How to insert item into array using PatchEvent in custom Sanity input?

I can see the issue! You're creating the PatchEvent but not actually calling the onChange handler to apply it. PatchEvent.from() just creates the action - it doesn't automatically update the document. You need to pass it to the onChange prop that your custom input component receives.

Here's the corrected version:

const handleAction = (field) => {
    const patches = [
      { path: [], type: "setIfMissing", value: [] }, 
      {
        type: "insert",
        items: [{ _type: field.name }],
        path: [-1]
      }
    ]

    onChange(PatchEvent.from(patches))  // ← Add this!
}

The onChange function should be passed as a prop to your custom input component from Sanity Studio. Make sure your component signature includes it:

const MyCustomInput = ({ value, onChange, type }) => {
  // your component code
}

Also, heads up: the import syntax you're using (part:@sanity/form-builder/patch-event) is from the older Sanity Studio v2. If you're on Studio v3, you'll want to update to the newer import style. Check out the PatchEvent documentation for the current approach.

For array insertions specifically, you might also want to chain setIfMissing using .prepend() to ensure the array exists before inserting:

const patches = insert([{ _type: field.name }], 'after', [-1])
const patchEvent = PatchEvent.from(patches).prepend(setIfMissing([]))
onChange(patchEvent)

This ensures you're initializing the array if it doesn't exist before trying to insert items into it.