diff --git a/src/_vendor/zod-to-json-schema/parsers/optional.ts b/src/_vendor/zod-to-json-schema/parsers/optional.ts
index 9b3e9731f..aee9606e4 100644
--- a/src/_vendor/zod-to-json-schema/parsers/optional.ts
+++ b/src/_vendor/zod-to-json-schema/parsers/optional.ts
@@ -3,6 +3,15 @@ import { JsonSchema7Type, parseDef } from '../parseDef';
 import { Refs } from '../Refs';
 
 export const parseOptionalDef = (def: ZodOptionalDef, refs: Refs): JsonSchema7Type | undefined => {
+  if (refs.openaiStrictMode) {
+    const fieldName = refs.propertyPath?.slice(-1)[0] || 'unknown';
+    console.warn(
+      `Warning: Field "${fieldName}" uses .optional() which is not supported by OpenAI API Structured Outputs. ` +
+        `Please use .nullable() instead. ` +
+        `See: https://platform.openai.com/docs/guides/structured-outputs#all-fields-must-be-required`,
+    );
+  }
+
   if (refs.currentPath.toString() === refs.propertyPath?.toString()) {
     return parseDef(def.innerType._def, refs);
   }
diff --git a/tests/helpers/zod.test.ts b/tests/helpers/zod.test.ts
index 493b4c0c8..e35a951ca 100644
--- a/tests/helpers/zod.test.ts
+++ b/tests/helpers/zod.test.ts
@@ -88,6 +88,24 @@ describe('zodResponseFormat', () => {
     `);
   });
 
+  it('warns when using optional fields with OpenAI API', () => {
+    const consoleSpy = jest.spyOn(console, 'warn');
+
+    zodResponseFormat(
+      z.object({
+        required: z.string(),
+        optional: z.string().optional(),
+      }),
+      'test',
+    );
+
+    expect(consoleSpy).toHaveBeenCalledWith(
+      expect.stringContaining('uses .optional() which is not supported by OpenAI API Structured Outputs'),
+    );
+
+    consoleSpy.mockRestore();
+  });
+
   it('automatically adds properties with defaults to `required`', () => {
     expect(
       zodResponseFormat(