Fix Zod v4 oneOf unions in strict schemas

src/helpers/zod.ts

@@ -34,6 +34,22 @@ function zodV4ToJsonSchema(schema: z4.ZodType): Record<string, unknown> {
   return toStrictJsonSchema(
     z4.toJSONSchema(schema, {
       target: 'draft-7',
+      override: ({ zodSchema, jsonSchema }) => {
+        const def = zodSchema._zod.def;
+
+        if (def.type === 'union' && 'discriminator' in def && Array.isArray(jsonSchema.oneOf)) {
+          if (jsonSchema.anyOf !== undefined) {
+            throw new Error(
+              'Zod discriminated union generated both `anyOf` and `oneOf`, which cannot be represented in an OpenAI strict schema',
+            );
+          }
+
+          // Discriminator values are mutually exclusive, so anyOf preserves the
+          // union while staying inside the API's supported JSON Schema subset.
+          jsonSchema.anyOf = jsonSchema.oneOf;
+          delete jsonSchema.oneOf;
+        }
+      },
     }) as JSONSchema,
   ) as Record<string, unknown>;
 }

tests/helpers/zod.test.ts

@@ -30,6 +30,20 @@ function expectDefinitionRefsToResolve(schema: Record<string, unknown>) {
   visit(schema, new Set());
 }
 
+it('converts Zod v4 discriminated unions to anyOf for strict schemas', () => {
+  const ResponseSchema = zv4.object({
+    data: zv4.discriminatedUnion('type', [
+      zv4.object({ type: zv4.literal('a') }),
+      zv4.object({ type: zv4.literal('b') }),
+    ]),
+  });
+
+  const schema = zodResponseFormat(ResponseSchema, 'choice').json_schema.schema as any;
+
+  expect(JSON.stringify(schema)).not.toContain('"oneOf"');
+  expect(schema.properties.data.anyOf).toHaveLength(2);
+});
+
 describe.each([
   { version: 'v3', z: zv3 },
   { version: 'v4', z: zv4 as any as typeof zv3 },