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that's useful to know, your videos are also very helpful

It's not clear to me why you need the API for this. I don't think it's an issue with your FeatureScript code (though you may want to rename it from "filletEverything")

I did my own test and got it working. Here's the FeatureScript and API body I used with glassworks.

FeatureScript

FeatureScript 2581;
import(path : "onshape/std/common.fs", version : "2581.0");

annotation { "Feature Type Name" : "Add a plane", "Feature Type Description" : "" }
export const addPlane = defineFeature(function(context is Context, id is Id, definition is map)
precondition
{
// Define the parameters of the feature type
}
{
opPlane(context, id + "plane1", {
"plane" : plane(vector(0, 0, 6) * inch, vector(0, 0, 1))
});
});

API Request Body for POST/partstudios/d/{did}/{wvm}/{wvmid}/e/{eid}/features

{
"btType": "BTFeatureDefinitionCall-1406",
"feature": {
"btType": "BTMFeature-134",
"namespace": "e548d56ece749371a5a6ad6ec::m3e236759fa16868e79d8699e",
"name": "Add a plane 1",
"suppressed": false,
"parameters": [],
"featureId": "FFdDMAZL9at4Q6m_0",
"nodeId": "KG/OVwqfZXVDmvzg",
"featureType": "addPlane",
"returnAfterSubfeatures": false,
"subFeatures": []
}
}