diff --git a/core/__init__.py b/core/__init__.py
new file mode 100644
index 0000000..2d99708
--- /dev/null
+++ b/core/__init__.py
@@ -0,0 +1,15 @@
+"""Core domain models for JIGAIDO."""
+
+from core.models import (
+    Bounty,
+    TrackedBounty,
+    RoomData,
+    TrackingData,
+)
+
+__all__ = [
+    "Bounty",
+    "TrackedBounty",
+    "RoomData",
+    "TrackingData",
+]
diff --git a/core/models.py b/core/models.py
new file mode 100644
index 0000000..fe57645
--- /dev/null
+++ b/core/models.py
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+"""Domain dataclasses for JIGAIDO bounty tracker."""
+
+from dataclasses import dataclass
+
+
+@dataclass
+class Bounty:
+    """A bounty created by a user.
+
+    The created_by_user_id field always refers to the user who created the bounty.
+    It does NOT indicate whether the bounty is a group or personal bounty.
+    """
+
+    id: int
+    text: str | None
+    link: str | None
+    due_date_ts: int | None
+    created_at: int
+    created_by_user_id: int
+
+
+@dataclass
+class TrackedBounty:
+    """A bounty that a user is tracking.
+
+    Lightweight relation/pointer - the actual tracking context (including room)
+    lives in TrackingData, not here.
+    """
+
+    bounty_id: int
+    created_at: int
+
+
+@dataclass
+class RoomData:
+    """All data for a room (group or DM).
+
+    The room_id can be negative for Telegram groups or positive for DMs.
+    The next_id field is used to generate unique bounty IDs within this room.
+    """
+
+    room_id: int
+    bounties: list[Bounty]
+    next_id: int
+
+
+@dataclass
+class TrackingData:
+    """User tracking state within a room (group or DM).
+
+    TrackingData vs TrackedBounty:
+    - Use TrackingData to store ALL tracked bounties for a user in a specific room.
+      It contains the room_id, user_id, and a list of TrackedBounty entries.
+    - Use TrackedBounty to represent a single tracked bounty entry within that list.
+
+    TrackingData is the container, TrackedBounty is the item.
+    """
+
+    room_id: int
+    user_id: int
+    tracked: list[TrackedBounty]
diff --git a/tests/test_models.py b/tests/test_models.py
new file mode 100644
index 0000000..ca45860
--- /dev/null
+++ b/tests/test_models.py
@@ -0,0 +1,143 @@
+"""Tests for core/models.py — domain dataclasses."""
+
+import time
+
+import pytest
+
+from core.models import (
+    Bounty,
+    TrackedBounty,
+    RoomData,
+    TrackingData,
+)
+
+
+class TestBounty:
+    def test_create_bounty(self):
+        b = Bounty(
+            id=1,
+            text="Fix the bug",
+            link="https://github.com/example/repo/issues/1",
+            due_date_ts=1735689600,
+            created_at=1735603200,
+            created_by_user_id=123,
+        )
+        assert b.id == 1
+        assert b.text == "Fix the bug"
+        assert b.link == "https://github.com/example/repo/issues/1"
+        assert b.due_date_ts == 1735689600
+        assert b.created_at == 1735603200
+        assert b.created_by_user_id == 123
+
+    def test_bounty_optional_fields_can_be_none(self):
+        b = Bounty(
+            id=1,
+            text=None,
+            link=None,
+            due_date_ts=None,
+            created_at=0,
+            created_by_user_id=123,
+        )
+        assert b.text is None
+        assert b.link is None
+        assert b.due_date_ts is None
+
+    def test_bounty_comparison_equal(self):
+        b1 = Bounty(
+            id=1,
+            text="a",
+            link=None,
+            due_date_ts=None,
+            created_at=0,
+            created_by_user_id=123,
+        )
+        b2 = Bounty(
+            id=1,
+            text="a",
+            link=None,
+            due_date_ts=None,
+            created_at=0,
+            created_by_user_id=123,
+        )
+        assert b1 == b2
+
+    def test_bounty_comparison_not_equal(self):
+        b1 = Bounty(
+            id=1,
+            text="a",
+            link=None,
+            due_date_ts=None,
+            created_at=0,
+            created_by_user_id=123,
+        )
+        b2 = Bounty(
+            id=2,
+            text="b",
+            link=None,
+            due_date_ts=None,
+            created_at=0,
+            created_by_user_id=456,
+        )
+        assert b1 != b2
+
+
+class TestTrackedBounty:
+    def test_create_tracked_bounty(self):
+        tb = TrackedBounty(bounty_id=5, created_at=1735600000)
+        assert tb.bounty_id == 5
+        assert tb.created_at == 1735600000
+
+    def test_tracked_bounty_comparison(self):
+        tb1 = TrackedBounty(bounty_id=1, created_at=0)
+        tb2 = TrackedBounty(bounty_id=1, created_at=0)
+        assert tb1 == tb2
+
+
+class TestRoomData:
+    def test_create_group_room_data(self):
+        rd = RoomData(
+            room_id=-1001,
+            bounties=[],
+            next_id=1,
+        )
+        assert rd.room_id == -1001
+        assert rd.bounties == []
+        assert rd.next_id == 1
+
+    def test_create_dm_room_data(self):
+        rd = RoomData(
+            room_id=123456,
+            bounties=[],
+            next_id=1,
+        )
+        assert rd.room_id == 123456
+        assert rd.bounties == []
+        assert rd.next_id == 1
+
+    def test_room_data_with_bounties(self):
+        b = Bounty(
+            id=1,
+            text="Task",
+            link=None,
+            due_date_ts=None,
+            created_at=0,
+            created_by_user_id=123,
+        )
+        rd = RoomData(room_id=-1001, bounties=[b], next_id=2)
+        assert len(rd.bounties) == 1
+        assert rd.bounties[0].text == "Task"
+        assert rd.bounties[0].created_by_user_id == 123
+
+
+class TestTrackingData:
+    def test_create_tracking_data(self):
+        td = TrackingData(room_id=-1001, user_id=123456, tracked=[])
+        assert td.room_id == -1001
+        assert td.user_id == 123456
+        assert td.tracked == []
+
+    def test_tracking_data_with_tracked(self):
+        tb = TrackedBounty(bounty_id=5, created_at=0)
+        td = TrackingData(room_id=-1001, user_id=123, tracked=[tb])
+        assert len(td.tracked) == 1
+        assert td.tracked[0].bounty_id == 5