a question on the word "so"

[marianren123]

here is a conversation between a father and a son:

son: Father, you promised!
Father: Yes, _______ . But it was you who didn't keep your word first.

what should be put into the blank? "so did I" or "so I did", and why?

[Tdol]

So I did= it's true that I did

[Doris]

The answer is "so I did" (as tdol was saying).

'So did I' = the same way did I = I did the same thing --> I promised too (like someone else did).

'So did I' would not work here since the "son" does not mention anyone else, only his father.

Regards,
Doris