preprocess makes usage difficult

[fregante]

I'm using zod to parse ENV variables and I've settled on this pattern to have FOO= use the default value:

const handleEmpty = (val: string | undefined) => (val === "" ? undefined : val);
const url = z.preprocess(handleEmpty, z.url());

const FOO = url.default('https://foo.baz/')
const BAR = url.default('https://bar.baz/')

Unfortunately I hit yet another wall with preprocess when trying to add one more constraint:

  const handleEmpty = (val: string | undefined) => (val === "" ? undefined : val);
  const url = z.preprocess(handleEmpty, z.url()
+   .endsWith("/")
  );

Types force me to add it there. Not after .preprocess(). Not after .default(). But the code above fails to reach the default value, so it throws.

So I have a few questions:

• can I have my "handleEmpty" logic without preprocess?
• why is preprocess so difficult to deal with? This is not the first limitation I hit

and finally:

• where do I add .endsWith if I have no alternative?

[dosubot[bot]]

Hi @fregante! I'm Dosu and I’m helping the zod team.

I'm a bot trained on Zod's codebase and issues.

There's no built-in way to treat empty strings as undefined in Zod without using z.preprocess. The pipeline in Zod runs preprocessors first, then core validation (like z.url()), then refinements (like .endsWith()), and finally applies .default() if the result is undefined. This means that if your input is an empty string, your handleEmpty logic (via preprocess) will convert it to undefined, but any refinements like .endsWith() are still evaluated before the default is applied, which can cause the issues you're seeing. This order is intentional to keep the parsing logic predictable and maintainable, but it can feel unintuitive in cases like this. You can read more about the pipeline and method order in the docs and changelog:
Zod API docs
Zod v4 changelog

A common workaround is to use .transform() after validation to enforce post-processing, but this doesn't help with defaulting, since .default() only triggers on undefined before transforms. If you want to combine all three—custom empty handling, defaulting, and a trailing slash constraint—you'll need to keep using z.preprocess for the empty string logic, and be aware that refinements will always run before defaults.

If you have a minimal reproduction that demonstrates a specific edge case, feel free to share it. If this answers your question, please close the issue.

To reply, just mention @dosu.

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[fregante]

If this is accurate, I wish it wasn't. I don't see why z.url() is applied after the default but z.endsWith() is not. Pipelines are not flexible enough.