Definition - What does Bulletproof mean?
In IT, algorithms or other tech products may be referred to as “bulletproof,” meaning that they are resistant to failure and/or adequately protected from threats. A bulletproof algorithm or piece of technology has been designed well, to withstand challenges and operate in a wide spectrum of scenarios.
Armor-plated is also used in the same sense as bulletproof.
Techopedia explains Bulletproof
Some experts point out specific successes associated with bulletproof technologies. One is that they're resistant to loss – with accurate design, they are capable of producing accurate results. Another characteristic of a bulletproof algorithm is that it is capable of successfully handling exceptions or error conditions. The idea is that the programmer has built abundant safeguards into the technology, so that it is resistant to failure and able to tackle tough problems or respond in tough situations.