As per choroba's answer, coercions are only triggered if the value fails the type constraint, and "1" is a valid string.
You could use Enum["yes","no"] as your type constraint instead of Str.
use 5.026;
use Test::Most;
{
package Thing;
use Moose;
use Types::Standard qw( Bool Enum );
has delete => (
coerce => 1,
is => 'ro',
isa => Enum->of(qw/yes no/)->plus_coercions( Bool, q{ $_ ?
+'yes' : 'no' } ),
);
}
{
cmp_methods(
new_ok( Thing => [ delete => 1 ] ),
[ delete => 'yes' ],
'attributes are coerced as expected',
);
}
done_testing;