diff --git a/Dockerfile b/Dockerfile
index 2de6192715..37070bd991 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -298,7 +298,7 @@ RUN uv pip install --no-cache-dir --no-deps -e "."
 USER root
 RUN mkdir -p /opt/hermes/bin && \
     cp /opt/hermes/docker/hermes-exec-shim.sh /opt/hermes/bin/hermes && \
-    chmod 0755 /opt/hermes/bin/hermes && \
+    chmod 0755 /opt/hermes /opt/hermes/bin/hermes && \
     printf 'docker\n' > /opt/hermes/.install_method
 # The ``.install_method`` stamp is baked next to the running code (the install
 # tree), NOT into $HERMES_HOME. $HERMES_HOME (/opt/data) is a shared data
diff --git a/hermes_constants.py b/hermes_constants.py
index e7af188397..9b6c08a156 100644
--- a/hermes_constants.py
+++ b/hermes_constants.py
@@ -1023,6 +1023,15 @@ def secure_parent_dir(path: Path) -> None:
     # Refuse root and its direct children (/usr, /home, /var, /tmp, …).
     if parent == Path("/") or len(parent.parts) < 3:
         return
+    # Refuse /opt/hermes. The install dir lives on the image layer;
+    # chmodding it to 0700 breaks hermes-user traversal and produces
+    # spurious "Permission denied" on every new exec until manual
+    # `chmod 0755 /opt/hermes`. Reproducer: any auth write to a file
+    # directly under /opt/hermes (e.g. /opt/hermes/auth.json when
+    # HERMES_HOME resolves there) triggers the 0700 chmod and locks
+    # out UID 10000. See issue #25821 follow-up.
+    if str(parent) == "/opt/hermes":
+        return
     try:
         os.chmod(parent, 0o700)
     except OSError: