Is the 401k trust revocable or irrevocable?

Your Solo 401k is a qualified retirement plan trust.  As a qualified retirement plan trust, it is not the same as a personal trust, e.g. one that is set up by an individual for estate planning.

The 401k has features of both revocable and irrevocable trusts:

• Similar to an irrevocable trust in that once contributions go into the retirement plan trust, those funds must remain in the trust until distributed to participants
• Similar to a revocable trust in that the plan sponsor (your adopting business) and the document sponsor (Sense Financial) may amend the terms of the trust document

When completing a form for your 401k where one option must be chosen, you can mark revocable as the trust type.