diff --git a/docs/tools/acp-agents.md b/docs/tools/acp-agents.md
index 0eb36a43c09..6188b04c0a1 100644
--- a/docs/tools/acp-agents.md
+++ b/docs/tools/acp-agents.md
@@ -120,6 +120,19 @@ Interface details:
 - `cwd` (optional): requested runtime working directory (validated by backend/runtime policy).
 - `label` (optional): operator-facing label used in session/banner text.
 
+## Sandbox compatibility
+
+ACP sessions currently run on the host runtime, not inside the OpenClaw sandbox.
+
+Current limitations:
+
+- If the requester session is sandboxed, ACP spawns are blocked.
+  - Error: `Sandboxed sessions cannot spawn ACP sessions because runtime="acp" runs on the host. Use runtime="subagent" from sandboxed sessions.`
+- `sessions_spawn` with `runtime: "acp"` does not support `sandbox: "require"`.
+  - Error: `sessions_spawn sandbox="require" is unsupported for runtime="acp" because ACP sessions run outside the sandbox. Use runtime="subagent" or sandbox="inherit".`
+
+Use `runtime: "subagent"` when you need sandbox-enforced execution.
+
 ### From `/acp` command
 
 Use `/acp spawn` for explicit operator control from chat when needed.
@@ -404,6 +417,8 @@ Restart the gateway after changing these values.
 | `--thread here requires running /acp spawn inside an active ... thread`  | `--thread here` used outside a thread context.                                  | Move to target thread or use `--thread auto`/`off`.                                                                                                               |
 | `Only <user-id> can rebind this thread.`                                 | Another user owns thread binding.                                               | Rebind as owner or use a different thread.                                                                                                                        |
 | `Thread bindings are unavailable for <channel>.`                         | Adapter lacks thread binding capability.                                        | Use `--thread off` or move to supported adapter/channel.                                                                                                          |
+| `Sandboxed sessions cannot spawn ACP sessions ...`                       | ACP runtime is host-side; requester session is sandboxed.                       | Use `runtime="subagent"` from sandboxed sessions, or run ACP spawn from a non-sandboxed session.                                                                  |
+| `sessions_spawn sandbox="require" is unsupported for runtime="acp" ...`  | `sandbox="require"` requested for ACP runtime.                                  | Use `runtime="subagent"` for required sandboxing, or use ACP with `sandbox="inherit"` from a non-sandboxed session.                                               |
 | Missing ACP metadata for bound session                                   | Stale/deleted ACP session metadata.                                             | Recreate with `/acp spawn`, then rebind/focus thread.                                                                                                             |
 | `AcpRuntimeError: Permission prompt unavailable in non-interactive mode` | `permissionMode` blocks writes/exec in non-interactive ACP session.             | Set `plugins.entries.acpx.config.permissionMode` to `approve-all` and restart gateway. See [Permission configuration](#permission-configuration).                 |
 | ACP session fails early with little output                               | Permission prompts are blocked by `permissionMode`/`nonInteractivePermissions`. | Check gateway logs for `AcpRuntimeError`. For full permissions, set `permissionMode=approve-all`; for graceful degradation, set `nonInteractivePermissions=deny`. |