Adapted from this recommendation by @jandockx

Cyclical objects

Despite supporting recursive schemas, passing cyclical data into Zod will cause an infinite loop in some cases.

You can protect against cyclical objects starting an infinite loop (at a performance cost) with the following approach (using the above jsonSchema as an example):

function isCircular(v: unknown, visited?: Set<unknown>): boolean {
  if (v === null || typeof v !== 'object') {
    return false;
  }

  if (visited?.has(v)) {
    return true;
  }

  const actualVisited = visited ?? new Set<unknown>();
  actualVisited.add(v);

  if (Array.isArray(v)) {
    return v.some(av => isCircular(av, actualVisited));
  }

  return Object.values(v).some(ov => isCircular(ov, actualVisited));
}

const NotCircular = z.unknown().superRefine((val, ctx) => {
  if (isCircular(val)) {
    ctx.addIssue({
      code: z.ZodIssueCode.custom,
      message: 'values cannot be circular data structures',
      fatal: true
    });

    return z.NEVER;
  }
})

const acircularJSONSchema = NotCircular.pipe(jsonSchema);

acircularJSONSchema.parse(data);

When NotCircular fails, pipe will not pass the value into the next schema for evaluation, preventing the infinite loop.