Synonyms of 'foolproof'
infallibleDictionary, certain, safe
The system is not 100 per cent foolproof.
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If you say that something is guaranteed to happen, you mean that you are certain that it will happen.
Reports of this kind are guaranteed to cause anxiety.
Thesaurus for foolproof from the Collins English Thesaurus
Something such as a plan or a machine that is foolproof is so well designed, easy to understand , or easy to use that it cannot go wrong or be used wrongly.