diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a336462..8650cde 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -5,7 +5,9 @@ on:
   pull_request:
 jobs:
   test:
-    runs-on: ubuntu-latest
+    # Pinned: ubuntu-latest moved to 26.04, which has no Python 3.9 build, and
+    # pyproject still declares requires-python >= 3.9. Unpin when 3.9 is dropped.
+    runs-on: ubuntu-24.04
     strategy:
       matrix:
         python: ['3.9', '3.10', '3.11', '3.12', '3.13']