diff --git a/src/dask_awkward/lib/io/columnar.py b/src/dask_awkward/lib/io/columnar.py
index 345d1c7e..88b8bb02 100644
--- a/src/dask_awkward/lib/io/columnar.py
+++ b/src/dask_awkward/lib/io/columnar.py
@@ -1,10 +1,11 @@
 from __future__ import annotations
 
 import logging
-from typing import TYPE_CHECKING, TypeVar, cast
+from typing import TYPE_CHECKING, Protocol, TypeVar, cast
 
 import awkward as ak
 from awkward import Array as AwkwardArray
+from awkward.forms import Form
 
 from dask_awkward.layers.layers import ImplementsNecessaryColumns
 from dask_awkward.lib.utils import (
@@ -26,6 +27,22 @@
 T = TypeVar("T")
 
 
+class ImplementsColumnProjectionMixin(ImplementsNecessaryColumns, Protocol):
+    @property
+    def form(self) -> Form:
+        ...
+
+    @property
+    def behavior(self) -> dict | None:
+        ...
+
+    def project_columns(self: T, columns: frozenset[str]) -> T:
+        ...
+
+
+S = TypeVar("S", bound=ImplementsColumnProjectionMixin)
+
+
 class ColumnProjectionMixin(ImplementsNecessaryColumns[FormStructure]):
     """A mixin to add column-centric buffer projection to an IO function.
 
@@ -35,11 +52,11 @@ class ColumnProjectionMixin(ImplementsNecessaryColumns[FormStructure]):
     when only metadata buffers are required.
     """
 
-    def mock(self) -> AwkwardArray:
+    def mock(self: S) -> AwkwardArray:
         return ak.typetracer.typetracer_from_form(self.form, behavior=self.behavior)
 
     def prepare_for_projection(
-        self,
+        self: S,
     ) -> tuple[AwkwardArray, TypeTracerReport, FormStructure]:
         form = form_with_unique_keys(self.form, "@")
 
@@ -58,7 +75,7 @@ def prepare_for_projection(
         )
 
     def necessary_columns(
-        self,
+        self: S,
         report: TypeTracerReport,
         state: FormStructure,
     ) -> frozenset[str]:
@@ -131,10 +148,10 @@ def necessary_columns(
         return frozenset({".".join(p) for p in paths if p})
 
     def project(
-        self: T,
+        self: S,
         report: TypeTracerReport,
         state: FormStructure,
-    ) -> T:
+    ) -> S:
         if not self.use_optimization:
             return self