I don’t understand why you think it would stop that, given how comprehensive a record of self-contradiction he already has.

https://www.nytimes.com/2025/08/08/business/japan-tariffs-us...

https://www.asahi.com/sp/ajw/articles/15953967

Well, having the paper agreement certainly helped last time. (A month ago.)

There's also a Language barrier. So making sure there is a written agreement ensures that both sides understand what was agreed upon.

Maybe more in that it gives the Japanese an easy out to change terms when America inevitably breaks the deal.

True, but the Japanese government is accountable to Japanese voters and their business community. If they see Trump reneging on or twisting an agreement they'll be more likely to blame Trump than the LDP. He's not popular there nowadays except among Sanseito members.