How do you provide custom error message for Ecto's cast errors?

Have you tried using gettext for this? You should have some examples in a generated phoenix project.

The metadata in the error would allow you to do the transformation you mention to the error in Ecto.Changeset.traverse_errors (which is likely already called somewhere in your codebase). You can ignore the existing error string and match on metadata[:validation] + metadata[:type].

I made a post discussing how we solved this issue.

It boils down to creating a function

  @doc """
  Overwrites the error message for `Ecto.Changeset.cast/4` from `"is invalid"` to the
  custom supplied error message.
  """
  def custom_cast_error_message(changeset, field, message) do
    errors = Enum.map(changeset.errors, fn {error_field, {_msg, opts}} = error ->
      if field == error_field && opts[:validation] == :cast do
        {field, {message, opts}}
      else
        error
      end
    end)

    %{changeset | errors: errors}
  end

and calling it like

message = "This is our custom error message for cast/4"

Ecto.Changeset.cast(answer, params, [:value])
|> Aw.Changeset.custom_cast_error_message(:value, message)