fix(zod): fall back to z.union for discriminated unions with empty-object members

.changeset/zod-discriminated-union-empty-object.md

@@ -0,0 +1,5 @@
+---
+"@hey-api/openapi-ts": patch
+---
+
+**fix**: avoid invalid `.extend()` on `z.record()` when a discriminated union member is an empty object. The union now falls back to `z.union()` for these branches so the generated schema compiles.

packages/openapi-ts-tests/zod/v3/__snapshots__/3.0.x/mini/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/v4-mini';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.intersection(z.object({
+        type: z.literal('Empty')
+    }), zEmpty),
+    z.intersection(z.object({
+        type: z.literal('WithValue')
+    }), zWithValue)
+]);

packages/openapi-ts-tests/zod/v3/__snapshots__/3.0.x/v3/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod';
+
+export const zEmpty = z.record(z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v3/__snapshots__/3.0.x/v4/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/v4';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/mini/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/v4-mini';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.intersection(z.object({
+        type: z.literal('Empty')
+    }), zEmpty),
+    z.intersection(z.object({
+        type: z.literal('WithValue')
+    }), zWithValue)
+]);

packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v3/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod';
+
+export const zEmpty = z.record(z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v3/__snapshots__/3.1.x/v4/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/v4';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v3/test/3.0.x.test.ts

@@ -34,6 +34,14 @@ for (const zodVersion of zodVersions) {
         }),
         description: 'generates circular schemas',
       },
+      {
+        config: createConfig({
+          input: 'discriminator-empty-object-member.yaml',
+          output: 'discriminator-empty-object-member',
+        }),
+        description:
+          'falls back to z.union() when a discriminated union member is an empty object (z.record cannot be extended)',
+      },
       {
         config: createConfig({
           input: 'enum-null.json',

packages/openapi-ts-tests/zod/v3/test/3.1.x.test.ts

@@ -138,6 +138,14 @@ for (const zodVersion of zodVersions) {
         }),
         description: 'generates discriminated union for oneOf with discriminator mapping',
       },
+      {
+        config: createConfig({
+          input: 'discriminator-empty-object-member.yaml',
+          output: 'discriminator-empty-object-member',
+        }),
+        description:
+          'falls back to z.union() when a discriminated union member is an empty object (z.record cannot be extended)',
+      },
       {
         config: createConfig({
           input: 'discriminator-any-of.yaml',

packages/openapi-ts-tests/zod/v4/__snapshots__/3.0.x/mini/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/mini';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.intersection(z.object({
+        type: z.literal('Empty')
+    }), zEmpty),
+    z.intersection(z.object({
+        type: z.literal('WithValue')
+    }), zWithValue)
+]);

packages/openapi-ts-tests/zod/v4/__snapshots__/3.0.x/v3/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod/v3';
+
+export const zEmpty = z.record(z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v4/__snapshots__/3.0.x/v4/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/mini/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod/mini';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.intersection(z.object({
+        type: z.literal('Empty')
+    }), zEmpty),
+    z.intersection(z.object({
+        type: z.literal('WithValue')
+    }), zWithValue)
+]);

packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v3/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import { z } from 'zod/v3';
+
+export const zEmpty = z.record(z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v4/__snapshots__/3.1.x/v4/discriminator-empty-object-member/zod.gen.ts

@@ -0,0 +1,18 @@
+// This file is auto-generated by @hey-api/openapi-ts
+
+import * as z from 'zod';
+
+export const zEmpty = z.record(z.string(), z.unknown());
+
+export const zWithValue = z.object({
+    value: z.string()
+});
+
+export const zTestResponse = z.union([
+    z.object({
+        type: z.literal('Empty')
+    }).and(zEmpty),
+    z.object({
+        type: z.literal('WithValue')
+    }).and(zWithValue)
+]);

packages/openapi-ts-tests/zod/v4/test/3.0.x.test.ts

@@ -42,6 +42,14 @@ for (const zodVersion of zodVersions) {
         description:
           'falls back to z.union() when discriminated union members have allOf (intersection)',
       },
+      {
+        config: createConfig({
+          input: 'discriminator-empty-object-member.yaml',
+          output: 'discriminator-empty-object-member',
+        }),
+        description:
+          'falls back to z.union() when a discriminated union member is an empty object (z.record cannot be extended)',
+      },
       {
         config: createConfig({
           input: 'enum-null.json',

packages/openapi-ts-tests/zod/v4/test/3.1.x.test.ts

@@ -171,6 +171,14 @@ for (const zodVersion of zodVersions) {
         description:
           'falls back to z.union() when discriminated union members have allOf (intersection)',
       },
+      {
+        config: createConfig({
+          input: 'discriminator-empty-object-member.yaml',
+          output: 'discriminator-empty-object-member',
+        }),
+        description:
+          'falls back to z.union() when a discriminated union member is an empty object (z.record cannot be extended)',
+      },
       {
         config: createConfig({
           input: 'discriminator-any-of.yaml',

packages/openapi-ts/src/plugins/zod/shared/discriminated-union.ts

@@ -16,6 +16,19 @@ export interface DiscriminatedUnionData {
   members: Array<DiscriminatedUnionMember>;
 }
 
+/**
+ * A schema is emitted as `z.record(...)` rather than `z.object({...})` when it
+ * has no `properties` but does declare `additionalProperties`. Such a schema
+ * cannot be extended with `.extend({...})`, so it cannot participate in a
+ * `z.discriminatedUnion` via the `ref.extend({ discriminator: literal })`
+ * pattern this builder produces.
+ */
+function isRecordShaped(schema: IR.SchemaObject | undefined): boolean {
+  if (!schema || schema.type !== 'object') return false;
+  const hasProperties = schema.properties && Object.keys(schema.properties).length > 0;
+  return !hasProperties && Boolean(schema.additionalProperties);
+}
+
 export function tryBuildDiscriminatedUnion({
   items,
   parentSchema,
@@ -57,6 +70,13 @@ export function tryBuildDiscriminatedUnion({
         tool: 'zod',
       };
       if ((plugin.querySymbol(query)?.meta as unknown as ZodMeta)?.isIntersection) return null;
+      let resolved: IR.SchemaObject | undefined;
+      try {
+        resolved = plugin.context.resolveIrRef<IR.SchemaObject>(refPart.$ref);
+      } catch {
+        // unresolvable refs fall through and will surface elsewhere
+      }
+      if (isRecordShaped(resolved)) return null;
       refExpression = $(plugin.referenceSymbol(query));
     } else {
       return null;

specs/3.0.x/discriminator-empty-object-member.yaml

@@ -0,0 +1,24 @@
+openapi: 3.0.3
+info:
+  title: OpenAPI 3.0.3 discriminator empty object member example
+  version: 1
+components:
+  schemas:
+    Empty:
+      type: object
+    WithValue:
+      type: object
+      required:
+        - value
+      properties:
+        value:
+          type: string
+    TestResponse:
+      oneOf:
+        - $ref: '#/components/schemas/Empty'
+        - $ref: '#/components/schemas/WithValue'
+      discriminator:
+        propertyName: type
+        mapping:
+          Empty: '#/components/schemas/Empty'
+          WithValue: '#/components/schemas/WithValue'

specs/3.1.x/discriminator-empty-object-member.yaml

@@ -0,0 +1,24 @@
+openapi: 3.1.0
+info:
+  title: OpenAPI 3.1.0 discriminator empty object member example
+  version: 1
+components:
+  schemas:
+    Empty:
+      type: object
+    WithValue:
+      type: object
+      required:
+        - value
+      properties:
+        value:
+          type: string
+    TestResponse:
+      oneOf:
+        - $ref: '#/components/schemas/Empty'
+        - $ref: '#/components/schemas/WithValue'
+      discriminator:
+        propertyName: type
+        mapping:
+          Empty: '#/components/schemas/Empty'
+          WithValue: '#/components/schemas/WithValue'