Is Inuyasha the source of all isekai?

A Connecticut Yankee In King Arthur's Court was the first isekai

What titles did he put in it?

it is

This is Yea Forums - Anime & Manga you stupid nigger, fuck off.

then don't ask questions where the answer isn't anime or manga, faggot

OP doesn't have to prefix his question with "in the context of anime & manga" because he didn't post on Yea Forums

then he's asking a pointless question that doesn't need to take up a space on the board

That's Alternate History. And you do understand isekai is a genre? There aren't multiple isekais. Or a "first". It's 1 type of story given the name "isekai".
When you call a title an isekai, you're using a figure of speech called a synecdoche. Refering to a thing as something else it's associated with. In this case the genre.
It's a noun. Not an adjective. It's not some quality that you call isekai-ness. It's what the story - in it's entirety - does. Got it? The story. Not the setting. Not the characters. Not the tropes or memes. The STORY. That's what you're distinguishing by putting it in a genre.

Are Inuyasha and Connecticut Yankee anything alike? No. They're completely different stories. There're completely different mediums to be exact. So they're definitely not in the same genre.

>Reading everything literal.
You're not smart enough to carry a discussion. And the board doesn't exist to entertain you. I'm happy to educate, and maybe one day you'll be able to join a conversation without changing the subject.

>And you do understand isekai is a genre
You're the only one saying that.

Urashima Taro is the first isekai anime