diff --git a/tests/unit/galaxy/core/test_events.py b/tests/unit/galaxy/core/test_events.py
new file mode 100644
index 000000000..c98813a8c
--- /dev/null
+++ b/tests/unit/galaxy/core/test_events.py
@@ -0,0 +1,337 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+"""
+Unit tests for galaxy/core/events.py
+
+Covers EventType enum, Event dataclasses, IEventObserver / IEventPublisher
+interfaces, and EventBus behaviour (subscribe, unsubscribe, publish).
+"""
+
+import asyncio
+import sys
+import os
+import time
+from unittest.mock import AsyncMock, Mock
+
+import pytest
+
+# Add project root to path for imports
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "..", ".."))
+
+from galaxy.core.events import (
+    AgentEvent,
+    ConstellationEvent,
+    DeviceEvent,
+    Event,
+    EventBus,
+    EventType,
+    IEventObserver,
+    IEventPublisher,
+    TaskEvent,
+    get_event_bus,
+)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_event(event_type: EventType = EventType.TASK_STARTED) -> Event:
+    return Event(
+        event_type=event_type,
+        source_id="test_source",
+        timestamp=time.time(),
+        data={"key": "value"},
+    )
+
+
+class ConcreteObserver(IEventObserver):
+    """Minimal concrete observer for testing the abstract interface."""
+
+    def __init__(self):
+        self.received: list = []
+
+    async def on_event(self, event: Event) -> None:
+        self.received.append(event)
+
+
+# ---------------------------------------------------------------------------
+# EventType
+# ---------------------------------------------------------------------------
+
+
+class TestEventType:
+    def test_all_values_are_strings(self):
+        for member in EventType:
+            assert isinstance(member.value, str)
+
+    def test_task_event_types_exist(self):
+        assert EventType.TASK_STARTED
+        assert EventType.TASK_COMPLETED
+        assert EventType.TASK_FAILED
+
+    def test_constellation_event_types_exist(self):
+        assert EventType.CONSTELLATION_STARTED
+        assert EventType.CONSTELLATION_COMPLETED
+        assert EventType.CONSTELLATION_FAILED
+        assert EventType.CONSTELLATION_MODIFIED
+
+    def test_device_event_types_exist(self):
+        assert EventType.DEVICE_CONNECTED
+        assert EventType.DEVICE_DISCONNECTED
+        assert EventType.DEVICE_STATUS_CHANGED
+
+    def test_agent_event_types_exist(self):
+        assert EventType.AGENT_RESPONSE
+        assert EventType.AGENT_ACTION
+
+    def test_unique_values(self):
+        values = [m.value for m in EventType]
+        assert len(values) == len(set(values)), "Duplicate EventType values detected"
+
+
+# ---------------------------------------------------------------------------
+# Event dataclasses
+# ---------------------------------------------------------------------------
+
+
+class TestEventDataclass:
+    def test_base_event_fields(self):
+        evt = _make_event(EventType.TASK_STARTED)
+        assert evt.event_type is EventType.TASK_STARTED
+        assert evt.source_id == "test_source"
+        assert isinstance(evt.timestamp, float)
+        assert evt.data == {"key": "value"}
+
+    def test_task_event_fields(self):
+        evt = TaskEvent(
+            event_type=EventType.TASK_COMPLETED,
+            source_id="src",
+            timestamp=time.time(),
+            data={},
+            task_id="task-1",
+            status="completed",
+            result={"output": "ok"},
+            error=None,
+        )
+        assert evt.task_id == "task-1"
+        assert evt.status == "completed"
+        assert evt.result == {"output": "ok"}
+        assert evt.error is None
+
+    def test_task_event_defaults(self):
+        evt = TaskEvent(
+            event_type=EventType.TASK_FAILED,
+            source_id="src",
+            timestamp=time.time(),
+            data={},
+            task_id="task-2",
+            status="failed",
+        )
+        assert evt.result is None
+        assert evt.error is None
+
+    def test_constellation_event_fields(self):
+        evt = ConstellationEvent(
+            event_type=EventType.CONSTELLATION_STARTED,
+            source_id="src",
+            timestamp=time.time(),
+            data={},
+            constellation_id="c-1",
+            constellation_state="executing",
+            new_ready_tasks=["t1", "t2"],
+        )
+        assert evt.constellation_id == "c-1"
+        assert evt.new_ready_tasks == ["t1", "t2"]
+
+    def test_agent_event_fields(self):
+        evt = AgentEvent(
+            event_type=EventType.AGENT_RESPONSE,
+            source_id="src",
+            timestamp=time.time(),
+            data={},
+            agent_name="app_agent",
+            agent_type="app",
+            output_type="response",
+            output_data={"thought": "thinking..."},
+        )
+        assert evt.agent_name == "app_agent"
+        assert evt.output_data == {"thought": "thinking..."}
+
+    def test_device_event_fields(self):
+        evt = DeviceEvent(
+            event_type=EventType.DEVICE_CONNECTED,
+            source_id="src",
+            timestamp=time.time(),
+            data={},
+            device_id="dev-1",
+            device_status="connected",
+            device_info={"os": "Windows"},
+            all_devices={"dev-1": {"status": "connected"}},
+        )
+        assert evt.device_id == "dev-1"
+        assert evt.device_status == "connected"
+
+
+# ---------------------------------------------------------------------------
+# IEventObserver interface
+# ---------------------------------------------------------------------------
+
+
+class TestIEventObserver:
+    def test_concrete_subclass_is_instantiable(self):
+        obs = ConcreteObserver()
+        assert isinstance(obs, IEventObserver)
+
+    @pytest.mark.asyncio
+    async def test_on_event_is_called(self):
+        obs = ConcreteObserver()
+        evt = _make_event()
+        await obs.on_event(evt)
+        assert obs.received == [evt]
+
+    def test_abstract_class_cannot_be_instantiated(self):
+        with pytest.raises(TypeError):
+            IEventObserver()  # type: ignore[abstract]
+
+
+# ---------------------------------------------------------------------------
+# IEventPublisher interface
+# ---------------------------------------------------------------------------
+
+
+class TestIEventPublisher:
+    def test_abstract_class_cannot_be_instantiated(self):
+        with pytest.raises(TypeError):
+            IEventPublisher()  # type: ignore[abstract]
+
+
+# ---------------------------------------------------------------------------
+# EventBus
+# ---------------------------------------------------------------------------
+
+
+class TestEventBus:
+    # ------------------------------------------------------------------
+    # subscribe / unsubscribe
+    # ------------------------------------------------------------------
+
+    def test_subscribe_all_events(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs)
+        assert obs in bus._all_observers
+
+    def test_subscribe_specific_event_types(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs, {EventType.TASK_STARTED, EventType.TASK_COMPLETED})
+        assert obs in bus._observers[EventType.TASK_STARTED]
+        assert obs in bus._observers[EventType.TASK_COMPLETED]
+
+    def test_unsubscribe_removes_from_all_observers(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs)
+        bus.unsubscribe(obs)
+        assert obs not in bus._all_observers
+
+    def test_unsubscribe_removes_from_typed_observers(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs, {EventType.TASK_STARTED})
+        bus.unsubscribe(obs)
+        assert obs not in bus._observers.get(EventType.TASK_STARTED, set())
+
+    def test_unsubscribe_nonexistent_observer_is_noop(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.unsubscribe(obs)  # should not raise
+
+    # ------------------------------------------------------------------
+    # publish_event
+    # ------------------------------------------------------------------
+
+    @pytest.mark.asyncio
+    async def test_all_events_subscriber_receives_every_event(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs)
+
+        for et in [EventType.TASK_STARTED, EventType.TASK_COMPLETED, EventType.DEVICE_CONNECTED]:
+            await bus.publish_event(_make_event(et))
+
+        assert len(obs.received) == 3
+
+    @pytest.mark.asyncio
+    async def test_typed_subscriber_receives_only_matching_events(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs, {EventType.TASK_STARTED})
+
+        await bus.publish_event(_make_event(EventType.TASK_STARTED))
+        await bus.publish_event(_make_event(EventType.TASK_COMPLETED))
+
+        assert len(obs.received) == 1
+        assert obs.received[0].event_type is EventType.TASK_STARTED
+
+    @pytest.mark.asyncio
+    async def test_multiple_observers_all_notified(self):
+        bus = EventBus()
+        obs1 = ConcreteObserver()
+        obs2 = ConcreteObserver()
+        bus.subscribe(obs1)
+        bus.subscribe(obs2)
+
+        await bus.publish_event(_make_event())
+
+        assert len(obs1.received) == 1
+        assert len(obs2.received) == 1
+
+    @pytest.mark.asyncio
+    async def test_no_subscribers_publish_is_noop(self):
+        bus = EventBus()
+        # Should not raise
+        await bus.publish_event(_make_event())
+
+    @pytest.mark.asyncio
+    async def test_observer_exception_does_not_propagate(self):
+        """A failing observer must not crash the event bus."""
+        bus = EventBus()
+
+        bad_obs = Mock()
+        bad_obs.on_event = AsyncMock(side_effect=RuntimeError("observer failure"))
+        bus.subscribe(bad_obs)
+
+        good_obs = ConcreteObserver()
+        bus.subscribe(good_obs)
+
+        # publish_event uses gather(return_exceptions=True) so this should not raise
+        await bus.publish_event(_make_event())
+
+        assert len(good_obs.received) == 1
+
+    @pytest.mark.asyncio
+    async def test_unsubscribed_observer_receives_no_events(self):
+        bus = EventBus()
+        obs = ConcreteObserver()
+        bus.subscribe(obs)
+        bus.unsubscribe(obs)
+
+        await bus.publish_event(_make_event())
+
+        assert obs.received == []
+
+    # ------------------------------------------------------------------
+    # Singleton helper
+    # ------------------------------------------------------------------
+
+    def test_get_event_bus_returns_same_instance(self):
+        bus1 = get_event_bus()
+        bus2 = get_event_bus()
+        assert bus1 is bus2
+
+    def test_get_event_bus_returns_event_bus_instance(self):
+        assert isinstance(get_event_bus(), EventBus)