Wiktionary
a. (context idiomatic of a person, plan, theory, etc. English) shrewd but flawed by overthinking or excessive complexity, with a resulting tendency to be unreliable or unsuccessful.
Usage examples of "too clever by half".
The man was too clever by half, a mocking smile never far from his lips.
Promise or no promise, he could not find it in him to trust Lord Petyr Baelish, who struck him as too clever by half.
Someone in the High House was being too clever by half, and she did not think it was wise but dozy old Molavil-isi, who had barely stirred during the course of the discussion.
Andry was too clever by half and had learned logical argument all too well.
It was wild, it was exceedingly dangerous, it meant gambling everything on one chance, but this box was too clever by half.
Tomas Nau was too clever by half, and he had some strange kind of edge.