diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 78a08825..cc65c283 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -19,7 +19,12 @@ permissions:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    # Pinned to 24.04: the ubuntu-latest label migrated to Ubuntu 26.04 (from
+    # 2026-10-19), where the mongo:8 (8.3.x rapid-release) replica-set container
+    # exits on startup — the mongo:7 service and the rest of the suite are fine.
+    # 24.04 is the last environment this gate was green on (2026-10-23). Revisit
+    # when mongo:8 runs cleanly on 26.04 (pin mongo to the 8.0 LTS line too).
+    runs-on: ubuntu-24.04
 
     # Real-service test gate. Every backend the suite can touch runs for real in
     # CI so the integration tests execute instead of skipping (the gap that let