Modify Post Content via the API

I'm trying to modify a post content via the API, but having a hard time.

Here's a basic query:

mutation UpdatePost {
    updatePost(id: "<ID>", 
    input: {
    mappingFields:
    [
        {
        key: "textContent",
        type: html,
        value: "Modified."
        }
    ]
    }
     ) 
     {
        textContent

and the response:

{
    "data": {
        "updatePost": {
            "textContent": "coming soon..."
        }
    },
    "extensions": {
        "complexity": 1
    }
}

As you can see, the textContent never modifies. I've modified the value to include html and tried various types like "text"... but still nothing.

What am I doing wrong? Is there another way?

Thanks!