[BUG][typescript-axios] Missing import for a $ref under additionalProperties on a 3.1 nullable property

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix

Description

When a property is nullable using an OpenAPI 3.1 type array ("type": ["null", "object"]) and its additionalProperties is a $ref, the generated model references the $ref'd type but never imports it, so the output does not compile.

Instead of importing the referenced model, the generator emits an import for a Null model that it never writes:

// @ts-ignore
import type { Null } from './null';        // <- ./null is never generated
export interface ParentNullableMap {
    'map'?: { [key: string]: Child; };     // <- Child referenced, never imported
}

tsc then fails with:

models/parent-nullable-map.ts(21,30): error TS2304: Cannot find name 'Child'.

Two things narrow it down:

1. The nullable type array is what triggers it. With "type": "object" instead of "type": ["null", "object"], the same property imports Child correctly.
2. additionalProperties is specific. A nullable array of the same $ref ("type": ["null", "array"] with items: {$ref}) is generated correctly, in the same run — included below as a control.

The nullable is also silently dropped on the affected property: 'map'?: { [key: string]: Child; } has no | null, while the control correctly produces 'list'?: Array<Child> | null.

Reproduces with withSeparateModelsAndApi=true.

openapi-generator version

7.24.0 (via the openapitools/openapi-generator-cli:v7.24.0 Docker image). Not tested against earlier versions.

OpenAPI declaration file content or url

{
  "openapi": "3.1.0",
  "info": {"title": "repro", "version": "1.0.0"},
  "paths": {},
  "components": {
    "schemas": {
      "Child": {"type": "object", "properties": {"name": {"type": "string"}}},
      "ParentNullableMap": {
        "type": "object",
        "properties": {
          "map": {"type": ["null", "object"], "additionalProperties": {"$ref": "#/components/schemas/Child"}}
        }
      },
      "ParentNullableArray": {
        "type": "object",
        "properties": {
          "list": {"type": ["null", "array"], "items": {"$ref": "#/components/schemas/Child"}}
        }
      }
    }
  }
}

validate reports no errors on this spec (only "unused model" recommendations, since paths is empty).

Command line used for generation

docker run --rm -v "$PWD:/local" openapitools/openapi-generator-cli:v7.24.0 \
  generate -i /local/minimal.json -g typescript-axios -o /local/out \
  --additional-properties=withSeparateModelsAndApi=true,modelPackage=models,apiPackage=api

Steps to reproduce

1. Save the spec above as minimal.json.
2. Run the command above.
3. models/parent-nullable-map.ts imports Null from ./null and references Child with no import for it. models/null.ts does not exist.
4. models/parent-nullable-array.ts — the control — imports Child correctly.
5. Type-check the output, e.g. tsc --noEmit --strict --skipLibCheck --target es2020 --module esnext --moduleResolution bundler models/*.ts, and it fails with TS2304: Cannot find name 'Child'.

Actual output

// models/parent-nullable-map.ts  (BROKEN)
// @ts-ignore
import type { Null } from './null';
export interface ParentNullableMap {
    'map'?: { [key: string]: Child; };
}

// models/parent-nullable-array.ts  (control, correct)
// @ts-ignore
import type { Child } from './child';
export interface ParentNullableArray {
    'list'?: Array<Child> | null;
}

Expected output

// models/parent-nullable-map.ts
import type { Child } from './child';
export interface ParentNullableMap {
    'map'?: { [key: string]: Child; } | null;
}

Related issues/PRs

• #5601 — same missing-import symptom in typescript-axios with withSeparateModelsAndApi, but a different trigger: additionalProperties of type array with a $ref, on OpenAPI 3.0. That report notes the plain additionalProperties: {$ref} case works, which matches what I see — it only breaks here once the 3.1 nullable type array is added.
• #9385 — missing import for a nested dictionary-of-collection type; possibly the same import-resolution path.

Suggest a fix

The nullable type array appears to be consumed before the $ref inside additionalProperties is registered as an import, and a Null model is substituted in its place. Two observations that may help locate it:

• The Null import is emitted for the nullable-type-array case generally, not only when a bare {"type": "null"} schema exists — the spec above contains no such schema, yet the import appears.
• The // @ts-ignore above generated imports masks the dangling ./null import, so the failure surfaces at the usage site rather than the import, which makes it harder to trace. In the array (control) path the same @ts-ignore line is present but the import is correct.

A workaround for anyone hitting this: pre-process the spec to drop "null" from the type array on properties whose additionalProperties is a $ref (accepting the loss of nullability on those properties), which restores a compiling client.

Start with minimal.json and run the listed openapi-generator Docker command for typescript-axios. Compare models/parent-nullable-map.ts with models/parent-nullable-array.ts and type-check with the supplied tsc command; done means Child is imported, nullability is preserved, no nonexistent Null model is referenced, and the generated models compile.