Synonyms for Unfailing (same or very similar meaning)
WordNet sense 1 (not liable to failure):
foolproof
WordNet sense 2 (incapable of failure or error):
infallible
WordNet sense 3 (incapable of being entirely consumed or used up):
inexhaustible
WordNet sense 4 (unceasing):
unflagging
WordNet sense 5 (steadfast in purpose or devotion or affection):
constant
From the ODE community, based on WordNet
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