diff --git a/tests/test_mission_bus.py b/tests/test_mission_bus.py
new file mode 100644
index 00000000..22bc3873
--- /dev/null
+++ b/tests/test_mission_bus.py
@@ -0,0 +1,105 @@
+from importlib import util
+from pathlib import Path
+
+import pytest
+
+
+ROOT = Path(__file__).resolve().parent.parent
+MODULE_PATH = ROOT / "nexus" / "mission_bus.py"
+CONFIG_PATH = ROOT / "config" / "mission_bus_profiles.json"
+
+
+def load_module():
+    spec = util.spec_from_file_location("mission_bus", MODULE_PATH)
+    module = util.module_from_spec(spec)
+    assert spec.loader is not None
+    spec.loader.exec_module(module)
+    return module
+
+
+def build_bus(module):
+    profiles = module.load_profiles(CONFIG_PATH)
+    bus = module.MissionBus("mission-883", title="multi-agent teaming", config=profiles)
+    bus.register_participant("timmy", module.MissionRole.LEAD)
+    bus.register_participant("ezra", module.MissionRole.WRITE)
+    bus.register_participant("bezalel", module.MissionRole.READ)
+    bus.register_participant("allegro", module.MissionRole.AUDIT)
+    return bus
+
+
+def test_role_permissions_gate_publish_checkpoint_and_handoff():
+    module = load_module()
+    bus = build_bus(module)
+
+    assert bus.allowed("timmy", "publish") is True
+    assert bus.allowed("ezra", "handoff") is True
+    assert bus.allowed("allegro", "audit") is True
+    assert bus.allowed("bezalel", "publish") is False
+
+    with pytest.raises(PermissionError):
+        bus.publish("bezalel", "mission.notes", {"text": "should fail"})
+
+    with pytest.raises(PermissionError):
+        bus.create_checkpoint("allegro", summary="audit cannot checkpoint", state={})
+
+
+def test_mission_bus_unified_stream_records_messages_checkpoints_and_handoffs():
+    module = load_module()
+    bus = build_bus(module)
+
+    msg = bus.publish("timmy", "mission.start", {"goal": "build the slice"})
+    checkpoint = bus.create_checkpoint(
+        "ezra",
+        summary="checkpoint before lead review",
+        state={"branch": "fix/883", "files": ["nexus/mission_bus.py"]},
+        artifacts=["docs/mission-bus.md"],
+    )
+    handoff = bus.handoff("ezra", "timmy", checkpoint.checkpoint_id, note="ready for lead review")
+
+    assert [event.event_type for event in bus.events] == ["message", "checkpoint", "handoff"]
+    assert [event.sequence for event in bus.events] == [1, 2, 3]
+    assert msg.topic == "mission.start"
+    assert handoff.recipient == "timmy"
+
+
+def test_handoff_resume_packet_contains_checkpoint_state_and_participants():
+    module = load_module()
+    bus = build_bus(module)
+    checkpoint = bus.create_checkpoint(
+        "ezra",
+        summary="handoff package",
+        state={"branch": "fix/883", "tests": ["tests/test_mission_bus.py"]},
+        artifacts=["config/mission_bus_profiles.json"],
+    )
+    handoff = bus.handoff("ezra", "timmy", checkpoint.checkpoint_id, note="pick up from here")
+
+    packet = bus.build_resume_packet(handoff.handoff_id)
+    assert packet["recipient"] == "timmy"
+    assert packet["checkpoint"]["state"]["branch"] == "fix/883"
+    assert packet["checkpoint"]["artifacts"] == ["config/mission_bus_profiles.json"]
+    assert packet["participants"]["ezra"]["role"] == "write"
+    assert packet["handoff_note"] == "pick up from here"
+
+
+def test_profiles_define_level2_mount_namespace_and_level3_rootless_podman():
+    module = load_module()
+    profiles = module.load_profiles(CONFIG_PATH)
+
+    levels = {entry["level"]: entry["mechanism"] for entry in profiles["isolation_profiles"]}
+    assert levels[2] == "mount_namespace"
+    assert levels[3] == "rootless_podman"
+    assert profiles["roles"]["audit"] == ["read", "audit"]
+
+
+def test_mission_bus_roundtrip_preserves_events_and_isolation_profile():
+    module = load_module()
+    bus = build_bus(module)
+    bus.publish("timmy", "mission.start", {"goal": "roundtrip"})
+    checkpoint = bus.create_checkpoint("ezra", summary="save state", state={"count": 1})
+    bus.handoff("ezra", "timmy", checkpoint.checkpoint_id, note="resume")
+
+    restored = module.MissionBus.from_dict(bus.to_dict())
+    assert restored.mission_id == "mission-883"
+    assert restored.events[-1].event_type == "handoff"
+    assert restored.events[-1].note == "resume"
+    assert restored.isolation_profiles[1].mechanism == "mount_namespace"