diff --git a/pyproject.toml b/pyproject.toml
index 3abe3f8..b743380 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -18,6 +18,8 @@ readme = "README.rst"
 keywords = [
     "image processing",
 ]
+# Keep the classic table for compatibility with older pip/Python versions.
+# PEP 639 requires Core Metadata 2.4 and setuptools>=77 (Python>=3.9).
 license = {text = "MIT"}
 requires-python = ">=3.7"
 classifiers = [