You want both. The harness and the sandbox do different things. The harness says "You have access to tool X, Y, and Z, but not A, B, C". The sandbox says "If

You want both. The harness and the sandbox do different things. The harness says "You have access to tool X, Y, and Z, but not A, B, C". The sandbox says "If you try to use X to access a forbidden resource, I'll prevent you from reaching it".