The stellar keys add <name> --seed-phrase command accepts a seed phrase, but does not validate it.

You can pass it an 11 word phrase, or a malformed string, and it'll accept it.

For example, you can enter gibberish:

$ stellar keys add a4 --seed-phrase
Type a 12 word seed phrase: 
asdflkjaslkdfjklasdjflk

$ cat ~/.stellar/identity/a4.toml
seed_phrase = "asdflkjaslkdfjklasdjflk"

The add command when used with the seed phrase should only allow entering a valid 12 or 24 word key phrase.