`_output` is not branded for top-level branded schema

[dzhu]

In the following code, bso evaluates to string, but it presumably should evaluate to the branded type just as the other types do.

I realize _input and _output are deprecated, but they are still there, and at least tRPC is still relying on them (though I have suggested migrating).

import { z } from 'zod';
const BrandedSchema = z.string().brand<'brand'>();
const WrappedSchema = z.object({ key: BrandedSchema });

type bso = typeof BrandedSchema._output;
type wso = (typeof WrappedSchema._output)['key'];
type bso2 = z.output<typeof BrandedSchema>;
type wso2 = z.output<typeof WrappedSchema>['key'];

// Type equality checker from https://github.com/microsoft/TypeScript/issues/27024#issuecomment-421529650.
type Equals<X, Y> =
  (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y ? 1 : 2 ? true : false;

const _1: Equals<string, bso> = true;
const _2: Equals<string, wso> = false;
const _3: Equals<string, bso2> = false;
const _4: Equals<string, wso2> = false;

Tested with TypeScript 5.9 and Zod v4.0.15.

[dosubot]

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

You're right—_output on a top-level branded schema like z.string().brand<'brand'> evaluates to just string, not the branded type. This is because Zod's internal type logic for _output doesn't track the brand for top-level schemas, while the recommended z.output<typeof Schema> utility type does, thanks to extra conditional type logic in its TypeScript definitions. For wrapped schemas (like objects containing branded schemas), _output does reflect the brand for the property, but not for the top-level schema itself.

The use of _input and _output is deprecated in Zod v4, and the maintainers recommend migrating to z.output<typeof Schema> for branded types and better type inference. This change is especially important for libraries like tRPC that still rely on _output—migrating will ensure branded types are preserved as expected. The branding mechanism in Zod is purely a TypeScript construct for nominal typing and doesn't affect runtime validation; it's only present in the static type system. A true fix for branded type inference in generics and with _output is currently blocked by TypeScript limitations, and Zod maintainers are aware of these broader inference issues [source].

For more details, see the Zod docs on branded types and the relevant issue discussing branded type inference and _output. If your question is answered, feel free to close the issue!

To reply, just mention @dosu.

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[colinhacks]

Yeah this is difficult to fix, as "properly" typing _output and _input causes issues with recursive types. Closing as there's nothing I can do about this. I encourage tRPC to prefentially read from the Standard Schema fields if possible.

[dzhu]

Thanks for looking into this! I've reported back on the tRPC issue.

If you've got a moment to explain more to a TypeScript/Zod noob: What is it about those attributes that makes them unfixable, compared to the other ones? Naively, I might think that they should be able to work the same way, or even just be an alias somehow.