Use any standard schema within zod3

gistfile1.ts

import type { StandardSchemaV1 } from '@standard-schema/spec';
import { ZodError, ZodIssueCode, type ZodType, type ZodTypeDef, z } from 'zod/v3';

/**
 * Wrap any Standard Schema as a Zod schema.
 * Always use `parseAsync` (or `safeParseAsync`) on the returned schema.
 */
export function z3FromStandard<S extends StandardSchemaV1>(
  schema: S
): ZodType<StandardSchemaV1.InferOutput<S>> {
  const base = z.any().transform(async (value) => {
    const res = await Promise.resolve(schema['~standard'].validate(value));
    if ('issues' in res) {
      // Map Standard Schema issues -> ZodError
      throw new ZodError(
        res.issues?.map((i) => ({
          code: ZodIssueCode.custom,
          message: i.message,
          path: (i.path ?? []).map((p) =>
            typeof p === 'object' && p && 'key' in p ? p.key : p
          ) as Array<string | number>,
        })) ?? []
      );
    }
    return res.value as StandardSchemaV1.InferOutput<S>;
  });

  // Cast to the cleaner ZodType<Output> surface
  return base as unknown as ZodType<StandardSchemaV1.InferOutput<S>>;
}

import { z as z4 } from 'zod/v4';

const testz4Schema = z4.object({
  a: z4.string(),
  b: z4.number(),
});

const testz3Schema = z.object({
  a: z.string(),
  b: z3FromStandard(testz4Schema)
});

type Testz3Schema = z.infer<typeof testz3Schema>;