diff --git a/ci/Jenkinsfile.hermes-agent-image b/ci/Jenkinsfile.hermes-agent-image
index b034de5e..10acae62 100644
--- a/ci/Jenkinsfile.hermes-agent-image
+++ b/ci/Jenkinsfile.hermes-agent-image
@@ -100,9 +100,14 @@ spec:
         requests:
           cpu: 250m
           memory: 1Gi
+          # The agent image build expands the SQLite and web build layers in
+          # its workspace. Reserve enough node-local space to keep it off the
+          # 25 GiB workers, whose unrequested Kaniko workspace was evicted.
+          ephemeral-storage: 32Gi
         limits:
           cpu: "2"
           memory: 4Gi
+          ephemeral-storage: 40Gi
 """
     }
   }
@@ -226,6 +231,21 @@ spec:
     stage('Build arm64 leg without a daemon') {
       steps {
         container('kaniko') {
+          sh '''#!/busybox/sh
+            set -eu
+            minimum_available_kib=16777216
+            available_kib="$(/busybox/df -Pk / | /busybox/awk 'NR == 2 { print $4 }')"
+            case "${available_kib}" in
+              ''|*[!0-9]*)
+                echo "cannot determine Kaniko ephemeral-storage availability" >&2
+                exit 2
+                ;;
+            esac
+            if [ "${available_kib}" -lt "${minimum_available_kib}" ]; then
+              echo "Kaniko requires at least 16 GiB of free ephemeral storage" >&2
+              exit 1
+            fi
+          '''
           withCredentials([usernamePassword(
             credentialsId: 'harbor-robot',
             usernameVariable: 'HARBOR_USER',
diff --git a/testing/tests/test_hermes_image_builder_fresh_review.py b/testing/tests/test_hermes_image_builder_fresh_review.py
index 6df1a15c..e731482b 100644
--- a/testing/tests/test_hermes_image_builder_fresh_review.py
+++ b/testing/tests/test_hermes_image_builder_fresh_review.py
@@ -77,6 +77,25 @@ def test_builder_prefers_rpi5_with_healthy_arm64_worker_fallback() -> None:
     }
 
 
+def test_builder_reserves_storage_before_kaniko_expands_the_workspace() -> None:
+    """The ARM build cannot be scheduled onto the 25 GiB worker disks."""
+    containers = {item["name"]: item for item in _pod_spec()["containers"]}
+    resources = containers["kaniko"]["resources"]
+    assert resources["requests"]["ephemeral-storage"] == "32Gi"
+    assert resources["limits"]["ephemeral-storage"] == "40Gi"
+    source = PIPELINE.read_text(encoding="utf-8")
+    arm_build = source.split("stage('Build arm64 leg without a daemon')", 1)[1].split(
+        "stage('Build amd64 leg without a daemon')", 1
+    )[0]
+    assert 'minimum_available_kib=16777216' in arm_build
+    assert '/busybox/df -Pk /' in arm_build
+    assert 'cannot determine Kaniko ephemeral-storage availability' in arm_build
+    assert 'Kaniko requires at least 16 GiB of free ephemeral storage' in arm_build
+    assert arm_build.index("minimum_available_kib=16777216") < arm_build.index(
+        "withCredentials([usernamePassword("
+    )
+
+
 @pytest.mark.parametrize(
     "unsupported",
     [