Surefire battery built in protection?

From the surefire website:

"Many counterfeit or inferior lithium batteries don't have the power output characteristics or built-in fault (short-circuit) protection that SureFire batteries have"

I assumed that all CR123a primaries were unprotected cells. What short-circuit protection is built into a Surefire battery? are they low-voltage protected?

Lithium primary batteries have PTC protection, which stands for Positive Temperature Coefficient (not Protection Circuitry). It will prevent excessive current draw, but since these cells aren't rechargeable, there is no danger of "overdischarging."

^ well said
which only leaves the possibility of a "reverse charge" situation in series use, with things that do not cut off (at some low voltage).
just pointing that out because:
you still don't want to "Mix" them (a partly used with a new or mix different brands).
You still want to use a set as a team, and stop using them when one is depleted (light gone dimer than normal)
Prefer to not let them sit around dead, because they may have been reversed.

OK, so basically the battery has a built in fuse. I was unaware of this. I did not think they had protection circuitry.

I am not comfortable that you cannot overdischarge them when used in series though. I believe that even with well matched pairs, in series one battery could possibly exhaust before the other, perhaps due to some difference in internal resistance, and then receive a reverse charge from the other in the set.

"Overdischarging" is a different danger for lithium primaries than for lithium-ion batteries.

With lithium-ion rechargeables, overdischarge will become hazardous the next time the battery is charged, so they use a protection circuitry that prevents voltage from getting too low. This type of overdischarge is not a concern with lithium primaries because they aren't designed to be recharged.

With lithium primaries in multi-cell lights, reverse charge can happen and is also dangerous. The PTC is essentially a fuse, so it won't prevent this from happening, but it is rare if you follow standard precautions (name brand cells, matched pairs, replace batteries before complete depletion, etc.).

When Surefire primaries become too hot in certain situations, like running the MN21 in an M6 for an extended period of time in a warm climate, they'll likely shut down.

I think that is the only protection in the batteries. It's not really technically an over discharge protection since it's based thermal conditions, but I guess you could say it functions as an over discharge protection in a way as well.