system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the rule for using "I'll" in English? Because sometimes it makes sense but sometimes it sounds awkward, for example, "Okay, I'll."
gpt
The contraction is acceptable when the auxiliary verb ("will") is combined with action verb ("do"), not when the action verb is simply implied. 

Will you do the thing?  
I'll do the thing.

Will you *really* do the thing.  
I will.