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;
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