diff --git a/stacks/fish-cpp/Dockerfile b/stacks/fish-cpp/Dockerfile
index f95723c..9ba714d 100644
--- a/stacks/fish-cpp/Dockerfile
+++ b/stacks/fish-cpp/Dockerfile
@@ -50,7 +50,13 @@ ENV DEBIAN_FRONTEND=noninteractive \
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
       python3 python3-pip python3-venv tini \
+      libgomp1 \
     && rm -rf /var/lib/apt/lists/*
+# libgomp1 = GNU OpenMP runtime — required by the s2 binary at runtime.
+# CMake auto-enabled OpenMP at build time (gcc has -fopenmp), so the
+# binary dynamically links libgomp.so.1; the slim cuda:runtime base
+# doesn't include it by default. Without this, s2 fails on first
+# invocation with "error while loading shared libraries: libgomp.so.1".
 
 # Pull the shim deps into an isolated venv so we don't fight system pip.
 RUN python3 -m venv /opt/venv