A descriptive activity name leads to a better developer experience:

- You can look at activity and instantly know what action is being performed.
- Servers that implement your FEP don't need to use heuristics, look up object, etc. to figure out what's going on.
- Servers that don't implement your FEP silently ignore unsupported activities, without wasting any resources.

I tried to use the existing vocabulary as much as possible. It seems like this fits the existing grammar, does it not?

{
  type:"Offer"
  object: {
    type:"Endorse"
  }
}

Also, Gilles makes a strong point to just use the word "Trust" -- it's simpler and better known around the world. What do you think of that, instead?

I suggest picking more descriptive names for activities:

- OfferEndorsement
- AcceptEndorsement
- RejectEndorsement
- UndoEndorsement

And maybe Endorse should be an object instead of activity? An endorser may want to update the endorsement metadata (content, image, tags).