Validation runs at construction. Assigning to a field afterwards, user.age = "not a number", is a plain attribute set: no coercion, no error, and the model now holds a value its own type annotation forbids. This has been true in both v1 and v2 and is the single most common source of models that pass validation and still contain garbage.
Enable it per model with model_config = ConfigDict(validate_assignment=True), after which assignment runs the same validators and raises ValidationError on bad input. It is off by default because validating every assignment is measurably slower and most models are constructed once and read many times.
The adjacent setting is frozen=True, which makes the model immutable and hashable and turns assignment into an error outright. For a model that crosses a trust boundary, frozen plus reconstructing through model_copy(update=...) is usually a better design than validate-on-assignment, because it makes the point of validation explicit rather than scattered across every mutation.