.

Happy to send a PR for this if a fix direction is agreed on — just wanted to raise the bug first.

Environment

• massimo / massimo-cli: 1.3.0
• OpenAPI version in spec: 3.1.0