diff --git a/ai_mouse/data_adapters/__init__.py b/ai_mouse/data_adapters/__init__.py
new file mode 100644
index 0000000..202278a
--- /dev/null
+++ b/ai_mouse/data_adapters/__init__.py
@@ -0,0 +1 @@
+"""Data adapters: convert external datasets to the project's traces.jsonl format."""
diff --git a/ai_mouse/data_adapters/balabit.py b/ai_mouse/data_adapters/balabit.py
new file mode 100644
index 0000000..1febc2e
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+++ b/ai_mouse/data_adapters/balabit.py
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+"""Adapter for the Balabit Mouse Dynamics Challenge dataset.
+
+Source: https://github.com/balabit/Mouse-Dynamics-Challenge
+
+Each session is a CSV file with columns:
+    record timestamp, client timestamp, button, state, x, y
+
+Where:
+    state ∈ {Move, Pressed, Released, Drag, Scroll}
+    button ∈ {NoButton, Left, Right, Wheel}
+
+We extract "click-anchored" trajectory segments: each Pressed event
+defines a target, and the W ms of Move events preceding it form one
+training trace.
+"""
+from __future__ import annotations
+
+import json
+import logging
+from dataclasses import dataclass
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class MouseEvent:
+    """A single mouse event from a Balabit CSV row."""
+
+    t_ms: int        # client timestamp in milliseconds (relative to session start)
+    button: str      # "NoButton", "Left", "Right", "Wheel"
+    state: str       # "Move", "Pressed", "Released", "Drag", "Scroll"
+    x: int
+    y: int
+
+
+@dataclass
+class Segment:
+    """A click-anchored trajectory segment ready to be written to JSONL."""
+
+    events: list[MouseEvent]   # only Move events, sorted by t_ms ascending
+    click_x: int               # the Pressed event's x coordinate
+    click_y: int               # the Pressed event's y coordinate
+    click_t_ms: int            # the Pressed event's timestamp
+    session_id: str            # e.g. "user7_session_42"
+
+
+def parse_session_csv(path: Path) -> list[MouseEvent]:
+    """Stub — implemented in Task 3."""
+    raise NotImplementedError
+
+
+def segment_by_clicks(
+    events: list[MouseEvent],
+    window_ms: int,
+    session_id: str,
+) -> list[Segment]:
+    """Stub — implemented in Task 4."""
+    raise NotImplementedError
+
+
+def filter_segments(
+    segments: list[Segment],
+    min_events: int,
+    min_dist: int,
+    max_span_ms: int,
+    max_gap_ms: int,
+) -> list[Segment]:
+    """Stub — implemented in Task 5."""
+    raise NotImplementedError
+
+
+def process_session(
+    csv_path: Path,
+    output_jsonl: Path,
+    config,
+) -> int:
+    """Stub — implemented in Task 6."""
+    raise NotImplementedError
diff --git a/tests/test_balabit_adapter.py b/tests/test_balabit_adapter.py
new file mode 100644
index 0000000..c959769
--- /dev/null
+++ b/tests/test_balabit_adapter.py
@@ -0,0 +1,36 @@
+"""Tests for Balabit Mouse Dynamics Challenge data adapter."""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+
+def test_module_exports():
+    """The adapter module must export the public functions used by CLI."""
+    from ai_mouse.data_adapters import balabit
+    assert hasattr(balabit, "parse_session_csv")
+    assert hasattr(balabit, "segment_by_clicks")
+    assert hasattr(balabit, "filter_segments")
+    assert hasattr(balabit, "process_session")
+    assert hasattr(balabit, "MouseEvent")
+    assert hasattr(balabit, "Segment")
+
+
+def test_mouse_event_dataclass():
+    """MouseEvent has expected fields."""
+    from ai_mouse.data_adapters.balabit import MouseEvent
+    e = MouseEvent(t_ms=100, button="NoButton", state="Move", x=300, y=400)
+    assert e.t_ms == 100
+    assert e.state == "Move"
+    assert e.x == 300
+
+
+def test_segment_dataclass():
+    """Segment has expected fields."""
+    from ai_mouse.data_adapters.balabit import MouseEvent, Segment
+    events = [MouseEvent(t_ms=0, button="NoButton", state="Move", x=10, y=20)]
+    s = Segment(events=events, click_x=100, click_y=200, click_t_ms=500, session_id="user1_s1")
+    assert s.events == events
+    assert s.click_x == 100
+    assert s.session_id == "user1_s1"