I carry a chain tool in my "everything" commute bag, but more and more I've been taking the lighter minimalist bag even when I'm riding the bikes with back racks. But how often does a chain break? For me, I think in 33 years of commuting and weekend rides maybe 3 times....so I guess I'm due. I always feel a little unprepared when taking the light bag...now I'll feel just a little more uneasy.