diff --git a/python/cudf_polars/cudf_polars/expressions.py b/python/cudf_polars/cudf_polars/expressions.py
index 0b3872bace9..c13e6b6d309 100644
--- a/python/cudf_polars/cudf_polars/expressions.py
+++ b/python/cudf_polars/cudf_polars/expressions.py
@@ -5,7 +5,7 @@
 
 from collections import defaultdict
 from functools import singledispatch
-from typing import TYPE_CHECKING, NamedTuple
+from typing import TYPE_CHECKING
 
 import cudf
 import cudf._lib as libcudf
@@ -35,10 +35,16 @@
     from cudf_polars.typing import ColumnType, Expr, Visitor
 
 
-class ExprVisitor(NamedTuple):
+class ExprVisitor:
     """Object holding rust visitor and utility methods."""
 
+    __slots__ = ("visitor", "in_groupby")
     visitor: Visitor
+    in_groupby: bool
+
+    def __init__(self, visitor: Visitor):
+        self.visitor = visitor
+        self.in_groupby = False
 
     def __call__(self, node: int, context: DataFrame) -> ColumnType:
         """
@@ -175,6 +181,8 @@ def _literal(
 
 @evaluate_expr.register
 def _sort(expr: expr_nodes.Sort, context: DataFrame, visitor: ExprVisitor):
+    if visitor.in_groupby:
+        raise NotImplementedError("sort inside groupby")
     to_sort = visitor(expr.expr, context)
     (stable, nulls_last, descending) = expr.options
     descending, column_order, null_precedence = sort_order(
@@ -198,6 +206,8 @@ def _sort(expr: expr_nodes.Sort, context: DataFrame, visitor: ExprVisitor):
 def _sort_by(
     expr: expr_nodes.SortBy, context: DataFrame, visitor: ExprVisitor
 ):
+    if visitor.in_groupby:
+        raise NotImplementedError("sort_by inside groupby")
     to_sort = visitor(expr.expr, context)
     descending = expr.descending
     sort_keys = [visitor(e, context) for e in expr.by]
@@ -238,6 +248,8 @@ def _gather(expr: expr_nodes.Gather, context: DataFrame, visitor: ExprVisitor):
 
 @evaluate_expr.register
 def _filter(expr: expr_nodes.Filter, context: DataFrame, visitor: ExprVisitor):
+    if visitor.in_groupby:
+        raise NotImplementedError("filter inside groupby")
     result = visitor(expr.input, context)
     mask = visitor(expr.by, context)
     (column,) = plc.stream_compaction.apply_boolean_mask(
@@ -262,6 +274,8 @@ def _column(expr: expr_nodes.Column, context: DataFrame, visitor: ExprVisitor):
 
 @evaluate_expr.register
 def _agg(expr: expr_nodes.Agg, context: DataFrame, visitor: ExprVisitor):
+    if visitor.in_groupby:
+        raise NotImplementedError("nested agg in group_by")
     name = expr.name
     column = visitor(expr.arguments, context)
     # TODO: handle options
@@ -425,56 +439,6 @@ def _binop(
     return plc.binaryop.binary_operation(_as_plc(lop), _as_plc(rop), op, dtype)
 
 
-# Aggregations, need to be shared between plan and expression
-# evaluation, but circular dep, so we put them here.
-# TODO: document approach here properly
-def agg_depth(agg, visitor: ExprVisitor) -> int:
-    """
-    Determine the depth of aggregations in an expression.
-
-    Parameters
-    ----------
-    agg
-        Expression containing aggregations
-    visitor
-        Callback visitor
-
-    Returns
-    -------
-    Depth in the expression tree that an aggregation request was observed.
-
-    Raises
-    ------
-    NotImplementedError
-        If an aggregation request is nested inside another aggregation
-        request, or an unhandled expression is seen.
-    """
-    agg = visitor.visitor.view_expression(agg)
-    if isinstance(agg, expr_nodes.Column):
-        return 0
-    elif isinstance(agg, expr_nodes.Alias):
-        return agg_depth(agg.expr, visitor)
-    elif isinstance(agg, expr_nodes.BinaryExpr):
-        ldepth = agg_depth(agg.left, visitor)
-        rdepth = agg_depth(agg.right, visitor)
-        maxdepth = max(ldepth, rdepth)
-        assert ldepth == rdepth
-        return maxdepth
-    elif isinstance(agg, expr_nodes.Len):
-        return 1
-    elif isinstance(agg, expr_nodes.Agg):
-        # TODO: currently only singleton arguments (that's all that
-        # the Agg object has right now)
-        depth = agg_depth(agg.arguments, visitor)
-        if depth >= 1:
-            raise NotImplementedError("Nesting aggregations not supported")
-        return depth + 1
-    else:
-        raise NotImplementedError(f"Unhandled agg {agg=}")
-
-
-# TODO: would love to find a way to avoid the multiple traversal
-# right now, we must run agg_depth first.
 def collect_agg(
     node: int, context: DataFrame, depth: int, visitor: ExprVisitor
 ) -> tuple[
@@ -483,6 +447,17 @@ def collect_agg(
     """
     Collect the aggregation requirements of a single aggregation request.
 
+    Parameters
+    ----------
+    node
+        Node representing aggregation to collect
+    context
+        DataFrame context
+    depth
+        Depth of the aggregation in tree
+    visitor
+        Visitor for translating nodes
+
     Returns
     -------
     tuple of list of columns, list of (libcudf-agg-name, agg-expression) pairs,
@@ -551,10 +526,14 @@ def collect_agg(
             rcol, rreq = collect_agg(agg.right, context, depth, visitor)
             return [*lcol, *rcol], [*lreq, *rreq]
         else:
-            # TODO: Inside an aggregation, this needs to disallow (for now)
-            # seeing an aggregation request.
+            # TODO: Ugly non-local method of saying "we're in a groupby, disallow"
+            visitor.in_groupby = True
             column = evaluate_expr(agg, context, visitor)
+            visitor.in_groupby = False
             return [column], [(plc.aggregation.collect_list(), node)]
+    elif isinstance(agg, expr_nodes.Literal):
+        # Scalar value, constant across the groups
+        return [], []
     else:
         raise NotImplementedError
 
@@ -602,6 +581,10 @@ def collect_aggs(
                 _, column_requests, to_replace = groups.setdefault(
                     colid, (column, [], defaultdict(list))
                 )
+            if request is None:
+                # Literals, which don't produce requests since they must be
+                # uniform across the group
+                continue
             # We're only going to ask libcudf for unique aggregation requests
             if request not in column_requests:
                 column_requests.append(request)