diff --git a/src/codeprobe/sandbox/Dockerfile.sg_only b/src/codeprobe/sandbox/Dockerfile.sg_only
index 82e2609..056d245 100644
--- a/src/codeprobe/sandbox/Dockerfile.sg_only
+++ b/src/codeprobe/sandbox/Dockerfile.sg_only
@@ -3,7 +3,7 @@
 # Provides python + git + ripgrep + coreutils so agents can run read/search/
 # script workloads inside a container whose only writable surfaces are
 # those explicitly bind-mounted :rw by the caller.
-FROM python:3.12-slim
+FROM python:3.12-slim@sha256:78387bc3881b8273120a12ebe6c1ab22b018ccc2c9adf565ae1ac9b536e184ea
 
 ENV DEBIAN_FRONTEND=noninteractive \
     PYTHONDONTWRITEBYTECODE=1 \