diff --git a/docs/gateway/sandboxing.md b/docs/gateway/sandboxing.md
index cc2c24e0d57..a2a3567d88d 100644
--- a/docs/gateway/sandboxing.md
+++ b/docs/gateway/sandboxing.md
@@ -369,6 +369,8 @@ Default Docker image: `openclaw-sandbox:bookworm-slim`
 
     The default image does **not** include Node. If a skill needs Node (or other runtimes), either bake a custom image or install via `sandbox.docker.setupCommand` (requires network egress + writable root + root user).
 
+    OpenClaw does not silently substitute plain `debian:bookworm-slim` when `openclaw-sandbox:bookworm-slim` is missing. Sandbox runs that target the default image fail fast with a build instruction until you run `scripts/sandbox-setup.sh`, because the bundled image carries `python3` for sandbox write/edit helpers.
+
   </Step>
   <Step title="Optional: build the common image">
     For a more functional sandbox image with common tooling (for example `curl`, `jq`, `nodejs`, `python3`, `git`):