Found a clean solution for adding custom error messages when using Ecto.Changeset.cast/4.

The issue presented is cast/4 does not allow :message for an opts unlike other changeset functions like validate_format/4 and other custom validator functions.

This presented a problem for us as we have a number of custom types and messages that we use to validate our inputs. This works fine when validating things like a phone number because we can simply pass our custom error message directly to validate_format/4, but for our :integer and :float values we simply rely on the base cast/4 for those types without the need for more validation. When given "foo" to input type :float the changeset error message returns "is invalid". Using Gettext and traverse_errors/2 we can fix this error message on the UI, but that ends up in needing to have branching error message logic and makes it more difficult with a dozen types to clearly see where the error message definitions exist.

Instead we have a function custom_cast_error_message(changeset, field, message) that replaces the given field error only for cast/4 validations to be our custom message

  @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

A sample usage of this looks 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)

Posting this because I saw numerous posts about this same issues and didn’t love the solutions found.