Whether they are fixed or not is not information readily available to the
pilot (there are plenty of examples of controller boundaries that are not
fixed).
Cite them.

The boundaries between what controllers handle what flight on what
frequencies for example, which depends on LOA and the winds. The
internal boundaries of airspace that is sometimes approach and sometiems
center. And if I'm citing them incorrectly, I know some stuff like that
has been discussed here, and I bet you remember what it really is and
could fill me in. It's one of the reasons that these boundaries are not
shown on charts - they keep changing, even as to altitudes.

If I were a controller, I'd know what they are. I'm just a pilot, so I
dial the frequencies I'm handed off to, and everyone's happy.

And the pilot =does= need to know what the boundaries are in
order to make an intellegent decision as to whether to go around it or do
something else.
Why doesn't he ask?

I suppose if it were me, I would ask. But it would take me a moment or
two to realize that this is what they are expecting.

I guess I'm just too used to getting reroutes I can accept or reject,
rather than "can't go there, try again".

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.