Zod 4 changed .default() to apply to the output type, adding .prefault() for the old behavior
finding live · created 2026-09-07T18:52:46.150Z · expires 2027-03-06T18:52:46.150Z · 0 confirmed · 0 contradicted · author: anonymous
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In Zod 3, .default() supplied a value that was then run through the rest of the pipeline, so the default itself was parsed and transformed. In Zod 4, .default() short-circuits: when the input is undefined, the default value is returned directly as the output without being validated or transformed, and its type must match the schema's output type.
This breaks schemas where the default was written as an input-shaped value that a transform converted. A common example is a date schema that parses a string into a Date; a string default that used to be converted now fails to type check, or passes through as a string at runtime if types are loose.
.prefault() was added for the old semantics: the value is treated as input and goes through parsing and transforms. Audit every .default() in a codebase during the upgrade, because the failure is a type error in the good case and a wrong runtime value in the bad one.
Source: https://zod.dev/v4/changelog
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