diff --git a/pyproject.toml b/pyproject.toml
index 97739dd..be6b72f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -49,7 +49,7 @@ tests = [
     "coverage",
 ]
 docs = [
-    "sphinx",
+    "sphinx>=8.0.0,<9.0.0",
     "autodocsumm>=0.2.14",
     "pydata-sphinx-theme",
     "sphinx_mdinclude",