diff --git a/src/dashboard/app.py b/src/dashboard/app.py
index 3176455..93f6d07 100644
--- a/src/dashboard/app.py
+++ b/src/dashboard/app.py
@@ -373,6 +373,12 @@ async def lifespan(app: FastAPI):
         except Exception as exc:
             logger.debug("Vault size check skipped: %s", exc)
 
+    # Start Workshop presence heartbeat
+    from timmy.workshop_state import WorkshopHeartbeat
+
+    workshop_heartbeat = WorkshopHeartbeat()
+    await workshop_heartbeat.start()
+
     # Start chat integrations in background
     chat_task = asyncio.create_task(_start_chat_integrations_background())
 
@@ -404,6 +410,8 @@ async def lifespan(app: FastAPI):
     except Exception as exc:
         logger.debug("MCP shutdown: %s", exc)
 
+    await workshop_heartbeat.stop()
+
     for task in [briefing_task, thinking_task, chat_task, loop_qa_task]:
         if task:
             task.cancel()
diff --git a/src/timmy/workshop_state.py b/src/timmy/workshop_state.py
new file mode 100644
index 0000000..f81a0ef
--- /dev/null
+++ b/src/timmy/workshop_state.py
@@ -0,0 +1,155 @@
+"""Workshop presence heartbeat — periodic writer for ``~/.timmy/presence.json``.
+
+Maintains Timmy's observable presence state for the Workshop 3D renderer.
+Writes the presence file every 30 seconds (or on cognitive state change),
+skipping writes when state is unchanged.
+
+See ADR-023 for the schema contract.
+"""
+
+import asyncio
+import hashlib
+import json
+import logging
+from datetime import UTC, datetime
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+PRESENCE_FILE = Path.home() / ".timmy" / "presence.json"
+HEARTBEAT_INTERVAL = 30  # seconds
+
+# Cognitive mood → presence mood mapping
+_MOOD_MAP = {
+    "curious": "exploring",
+    "settled": "focused",
+    "hesitant": "uncertain",
+    "energized": "excited",
+}
+
+
+def get_state_dict() -> dict:
+    """Build presence state dict from current cognitive state.
+
+    Returns a v1 presence schema dict suitable for JSON serialisation.
+    """
+    from timmy.cognitive_state import cognitive_tracker
+
+    state = cognitive_tracker.get_state()
+
+    # Map cognitive engagement to presence mood fallback
+    mood = _MOOD_MAP.get(state.mood, "focused")
+    if state.engagement == "idle" and state.mood == "settled":
+        mood = "idle"
+
+    # Build active threads from commitments
+    threads = []
+    for commitment in state.active_commitments[:10]:
+        threads.append({"type": "thinking", "ref": commitment[:80], "status": "active"})
+
+    return {
+        "version": 1,
+        "liveness": datetime.now(UTC).strftime("%Y-%m-%dT%H:%M:%SZ"),
+        "current_focus": state.focus_topic or "",
+        "active_threads": threads,
+        "recent_events": [],
+        "concerns": [],
+        "mood": mood,
+    }
+
+
+def write_state(state_dict: dict | None = None, path: Path | None = None) -> None:
+    """Write presence state to ``~/.timmy/presence.json``.
+
+    Gracefully degrades if the file cannot be written.
+    """
+    if state_dict is None:
+        state_dict = get_state_dict()
+    target = path or PRESENCE_FILE
+    try:
+        target.parent.mkdir(parents=True, exist_ok=True)
+        target.write_text(json.dumps(state_dict, indent=2) + "\n")
+    except OSError as exc:
+        logger.warning("Failed to write presence state: %s", exc)
+
+
+def _state_hash(state_dict: dict) -> str:
+    """Compute hash of state dict, ignoring liveness timestamp."""
+    stable = {k: v for k, v in state_dict.items() if k != "liveness"}
+    return hashlib.md5(json.dumps(stable, sort_keys=True).encode()).hexdigest()
+
+
+class WorkshopHeartbeat:
+    """Async background task that keeps ``presence.json`` fresh.
+
+    - Writes every ``interval`` seconds (default 30).
+    - Reacts to cognitive state changes via sensory bus.
+    - Skips write if state hasn't changed (hash comparison).
+    """
+
+    def __init__(
+        self,
+        interval: int = HEARTBEAT_INTERVAL,
+        path: Path | None = None,
+    ) -> None:
+        self._interval = interval
+        self._path = path or PRESENCE_FILE
+        self._last_hash: str | None = None
+        self._task: asyncio.Task | None = None
+        self._trigger = asyncio.Event()
+
+    async def start(self) -> None:
+        """Start the heartbeat background loop."""
+        self._subscribe_to_events()
+        self._task = asyncio.create_task(self._run())
+
+    async def stop(self) -> None:
+        """Cancel the heartbeat task gracefully."""
+        if self._task:
+            self._task.cancel()
+            try:
+                await self._task
+            except asyncio.CancelledError:
+                pass
+            self._task = None
+
+    def notify(self) -> None:
+        """Signal an immediate state write (e.g. on cognitive state change)."""
+        self._trigger.set()
+
+    async def _run(self) -> None:
+        """Main loop: write state on interval or trigger."""
+        await asyncio.sleep(1)  # Initial stagger
+        while True:
+            try:
+                # Wait for interval OR early trigger
+                try:
+                    await asyncio.wait_for(self._trigger.wait(), timeout=self._interval)
+                    self._trigger.clear()
+                except TimeoutError:
+                    pass  # Normal periodic tick
+
+                self._write_if_changed()
+            except asyncio.CancelledError:
+                raise
+            except Exception as exc:
+                logger.error("Workshop heartbeat error: %s", exc)
+
+    def _write_if_changed(self) -> None:
+        """Build state, compare hash, write only if changed."""
+        state_dict = get_state_dict()
+        current_hash = _state_hash(state_dict)
+        if current_hash == self._last_hash:
+            return
+        self._last_hash = current_hash
+        write_state(state_dict, self._path)
+
+    def _subscribe_to_events(self) -> None:
+        """Subscribe to cognitive state change events on the sensory bus."""
+        try:
+            from timmy.event_bus import get_sensory_bus
+
+            bus = get_sensory_bus()
+            bus.subscribe("cognitive_state_changed", lambda _: self.notify())
+        except Exception as exc:
+            logger.debug("Heartbeat event subscription skipped: %s", exc)
diff --git a/tests/timmy/test_workshop_state.py b/tests/timmy/test_workshop_state.py
new file mode 100644
index 0000000..f55c07e
--- /dev/null
+++ b/tests/timmy/test_workshop_state.py
@@ -0,0 +1,179 @@
+"""Tests for Workshop presence heartbeat."""
+
+import json
+from unittest.mock import patch
+
+import pytest
+
+from timmy.workshop_state import (
+    WorkshopHeartbeat,
+    _state_hash,
+    get_state_dict,
+    write_state,
+)
+
+# ---------------------------------------------------------------------------
+# get_state_dict
+# ---------------------------------------------------------------------------
+
+
+def test_get_state_dict_returns_v1_schema():
+    state = get_state_dict()
+    assert state["version"] == 1
+    assert "liveness" in state
+    assert "current_focus" in state
+    assert "mood" in state
+    assert isinstance(state["active_threads"], list)
+    assert isinstance(state["recent_events"], list)
+    assert isinstance(state["concerns"], list)
+
+
+def test_get_state_dict_idle_mood():
+    """Idle engagement + settled mood → 'idle' presence mood."""
+    from timmy.cognitive_state import CognitiveState, CognitiveTracker
+
+    tracker = CognitiveTracker.__new__(CognitiveTracker)
+    tracker.state = CognitiveState(engagement="idle", mood="settled")
+    with patch("timmy.cognitive_state.cognitive_tracker", tracker):
+        state = get_state_dict()
+    assert state["mood"] == "idle"
+
+
+def test_get_state_dict_maps_mood():
+    """Cognitive moods map to presence moods."""
+    from timmy.cognitive_state import CognitiveState, CognitiveTracker
+
+    for cog_mood, expected in [
+        ("curious", "exploring"),
+        ("hesitant", "uncertain"),
+        ("energized", "excited"),
+    ]:
+        tracker = CognitiveTracker.__new__(CognitiveTracker)
+        tracker.state = CognitiveState(engagement="deep", mood=cog_mood)
+        with patch("timmy.cognitive_state.cognitive_tracker", tracker):
+            state = get_state_dict()
+        assert state["mood"] == expected, f"Expected {expected} for {cog_mood}"
+
+
+# ---------------------------------------------------------------------------
+# write_state
+# ---------------------------------------------------------------------------
+
+
+def test_write_state_creates_file(tmp_path):
+    target = tmp_path / "presence.json"
+    state = {"version": 1, "liveness": "2026-01-01T00:00:00Z", "current_focus": ""}
+    write_state(state, path=target)
+
+    assert target.exists()
+    data = json.loads(target.read_text())
+    assert data["version"] == 1
+
+
+def test_write_state_creates_parent_dirs(tmp_path):
+    target = tmp_path / "deep" / "nested" / "presence.json"
+    write_state({"version": 1}, path=target)
+    assert target.exists()
+
+
+# ---------------------------------------------------------------------------
+# _state_hash
+# ---------------------------------------------------------------------------
+
+
+def test_state_hash_ignores_liveness():
+    a = {"version": 1, "mood": "focused", "liveness": "2026-01-01T00:00:00Z"}
+    b = {"version": 1, "mood": "focused", "liveness": "2026-12-31T23:59:59Z"}
+    assert _state_hash(a) == _state_hash(b)
+
+
+def test_state_hash_detects_mood_change():
+    a = {"version": 1, "mood": "focused", "liveness": "t1"}
+    b = {"version": 1, "mood": "idle", "liveness": "t1"}
+    assert _state_hash(a) != _state_hash(b)
+
+
+# ---------------------------------------------------------------------------
+# WorkshopHeartbeat — _write_if_changed
+# ---------------------------------------------------------------------------
+
+
+def test_write_if_changed_writes_on_first_call(tmp_path):
+    target = tmp_path / "presence.json"
+    hb = WorkshopHeartbeat(interval=60, path=target)
+
+    with patch("timmy.workshop_state.get_state_dict") as mock_state:
+        mock_state.return_value = {
+            "version": 1,
+            "liveness": "t1",
+            "current_focus": "testing",
+            "mood": "focused",
+        }
+        hb._write_if_changed()
+
+    assert target.exists()
+    data = json.loads(target.read_text())
+    assert data["version"] == 1
+    assert data["current_focus"] == "testing"
+
+
+def test_write_if_changed_skips_when_unchanged(tmp_path):
+    target = tmp_path / "presence.json"
+    hb = WorkshopHeartbeat(interval=60, path=target)
+
+    fixed_state = {"version": 1, "liveness": "t1", "mood": "idle"}
+
+    with patch("timmy.workshop_state.get_state_dict", return_value=fixed_state):
+        hb._write_if_changed()  # First write
+        target.write_text("")  # Clear to detect if second write happens
+        hb._write_if_changed()  # Should skip — state unchanged
+
+    # File should still be empty (second write was skipped)
+    assert target.read_text() == ""
+
+
+def test_write_if_changed_writes_on_state_change(tmp_path):
+    target = tmp_path / "presence.json"
+    hb = WorkshopHeartbeat(interval=60, path=target)
+
+    state_a = {"version": 1, "liveness": "t1", "mood": "idle"}
+    state_b = {"version": 1, "liveness": "t2", "mood": "focused"}
+
+    with patch("timmy.workshop_state.get_state_dict", return_value=state_a):
+        hb._write_if_changed()
+
+    with patch("timmy.workshop_state.get_state_dict", return_value=state_b):
+        hb._write_if_changed()
+
+    data = json.loads(target.read_text())
+    assert data["mood"] == "focused"
+
+
+# ---------------------------------------------------------------------------
+# WorkshopHeartbeat — notify
+# ---------------------------------------------------------------------------
+
+
+def test_notify_sets_trigger():
+    hb = WorkshopHeartbeat(interval=60)
+    assert not hb._trigger.is_set()
+    hb.notify()
+    assert hb._trigger.is_set()
+
+
+# ---------------------------------------------------------------------------
+# WorkshopHeartbeat — start/stop lifecycle
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_heartbeat_start_stop_lifecycle(tmp_path):
+    target = tmp_path / "presence.json"
+    hb = WorkshopHeartbeat(interval=60, path=target)
+
+    with patch("timmy.workshop_state.get_state_dict", return_value={"version": 1}):
+        await hb.start()
+        assert hb._task is not None
+        assert not hb._task.done()
+        await hb.stop()
+        assert hb._task is None