New pull request. Leaping into action...

  ^{｜ View workflow run ｜ via Pullfrog ｜ Using Claude Opus ｜ 𝕏}

Heads-up: this PR also resolves #5694 (and effectively #4778). The new overload's core.output<this> extends core.input<T> check covers both the "branded output → unbranded input" case in your tests and the "stricter source → looser target" case (z.number().pipe(z.number().optional()), .transform().pipe(nullable_schema)) reported there.

Verified locally: both #5694 reproductions fail on main with the documented TypeCheckError, and pass on this branch with no extra changes. If it'd help reviewer confidence to lock in those cases too, here's a drop-in test file you can include — feel free to ignore if you'd rather keep the PR scope narrow.

// packages/zod/src/v4/classic/tests/pipe-compat.test.ts (or merge into pipe.test.ts)
import { expectTypeOf, test } from "vitest";
import * as z from "zod/v4";

test("pipe stricter source into looser target (issue #5694)", () => {
  const maybeNumber = z.number().optional();
  const out = z.number().pipe(maybeNumber).parse(42);
  expectTypeOf(out).toEqualTypeOf<number | undefined>();
});

test("pipe transform output into nullable target (issue #5694)", () => {
  const backEnd = z.object({ field: z.number().min(1).max(100).nullable() });
  backEnd.extend({
    field: z.string().nonempty().transform(Number).pipe(backEnd.shape.field),
  });
});

Either way, two more issues fall behind this PR — worth flagging in the description if you'd like.