diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index bc716135..5c760131 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -16,28 +16,33 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         python-version: ["3.9", "3.10", "3.11", "3.12"]
-        sphinx: [">=5,<8"]  # Newest Sphinx (any)
+        sphinx: [">=5,<9"]  # Newest Sphinx (any)
         myst-parser: [">=1,<3"]  # Newest MyST Parser (any)
         include:
           # Just check the other platforms once
           - os: windows-latest
-            python-version: "3.10"
-            sphinx: "~=7.0"
-            myst-parser: "~=2.0"
+            python-version: "3.12"
+            sphinx: "~=8.0"
+            myst-parser: "~=4.0"
           - os: macos-latest
-            python-version: "3.10"
-            sphinx: "~=7.0"
-            myst-parser: "~=2.0"
+            python-version: "3.12"
+            sphinx: "~=8.0"
+            myst-parser: "~=4.0"
           # Oldest known-compatible dependencies
           - os: ubuntu-latest
             python-version: "3.9"
             sphinx: "==5.0.0"
             myst-parser: "==1.0.0"
-          # Newest known-compatible dependencies
+          # Mid-range dependencies
           - os: ubuntu-latest
-            python-version: "3.12"
+            python-version: "3.11"
             sphinx: "==7.0.0"
             myst-parser: "==2.0.0"
+          # Newest known-compatible dependencies
+          - os: ubuntu-latest
+            python-version: "3.12"
+            sphinx: "==8.0.2"
+            myst-parser: "==4.0.0"
 
     runs-on: ${{ matrix.os }}
 
diff --git a/myst_nb/core/read.py b/myst_nb/core/read.py
index 2b3dae4b..6951f34a 100644
--- a/myst_nb/core/read.py
+++ b/myst_nb/core/read.py
@@ -68,7 +68,7 @@ def create_nb_reader(
     # we check suffixes ordered by longest first, to ensure we get the "closest" match
     iterator = sorted(readers.items(), key=lambda x: len(x[0]), reverse=True)
     for suffix, (reader, reader_kwargs, commonmark_only) in iterator:
-        if path.endswith(suffix):
+        if Path(path).suffix == suffix:
             if isinstance(reader, str):
                 # attempt to load the reader as an object path
                 reader = import_object(reader)
diff --git a/tox.ini b/tox.ini
index d8008132..464ea42a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -16,12 +16,13 @@ envlist = py311-sphinx7
 [testenv]
 usedevelop = true
 
-[testenv:py{39,310,311,312}-sphinx{5,6,7}]
+[testenv:py{39,310,311,312}-sphinx{5,6,7,8}]
 extras = testing
 deps =
     sphinx5: sphinx>=5,<6
     sphinx6: sphinx>=6,<7
     sphinx7: sphinx>=7,<8
+    sphinx8: sphinx>=8,<9
 commands = pytest {posargs}
 
 [testenv:docs-{update,clean}]