fix(core): extract named enum type for anyOf nullable enum composition

packages/core/src/getters/combine.test.ts

@@ -38,6 +38,10 @@ const context = {
             name: { type: 'string' },
           },
         },
+        Status: {
+          type: 'string',
+          enum: ['new', 'in_progress'],
+        },
       },
     },
   },
@@ -85,6 +89,185 @@ describe('combineSchemas (allOf required handling)', () => {
     expect(result.value).toContain('Required<Pick');
   });
 
+  // OAS 3.1's `{type: 'null'}` variant inside an anyOf/oneOf is the
+  // nullable-enum spelling used by code generators like FastAPI. The result
+  // should be flagged as an enum so the caller can extract a named type,
+  // matching the equivalent `{type: ['string','null'], enum: [...]}` form.
+  // See issue #2710.
+  describe('nullable enum composition (#2710)', () => {
+    it('flags anyOf [enum, null] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ enum: ['new', 'in_progress'] }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(true);
+      expect(result.value).toContain(`'new' | 'in_progress'`);
+      expect(result.value).toContain('null');
+    });
+
+    it('flags oneOf [enum, null] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        oneOf: [{ enum: ['new', 'in_progress'] }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'oneOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(true);
+      expect(result.value).toContain(`'new' | 'in_progress'`);
+      expect(result.value).toContain('null');
+    });
+
+    // Detection must not depend on the order of subschemas — `{type: 'null'}`
+    // can appear before or after the enum.
+    it('flags anyOf [null, enum] (null first) as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ type: 'null' }, { enum: ['new', 'in_progress'] }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(true);
+    });
+
+    // The pattern is type-agnostic: numeric enums combined with null should
+    // also be recognized.
+    it('flags anyOf [numeric enum, null] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ type: 'integer', enum: [1, 2, 3] }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Code',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(true);
+      expect(result.value).toContain('1 | 2 | 3');
+      expect(result.value).toContain('null');
+    });
+
+    // Pin behavior for the multi-enum + null variant. Each enum branch
+    // contributes its values; the result is still a nullable enum union.
+    it('flags multiple inline enums + null as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ enum: ['a', 'b'] }, { enum: ['c', 'd'] }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(true);
+      expect(result.value).toContain(`'a' | 'b'`);
+      expect(result.value).toContain(`'c' | 'd'`);
+      expect(result.value).toContain('null');
+    });
+
+    // Negative: a plain nullable string (no enum) must stay a generic union
+    // and not be flagged as an enum. This is the case the existing
+    // query-params.test.ts:169 test already pins at the integration level.
+    it('does not flag anyOf [non-enum scalar, null] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ type: 'string', format: 'uuid' }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'AffiliationId',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(false);
+    });
+
+    // Negative: a `$ref` branch already resolves to an existing named enum
+    // schema. Treating this composition as an inline-enum would route the
+    // caller through `getEnum`, which emits a parallel const that nests the
+    // original ref (e.g. `{Status: Status}`) instead of reusing it.
+    it('does not flag anyOf [$ref enum, null] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ $ref: '#/components/schemas/Status' }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(false);
+    });
+
+    // Negative: `allOf` is an intersection, not a union. `allOf: [{enum}, {null}]`
+    // is semantically empty (no value can satisfy both); regardless, it must
+    // not be misclassified as a nullable enum union.
+    it('does not flag allOf [enum, null] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        allOf: [{ enum: ['new', 'in_progress'] }, { type: 'null' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'allOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(false);
+    });
+
+    // Negative: an enum combined with a non-null scalar is a genuine union,
+    // not a nullable enum. Extracting it as a named enum would change the
+    // semantics (the other branch's values would be lost).
+    it('does not flag anyOf [enum, non-null scalar] as a nullable enum', () => {
+      const schema: OpenApiSchemaObject = {
+        anyOf: [{ enum: ['new', 'in_progress'] }, { type: 'string' }],
+      };
+
+      const result = combineSchemas({
+        schema,
+        name: 'Status',
+        separator: 'anyOf',
+        context,
+        nullable: '',
+      });
+
+      expect(result.isEnum).toBe(false);
+    });
+  });
+
   it('normalizes inline object in allOf to match parent object form', () => {
     const variantA: OpenApiSchemaObject = {
       allOf: [

packages/core/src/getters/combine.ts

@@ -344,6 +344,32 @@ export function combineSchemas({
   }
 
   const isAllEnums = resolvedData.isEnum.every(Boolean);
+  // OAS 3.1 spells a nullable enum as `anyOf: [{enum: [...]}, {type: 'null'}]`.
+  // Without this, the {type: 'null'} variant flips `isEnum` to false and the
+  // enum gets inlined instead of extracted as a named type — the
+  // {type: ['string','null'], enum: [...]} spelling already extracts. Treat
+  // null-only variants as transparent so the caller's `isEnum && !isRef`
+  // branch (query-params, schema-definition, resolvers/object) extracts via
+  // getEnum, whose `stripNullUnion` handling already preserves the trailing
+  // ` | null`. See issue #2710.
+  //
+  // Guards:
+  // - `allOf` semantics are intersection, not union — `allOf: [{enum}, {null}]`
+  //   does not describe a nullable enum, so restrict to `anyOf`/`oneOf`.
+  // - Non-null branches must be inline enums (`!isRef`). For `$ref + null`
+  //   the existing referenced enum should be reused; routing through
+  //   `getEnum` would emit a parallel const that nests the original ref
+  //   (e.g. `{Status: Status}`) instead of spreading or aliasing it.
+  const isUnionLikeSeparator = separator === 'anyOf' || separator === 'oneOf';
+  const isNullableEnumComposition =
+    isUnionLikeSeparator &&
+    !isAllEnums &&
+    resolvedData.isEnum.some(Boolean) &&
+    resolvedData.isEnum.every(
+      (isEnum, index) =>
+        (isEnum && !resolvedData.isRef[index]) ||
+        resolvedData.types[index] === 'null',
+    );
   const isAvailableToGenerateCombinedEnum =
     isAllEnums &&
     name &&
@@ -460,7 +486,7 @@ export function combineSchemas({
     dependencies: resolvedValue
       ? [...resolvedData.dependencies, ...resolvedValue.dependencies]
       : resolvedData.dependencies,
-    isEnum: false,
+    isEnum: isNullableEnumComposition,
     type: 'object' as SchemaType,
     isRef: false,
     hasReadonlyProps: