diff --git a/scripts/b3_isolated_policy.sh b/scripts/b3_isolated_policy.sh
index ccf3203..cb4eb3e 100755
--- a/scripts/b3_isolated_policy.sh
+++ b/scripts/b3_isolated_policy.sh
@@ -79,9 +79,13 @@ for gpu in $GPU_INDICES; do
             $EXTRA_VLLM_ARGS \
             > "$RUNDIR/logs/vllm_inst_${i}_gpu${gpu}.log" 2>&1 &
     elif [ "$ENABLE_KV_BOTH" = "1" ] && [ "$KV_CONNECTOR" = "Nixl" ]; then
-        # NixlConnector uses UCX side-channels for handshake (no bootstrap
-        # port needed). Side-channel host defaults to NIC IP discovered
-        # at register_kv_caches time.
+        # NixlConnector's handshake listener binds to a fixed default
+        # port 5600 unless VLLM_NIXL_SIDE_CHANNEL_PORT is overridden.
+        # Multiple instances on the same host MUST use distinct ports
+        # or only one will start; the rest hit
+        # `zmq.error.ZMQError: Address already in use`.
+        nixl_port=$((5600 + i))
+        VLLM_NIXL_SIDE_CHANNEL_PORT=$nixl_port \
         AGENTIC_STEP_LOG_PATH="$RUNDIR/engine_state/engine_${i}.jsonl" \
         AGENTIC_WORKER_ID="engine_${i}" \
         CUDA_VISIBLE_DEVICES=$gpu \