Problem

OpenAPI 3.1 expresses a nullable $ref as a union with the JSON Schema null type:

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

The generator treated anyOf / oneOf as a plain union, so { "type": "null" } was
converted to unknown and the $ref lost its component identity:

(T | unknown) collapses to unknown in TypeScript, which is the reported
TS2322: Type 'unknown' is not assignable to .... On the zod side the result became a
ZodUnion, which has no .unwrap().

Fix

anyOf: [S, { type: "null" }] is 3.1's nullable notation, not a union. Two helpers in
openapi-utils.ts normalize it to the OpenAPI 3.0 equivalent { ...S, nullable: true }
before any union handling, so the existing nullable machinery applies unchanged:

• isNullTypeSchema() — detects a { "type": "null" } member
• splitNullableUnion() — splits union members from the nullability they contribute
• normalizeNullableUnion() — collapses a single-member null union to the 3.0 shape,
  preserving sibling keywords (description, default) and returning the input
  unchanged when there is nothing to collapse

Applied on all three paths:

• generate-schema.ts — getTypeFromSchema() union branch
• generate-zod.ts — schemaToZod(), schemaToZodType(), and the request-body schema
  name lookup that fed postPathSchemas
• generate-interface.ts — response content and request-body branches, so the component
  reference shortcut and the raw-multipart check see the normalized schema

Unions that keep two or more members stay unions and now additionally reflect a removed
null member, so anyOf: [string, number, null] generates (string | number | null)
instead of (string | number | unknown).

Not regressed

• Genuine unions are untouched: anyOf: [{type:"string"},{type:"number"}] still generates
  (string | number) / z.union([...]) / z.ZodUnion<...>.
• OpenAPI 3.0 nullable: true input keeps working.
• type: ["string","null"] arrays were already handled by getPrimaryType() /
  isNullableSchema() and are unchanged.

Tests

New nullable-union.test.ts asserts that every 3.1 nullable form generates a
byte-identical result to its 3.0 counterpart across generateInterface,
generateZodSchemas, and generateZodTypeDeclarations, in both a request-body and a
component-property position. Verified TDD: 14 of the new tests fail without the fix and
all 31 pass with it, while the genuine-union guards pass in both states.

bun test 1148 pass / 0 fail, 100% line and function coverage on all four changed files,
bun run lint and bun run build clean.