diff --git a/packages/cloud-platform/cloud/internal_api/models.py b/packages/cloud-platform/cloud/internal_api/models.py
index b72049c..87bf7b3 100644
--- a/packages/cloud-platform/cloud/internal_api/models.py
+++ b/packages/cloud-platform/cloud/internal_api/models.py
@@ -40,6 +40,7 @@ class HeartbeatRequest(BaseModel):
     devices: list[DeviceSnapshotModel] = Field(default_factory=list)
     policy_revision: int = Field(default=0, ge=0)
     planner_transport: Literal["direct", "cloud"] = "direct"
+    mcp_busy_device_ids: list[str] = Field(default_factory=list)
 
 
 class HostGovernancePolicyModel(BaseModel):
diff --git a/packages/cloud-platform/tests/test_internal_api_models.py b/packages/cloud-platform/tests/test_internal_api_models.py
new file mode 100644
index 0000000..b73e26c
--- /dev/null
+++ b/packages/cloud-platform/tests/test_internal_api_models.py
@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from cloud.internal_api.models import HeartbeatRequest
+
+
+def test_heartbeat_request_defaults_mcp_busy_device_ids_to_empty() -> None:
+    req = HeartbeatRequest(host_id="h1")
+    assert req.mcp_busy_device_ids == []
+
+
+def test_heartbeat_request_accepts_mcp_busy_device_ids() -> None:
+    req = HeartbeatRequest(host_id="h1", mcp_busy_device_ids=["phone-1"])
+    assert req.mcp_busy_device_ids == ["phone-1"]
+
+
+def test_heartbeat_request_omitting_field_is_backward_compatible() -> None:
+    """Old host-agents that don't send the field must still validate."""
+    raw = {"host_id": "h1", "devices": []}
+    req = HeartbeatRequest.model_validate(raw)
+    assert req.mcp_busy_device_ids == []