feat(box): support explicit host backend (#2498)

skills/skills.index.json

@@ -1349,7 +1349,8 @@
             "local-agent",
             "tools",
             "e2b",
-            "nsjail"
+            "nsjail",
+            "host"
           ],
           "automation": "",
           "setup_automation": [],

skills/skills/langbot-deploy/SKILL.md

@@ -63,7 +63,7 @@ Key settings:
 | `api.global_api_key` | **Global API key** for the HTTP API + MCP server. Non-empty = accepted with no login/DB record; no `lbk_` prefix required. Empty = disabled. Plaintext — trusted/internal only, serve over HTTPS. |
 | `plugin.runtime_ws_url` | Standalone plugin runtime WS URL (e.g. `ws://langbot_plugin_runtime:5400/control/ws`) |
 | `box.enabled` | Master switch for the Box sandbox runtime |
-| `box.backend` | `local` (Docker/nsjail autopick) / `docker` / `nsjail` / `e2b`; env override `BOX__BACKEND` |
+| `box.backend` | `local` (Docker/nsjail autopick) / `docker` / `nsjail` / `e2b` / explicit unsafe `host`; env override `BOX__BACKEND` |
 | `box.runtime.endpoint` | External Box runtime URL (e.g. `ws://127.0.0.1:5410`); empty = local auto-managed |
 
 Many keys have `ENV__SUBKEY` overrides (e.g. `BOX__BACKEND`, `BOX__ENABLED`).
@@ -75,6 +75,10 @@ Many keys have `ENV__SUBKEY` overrides (e.g. `BOX__BACKEND`, `BOX__ENABLED`).
   with `--standalone-runtime`.
 - Box has a parallel `--standalone-box` flag; the Docker box host is
   `langbot_box:5410`.
+- `box.backend: host` runs commands directly as the Box Runtime system user.
+  It is never auto-selected, provides no sandbox isolation, and is only for
+  trusted local development. A WebSocket-controlled host backend requires
+  `LANGBOT_BOX_CONTROL_TOKEN`; local stdio control is allowed.
 
 ## Global API key — enabling for agents/automation
 
@@ -93,5 +97,7 @@ login session. See `langbot-mcp-ops` for using it, and `docs/API_KEY_AUTH.md`.
 - "No supported sandbox backend (Docker / nsjail / E2B)" with Docker running
   usually means the user isn't in the `docker` group →
   `sudo usermod -aG docker <user>` and restart in a new shell.
+- Do not use `box.backend: host` as a production fallback. It cannot enforce
+  image, filesystem, network, PID, CPU, memory, or storage isolation.
 - Box root host/container path mismatch breaks sandbox container creation.
 - Don't commit a non-empty `api.global_api_key` to version control.

skills/skills/langbot-testing/cases/sandbox-skill-authoring-e2e.yaml

@@ -13,6 +13,7 @@ tags:
   - tools
   - e2b
   - nsjail
+  - host
 skills:
   - langbot-env-setup
   - langbot-testing
@@ -23,7 +24,7 @@ env:
   - LANGBOT_LOCAL_AGENT_PIPELINE_NAME
 preconditions:
   - "LANGBOT_LOCAL_AGENT_PIPELINE_URL or LANGBOT_LOCAL_AGENT_PIPELINE_NAME points to the local-agent pipeline under test."
-  - "LangBot is started with the sandbox backend intended for this run, such as e2b or nsjail."
+  - "LangBot is started with the Box backend intended for this run, such as e2b, nsjail, or explicit host development mode."
   - "The selected model route supports tool/function calling strongly enough to invoke sandbox tools."
 steps:
   - "Start LangBot with the target sandbox backend and confirm the Box status UI or LANGBOT_BACKEND_URL /api/v1/box/status reports the expected backend."
@@ -33,7 +34,7 @@ steps:
 checks:
   - "UI: Debug Chat final assistant response contains E2E_OK:<skill-name>."
   - "Logs: The model called exec, register_skill, activate, then exec again from the activated skill path."
-  - "Logs: The selected backend name is the expected one, such as e2b or nsjail."
+  - "Logs: The selected backend name is the expected one, such as e2b, nsjail, or host."
   - "Skill store: The registered package and activated writeback match references/sandbox-skill-authoring.md."
   - "Box status: recent_error_count is 0 after the run."
 evidence_required:

skills/skills/langbot-testing/references/sandbox-skill-authoring.md

@@ -4,7 +4,7 @@
 
 Verify that Local Agent can use sandbox tools to create, register, activate, and use a LangBot skill package through the same path a user would exercise in Debug Chat.
 
-This flow applies to Docker, nsjail, and E2B backends. API calls are useful diagnostics, but the primary pass/fail signal is the model-driven Debug Chat tool sequence.
+This flow applies to Docker, nsjail, E2B, and the explicit host development backend. Host runs commands directly as the Box Runtime user and must never be treated as sandbox-isolation coverage. API calls are useful diagnostics, but the primary pass/fail signal is the model-driven Debug Chat tool sequence.
 
 ## Preconditions
 
@@ -13,6 +13,7 @@ This flow applies to Docker, nsjail, and E2B backends. API calls are useful diag
    - `BOX_BACKEND=e2b` when validating E2B.
    - `BOX_BACKEND=nsjail` when validating nsjail.
    - `BOX_BACKEND=local` or `docker` when validating local container fallback.
+   - `BOX_BACKEND=host` only when validating explicit, trusted local direct execution.
 3. Confirm `/api/v1/box/status` reports `available: true` and the expected backend name.
 4. Confirm Debug Chat uses a model with function-calling ability.
 5. Confirm backend logs say native sandbox tools are available.
@@ -71,7 +72,7 @@ Backend logs should show:
 - `register_skill`
 - `activate`
 - a second `exec` whose workdir is `/workspace/.skills/<skill-name>`
-- `backend=e2b`, `backend=nsjail`, or the expected local backend
+- `backend=e2b`, `backend=nsjail`, `backend=host`, or the expected local backend
 
 After the run, verify the skill store through the UI or API:
 
@@ -125,6 +126,8 @@ For E2B raw HTTP diagnostics, include a valid template id such as `base`; a miss
 - Session metadata should keep LangBot logical paths such as `/workspace`; storing provider-internal paths can make later requests look incompatible.
 - nsjail versions differ. Some expose only `--disable_clone_new*` flags and use `--bindmount` instead of `--rw_bind`.
 - On WSL, cgroup v2 may exist but not be writable. The backend should warn and fall back to rlimits rather than fail the sandbox.
+- The host backend does not honor sandbox image, network, rootfs, process, or
+  resource isolation. Use a disposable workspace and low-privilege account.
 - If `ALL_PROXY` uses a SOCKS URL and `socksio` is not installed, some Python HTTP clients can fail during startup. Prefer consistent HTTP proxy variables unless SOCKS support is installed.
 
 ## Related Troubleshooting

skills/skills/langbot-testing/troubleshooting/sandbox-native-tools-unavailable.yaml

@@ -3,7 +3,7 @@ title: "Native sandbox tools are unavailable even though a backend is configured
 date: 2026-05-18
 symptoms:
   - "Backend logs show Native sandbox tools (exec/read/write/edit/glob/grep) are NOT available."
-  - "The Box runtime later reports that E2B, nsjail, or Docker is configured."
+  - "The Box runtime later reports that E2B, nsjail, Docker, or explicit host mode is configured."
   - "Debug Chat does not expose exec, register_skill, or activate as usable tools."
 patterns:
   - "Native sandbox tools ... are NOT available"
@@ -19,6 +19,7 @@ fix_steps:
   - "Ensure the Box runtime reselects a backend when get_backend_info is called and the cached backend is empty."
   - "For E2B, verify the key without printing it and confirm any required template setting."
   - "For nsjail, run nsjail --help and confirm the binary is on PATH for the LangBot process."
+  - "For trusted local development only, explicitly set box.backend=host; never use host as a production sandbox fallback."
 verification: "Run sandbox-skill-authoring-e2e. Logs should show Native sandbox tools are available and /api/v1/box/status should report available=true with the expected backend."
 related_cases:
   - sandbox-skill-authoring-e2e

src/langbot/pkg/box/service.py

@@ -455,7 +455,9 @@ class BoxService:
 
     async def _require_validated_workspace_sandbox(self, execution_context: ExecutionContext) -> None:
         if not self._available:
-            raise BoxError('Box runtime is not available. Install and start Docker to use sandbox features.')
+            raise BoxError(
+                'Box runtime is not available. Configure an available Box backend before using Box features.'
+            )
         if self._cloud_managed:
             if self._admission is None:
                 raise BoxAdmissionError('Cloud Box sandbox admission is unavailable')
@@ -565,7 +567,9 @@ class BoxService:
         skip_host_mount_validation: bool = False,
     ) -> dict:
         if not self._available:
-            raise BoxError('Box runtime is not available. Install and start Docker to use sandbox features.')
+            raise BoxError(
+                'Box runtime is not available. Configure an available Box backend before using Box features.'
+            )
         execution_context = await self._validated_execution_context(self._query_execution_context(query))
         spec_payload = self._managed_policy_payload(execution_context, spec_payload)
         await self._require_validated_workspace_sandbox(execution_context)
@@ -2142,5 +2146,8 @@ class BoxService:
             if backend_name:
                 payload['connector_error'] = f'Configured sandbox backend "{backend_name}" is unavailable'
             else:
-                payload['connector_error'] = 'No supported sandbox backend (Docker / nsjail / E2B) is available'
+                payload['connector_error'] = (
+                    'No supported sandbox backend (Docker / nsjail / E2B) is available. '
+                    'Trusted local development may explicitly select the unsafe host backend.'
+                )
         return payload

src/langbot/pkg/provider/tools/loaders/native.py

@@ -222,6 +222,7 @@ class NativeToolLoader(loader.ToolLoader):
             self.ap.logger.warning(
                 'Native sandbox tools (exec/read/write/edit/glob/grep) are NOT available. '
                 'No sandbox backend (Docker/nsjail/E2B) is ready. '
+                'Trusted local development may explicitly select box.backend=host. '
                 'The LLM will not have access to code execution or file operation tools.'
             )
 

src/langbot/pkg/provider/tools/loaders/skill_authoring.py

@@ -43,6 +43,7 @@ class SkillToolLoader(loader.ToolLoader):
             self.ap.logger.info(
                 'Skill tools (activate/register_skill) are NOT available. '
                 'No sandbox backend (Docker/nsjail/E2B) is ready. '
+                'Trusted local development may explicitly select box.backend=host.'
             )
 
     async def _check_sandbox_available(self) -> bool:

src/langbot/templates/config.yaml

@@ -331,7 +331,10 @@ box:
     # skill tool, skill add/edit, and stdio-mode MCP servers. Skills can still
     # be listed read-only and http/sse MCP servers continue to work.
     enabled: true
-    backend: 'local'  # 'local' (Docker/nsjail), 'docker', 'nsjail', or 'e2b'. Can be written via BOX__BACKEND.
+    # 'host' runs commands directly as the Box Runtime user without sandbox
+    # isolation. It is never auto-selected and is only for trusted local
+    # development. Can be written via BOX__BACKEND.
+    backend: 'local'  # 'local' (Docker/nsjail), 'docker', 'nsjail', 'e2b', or explicit unsafe 'host'.
     runtime:
         # LANGBOT_BOX_CONTROL_TOKEN is optional for OSS external WebSocket
         # runtimes. To protect an exposed endpoint, set the same strong secret