diff --git a/src/agent_env/task_step/task_steps/sync_env_clock.py b/src/agent_env/task_step/task_steps/sync_env_clock.py
index aacdc5c40..6f1ed4c3c 100644
--- a/src/agent_env/task_step/task_steps/sync_env_clock.py
+++ b/src/agent_env/task_step/task_steps/sync_env_clock.py
@@ -3,6 +3,8 @@
 Arms the clock (set_time) and reads the gateway's server-reachable env_get_time_url (state), both as the
 stored env card declares clock/v1, then invokes sync_time on each backing server, MCP server or website, the
 env card lists (``legacy_protocol.backing_servers``) whose own card advertises it, at the route that card gives.
+An env whose card offers no set_time has no gateway clock to arm, as a computer deployed alone, and is refused
+saying to deploy it in a multi env; a record that kept no card is refused saying so.
 Non-advertisers are skipped when tolerate_missing_sync_time (default True), and websites always. Place right
 before prompt_agent so the virtual timeline starts at agent-start; when setup needs the clock armed, arm it at rate 0
 and start it with set_env_clock_rate right before prompt_agent.
@@ -19,6 +21,7 @@
 import tzdata  # noqa: F401
 from agentenv_protocol import client as protocol_v1
 
+from agent_env.env.env import EnvCapabilityUnsupported
 from agent_env.env.gateway.constants import EXT_CLOCK_URI
 from agent_env.task_step.context import StepContextError, TaskStepContext
 from agent_env.entity.refs import EntityRef
@@ -132,6 +135,13 @@ async def execute(self, context: TaskStepContext) -> TaskStepContext:
                                   timeout=self.timeout_seconds)
         except httpx.HTTPStatusError as e:
             raise RuntimeError(f"clock arm failed (HTTP {e.response.status_code}): {e.response.text}") from e
+        except EnvCapabilityUnsupported as e:
+            if not deployed.environment_card or not deployed.environment_url:
+                raise StepContextError(f"env '{self.env_id}': its deploy record keeps no env card or card URL, so "
+                                       "its clock can't be reached; deploy it again") from e
+            raise StepContextError(f"env '{self.env_id}' has no clock to arm: its card offers no {EXT_CLOCK_URI} "
+                                   "set_time, which a gateway serves. A computer deployed alone follows no clock; "
+                                   "deploy it in a multi env, whose gateway it follows") from e
         state = await deployed.invoke(EXT_CLOCK_URI, "state", timeout=self.timeout_seconds)
         env_get_time_url = state.get("env_get_time_url")
 
diff --git a/tst/unit/task_step/sync_env_clock_test.py b/tst/unit/task_step/sync_env_clock_test.py
index e8d01e65c..8c6d57b93 100644
--- a/tst/unit/task_step/sync_env_clock_test.py
+++ b/tst/unit/task_step/sync_env_clock_test.py
@@ -4,8 +4,8 @@
 
 import pytest
 
-from agent_env.env.env import DeployedEnv, DeployedGatewayEnv, EnvCapabilityUnsupported
-from agent_env.env.gateway.constants import EXT_CLOCK_URI
+from agent_env.env.env import DeployedEnv, DeployedGatewayEnv, DeployedSandboxEnv
+from agent_env.env.gateway.constants import EXT_CLOCK_URI, WELL_KNOWN_PATH
 from agent_env.task_step.context import StepContextError, TaskStepContext
 from agent_env.task_step.task_steps import sync_env_clock as mod
 from agent_env.task_step.task_steps.sync_env_clock import SyncEnvClockTaskStep, resolve_virtual_time
@@ -108,15 +108,6 @@ async def test_execute_syncs_each_child_env_through_the_stored_card(monkeypatch)
     assert [s["service"] for s in context.metadata["clock_configurations"][0]["synced"]] == ["email"]
 
 
-@pytest.mark.asyncio
-async def test_execute_raises_before_any_request_when_the_card_lacks_set_time(monkeypatch):
-    sent = _mock_http(monkeypatch)
-    record = _carded({"name": "gw", "capabilities": {"extensions": []}, "children_environments": []})
-    with pytest.raises(EnvCapabilityUnsupported, match="does not offer 'set_time' on urn:agentenv:clock/v1"):
-        await _step().execute(TaskStepContext(deployed_envs=[record]))
-    assert sent == []
-
-
 @pytest.mark.asyncio
 async def test_execute_keeps_the_arm_failure_message(monkeypatch):
     _mock_http(monkeypatch, set_time_status=409)
@@ -466,6 +457,31 @@ async def test_a_multi_envs_computer_is_synced_at_its_own_address_with_its_own_c
         computer.control_headers
 
 
+@pytest.mark.asyncio
+async def test_an_env_with_no_gateway_clock_as_a_computer_deployed_alone_is_refused_saying_to_deploy_it_in_a_multi_env(
+        monkeypatch):
+    sent = _mock_http(monkeypatch)
+    alone = DeployedSandboxEnv(env_id="e1", env_version=1, sandbox_id="sb-desk", control_token="desk-token",
+                               environment_card_url=f"https://desk.example{WELL_KNOWN_PATH}",
+                               environment_card={"name": "desktop", "url": "/agentenv",
+                                                 "capabilities": {"extensions": [_CLOCK_EXT]}})
+
+    with pytest.raises(StepContextError, match="has no clock to arm") as refused:
+        await _step().execute(TaskStepContext(deployed_envs=[alone]))
+
+    assert "deploy it in a multi env" in str(refused.value) and sent == []
+
+
+@pytest.mark.asyncio
+async def test_a_record_that_kept_no_card_is_refused_saying_so_not_told_to_deploy_a_multi_env(monkeypatch):
+    sent = _mock_http(monkeypatch)
+
+    with pytest.raises(StepContextError, match="its deploy record keeps no env card") as refused:
+        await _step().execute(TaskStepContext(deployed_envs=[_cardless()]))
+
+    assert "multi env" not in str(refused.value) and sent == []
+
+
 def test_a_padded_zone_is_stored_and_sent_stripped():
     s = _zoned(timezone=" America/New_York ")
     assert s.timezone == "America/New_York" and s.to_dict()["timezone"] == "America/New_York"