End of Life Ruby 3.0 has reached end of life and gets no more fixes. Switch to Ruby 4.0 →

class method

Thread.DEBUG=

Thread.DEBUG = num

static VALUE
rb_thread_s_debug_set(VALUE self, VALUE val)
{
    rb_thread_debug_enabled = RTEST(val) ? NUM2INT(val) : 0;
    return val;
}

Sets the thread debug level. Available only if compiled with THREAD_DEBUG=-1.

See also .DEBUG .exit .current .fork also in 3.1