diff --git a/functions/compose-model-cache/function/fn.py b/functions/compose-model-cache/function/fn.py
index b47f9845c..e81b370e2 100644
--- a/functions/compose-model-cache/function/fn.py
+++ b/functions/compose-model-cache/function/fn.py
@@ -80,10 +80,13 @@ def _namespace(meta: metav1.ObjectMeta | None) -> str:
 # name is a cross-function contract with it.
 _NS_PREFIX = "mp"
 
-# Hydration container. python:3.11-slim has pip; we install huggingface_hub
-# at runtime. A Modelplane-owned image with the tool preinstalled is a
-# follow-up (#115).
-HYDRATION_IMAGE = "python:3.11-slim"
+# Hydration container. It has pip; we install huggingface_hub at runtime. A
+# Modelplane-owned image with the tool preinstalled is a follow-up (#115).
+#
+# Pinned by digest to an image whose pip (26.2.2) distrusts the compromised
+# eMudhra intermediate CAs. Under a bare tag, a node that had already pulled an
+# older image would keep running it.
+HYDRATION_IMAGE = "python:3.13-slim@sha256:57b29a7c050c4e679e4bea572e14b3965cf4b5839de2da49d55e49ffc4024f39"
 HYDRATION_MOUNT = "/mnt/artifact"
 
 # ttlSecondsAfterFinished governs how long the completed Job (and its
diff --git a/functions/compose-model-cache/tests/test_fn.py b/functions/compose-model-cache/tests/test_fn.py
index 33e460630..1b9531ea6 100644
--- a/functions/compose-model-cache/tests/test_fn.py
+++ b/functions/compose-model-cache/tests/test_fn.py
@@ -248,7 +248,7 @@ def _job_object(pc: str, *, command: str = _HYDRATE_CMD, env: list | None = None
                                 "containers": [
                                     {
                                         "name": "hydrate",
-                                        "image": "python:3.11-slim",
+                                        "image": "python:3.13-slim@sha256:57b29a7c050c4e679e4bea572e14b3965cf4b5839de2da49d55e49ffc4024f39",
                                         "command": ["/bin/sh", "-c", command],
                                         "env": env,
                                         "volumeMounts": [{"name": "artifact", "mountPath": "/mnt/artifact"}],