system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] Why wasn't Stormbreaker an ace-in-the-hole against Thanos in Endgame like it seemed to be in Infinity War?
gpt
Thor took Thanos by surprise in Wakanda.

Stormbreaker is an immensely powerful weapon, but...the gauntlet could've countered it regardless. It's just that Thanos didn't know what he was dealing with there, so dealt with it like he would anything else, only it didn't work. The element of surprise can be an amazing tool.

Granted, he didn't have the gauntlet for most of Endgame, but he *was* aware of the weapon this time around (through Nebula's footage), and Thor didn't get the drop on him. That's the crucial bit.

There's never a point in Endgame where Stormbreaker *fails* to damage Thanos (it cuts through him just fine on Thanos' farm). He just avoids getting hit, disarms the weapon, turns it on Thor, etc. He's strong and skilled and smart enough to do so, and he knows what he's dealing with. 

Plus, he has his own nigh-indestructible blade to work with that presents a great threat to Thor in turn, and is useful as a defensive tool as well.