Summary

The second example in the Prefaults section of api.mdx is not runnable:

z.string().transform(val => val.length).prefault("tuna");
schema.parse(undefined); // => 4

The first line is a dangling expression — schema is never declared — and the second line then calls schema.parse(...) on an undefined binding. Copy-pasting this throws a ReferenceError (or, if a schema from a previous block is in scope, silently uses that one, which returns 0, contradicting the // => 4 comment).

Every parallel example in the surrounding Defaults/Prefaults/Catch sections binds const schema = ... — including the default(0) example directly above this one.

Change

Prepend const schema = so the example is self-contained and correct:

const schema = z.string().transform(val => val.length).prefault("tuna");
schema.parse(undefined); // => 4

Now prefault feeds "tuna" through the string → length transform, yielding 4 as the comment states. Docs-only change.