fix(cloud): allow explicitly disabled box runtime

deploy/prod/docker-compose.yml

@@ -67,7 +67,10 @@ services:
       PLUGIN__WORKER__MAX_TOTAL_MEMORY_MB: "4096"
       PLUGIN__WORKER__REQUIRE_HARD_LIMITS: "true"
       LANGBOT_PLUGIN_RUNTIME_CONTROL_TOKEN: ${PLUGIN_RUNTIME_CONTROL_TOKEN}
-      BOX__ENABLED: "true"
+      # Cloud v2 currently grants no managed Box capability. Keep the shared
+      # runtime deployed but disable Core integration until a hard-quota-capable
+      # backend can satisfy the fail-closed Cloud readiness contract.
+      BOX__ENABLED: "false"
       BOX__BACKEND: nsjail
       BOX__RUNTIME__ENDPOINT: ws://box:5410
       BOX__ADMISSION__REQUIRED: "true"

src/langbot/pkg/cloud/bootstrap.py

@@ -138,8 +138,14 @@ class VerifiedCloudDeployment:
         if plugin_worker.get('require_hard_limits') is not True:
             raise CloudBootstrapError('Cloud Runtime requires plugin.worker.require_hard_limits=true')
         box_config = config.get('box', {})
-        if box_config.get('enabled') is not True:
-            raise CloudBootstrapError('Cloud runtime requires box.enabled=true')
+        box_enabled = box_config.get('enabled')
+        if box_enabled is False:
+            # Explicitly disabling Box removes the sandbox surface entirely and
+            # therefore does not weaken tenant isolation. Validate the strict
+            # runtime/admission contract only when the surface is enabled.
+            return
+        if box_enabled is not True:
+            raise CloudBootstrapError('Cloud runtime requires box.enabled to be an explicit boolean')
         if box_config.get('backend') != 'nsjail':
             raise CloudBootstrapError('Cloud runtime requires box.backend=nsjail')
         runtime_endpoint = str(box_config.get('runtime', {}).get('endpoint', '') or '').strip()

tests/unit_tests/cloud/test_bootstrap.py

@@ -228,10 +228,23 @@ async def test_cloud_pgvector_contract_is_fail_closed(pgvector_config, message):
         )
 
 
+async def test_cloud_runtime_allows_explicitly_disabled_box():
+    config = _cloud_config()
+    config['box']['enabled'] = False
+
+    deployment = await resolve_deployment(
+        instance_uuid='instance-a',
+        instance_config=config,
+        entry_points=lambda: _EntryPoints([_EntryPoint(_Provider())]),
+        now=1_000,
+    )
+
+    assert isinstance(deployment, VerifiedCloudDeployment)
+
+
 @pytest.mark.parametrize(
     ('mutate', 'message'),
     [
-        (lambda config: config['box'].update(enabled=False), 'box.enabled=true'),
         (lambda config: config['box'].update(backend='docker'), 'box.backend=nsjail'),
         (lambda config: config['box']['runtime'].update(endpoint=''), 'box.runtime.endpoint'),
         (