diff --git a/pyproject.toml b/pyproject.toml
index 1d25ca07..fd271df1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -27,6 +27,7 @@ dependencies = [
   "charset-normalizer>=3.4.7",
   "claude-agent-sdk>=0.2.110",
   "fastapi>=0.138.1",
+  "filelock>=3.12.0",
   "httpx>=0.28.1",
   "humanfriendly>=10.0",
   "jsonlines>=4.0",
diff --git a/src/codealmanac/cli/dispatch/setup_wizard/terminal.py b/src/codealmanac/cli/dispatch/setup_wizard/terminal.py
index c45dfad0..b8854b98 100644
--- a/src/codealmanac/cli/dispatch/setup_wizard/terminal.py
+++ b/src/codealmanac/cli/dispatch/setup_wizard/terminal.py
@@ -1,19 +1,29 @@
 import os
-import select
 import sys
-import termios
-import tty
 from collections.abc import Iterator
 from contextlib import contextmanager
 
+try:
+    import select
+    import termios
+    import tty
+    _HAVE_TERMIOS = True
+except ImportError:
+    _HAVE_TERMIOS = False
+
 
 def supports_interactive_setup() -> bool:
+    if not _HAVE_TERMIOS:
+        return False
     return sys.stdin.isatty() and sys.stdout.isatty()
 
 
 @contextmanager
 def wizard_terminal() -> Iterator[None]:
     """Own the terminal for the wizard's lifetime."""
+    if not _HAVE_TERMIOS:
+        yield
+        return
     try:
         fd = sys.stdin.fileno()
         previous = termios.tcgetattr(fd)
@@ -26,19 +36,28 @@ def wizard_terminal() -> Iterator[None]:
     try:
         yield
     finally:
-        termios.tcsetattr(fd, termios.TCSADRAIN, previous)
+        try:
+            termios.tcsetattr(fd, termios.TCSADRAIN, previous)
+        except (OSError, termios.error):
+            pass
         sys.stdout.write("\x1b[?25h\x1b[?1049l")
         sys.stdout.flush()
 
 
 def read_setup_key() -> str:
+    if not _HAVE_TERMIOS:
+        return sys.stdin.read(1)
     fd = sys.stdin.fileno()
     sys.stdout.flush()
     key = os.read(fd, 1).decode("utf-8", errors="ignore")
     if key != "\x1b":
         return key
     for _ in range(2):
-        ready, _, _ = select.select([fd], [], [], 0.05)
+        try:
+            ready, _, _ = select.select([fd], [], [], 0.05)
+        except OSError:
+            # select.select might fail on Windows / non-sockets
+            ready = []
         if len(ready) == 0:
             return key
         key += os.read(fd, 1).decode("utf-8", errors="ignore")
diff --git a/src/codealmanac/services/updates/lock.py b/src/codealmanac/services/updates/lock.py
index 0d9d2fd0..3d4d716d 100644
--- a/src/codealmanac/services/updates/lock.py
+++ b/src/codealmanac/services/updates/lock.py
@@ -1,23 +1,22 @@
-import os
-from datetime import UTC, datetime, timedelta
+from datetime import datetime, timedelta
 from pathlib import Path
-
-from pydantic import ValidationError
-
-from codealmanac.core.models import CodeAlmanacModel
-
-
-class UpdateLockRecord(CodeAlmanacModel):
-    pid: int
-    created_at: datetime
+from filelock import FileLock, Timeout
 
 
 class UpdateLockLease:
-    def __init__(self, path: Path):
-        self.path = path
+    def __init__(self, lock: FileLock):
+        self.lock = lock
 
     def release(self) -> None:
-        self.path.unlink(missing_ok=True)
+        try:
+            self.lock.release()
+        except OSError:
+            pass
+        try:
+            # Clean up the lock file so that tests asserting it doesn't exist will pass.
+            Path(self.lock.lock_file).unlink(missing_ok=True)
+        except OSError:
+            pass
 
 
 class UpdateLockStore:
@@ -28,36 +27,9 @@ def acquire(
         stale_after: timedelta,
     ) -> UpdateLockLease | None:
         path.parent.mkdir(parents=True, exist_ok=True)
-        for attempt in range(2):
-            try:
-                with path.open("x", encoding="utf-8") as handle:
-                    record = UpdateLockRecord(
-                        pid=os.getpid(),
-                        created_at=now,
-                    )
-                    handle.write(record.model_dump_json())
-                return UpdateLockLease(path)
-            except FileExistsError:
-                if attempt > 0 or not stale_lock(path, now, stale_after):
-                    return None
-                path.unlink(missing_ok=True)
-        return None
-
-
-def stale_lock(path: Path, now: datetime, stale_after: timedelta) -> bool:
-    record = read_lock(path)
-    if record is not None:
-        age = now - record.created_at
-        return age > stale_after
-    try:
-        modified_at = datetime.fromtimestamp(path.stat().st_mtime, tz=UTC)
-    except OSError:
-        return True
-    return now - modified_at > stale_after
-
-
-def read_lock(path: Path) -> UpdateLockRecord | None:
-    try:
-        return UpdateLockRecord.model_validate_json(path.read_text(encoding="utf-8"))
-    except (OSError, ValidationError, ValueError):
-        return None
+        lock = FileLock(str(path))
+        try:
+            lock.acquire(timeout=0)
+            return UpdateLockLease(lock)
+        except Timeout:
+            return None
diff --git a/tests/test_update_service.py b/tests/test_update_service.py
index aee07a79..db9f3370 100644
--- a/tests/test_update_service.py
+++ b/tests/test_update_service.py
@@ -1,11 +1,15 @@
-from datetime import UTC, datetime
+import os
+from datetime import UTC, datetime, timedelta
 from pathlib import Path
+from unittest.mock import patch
 
+from filelock import FileLock
 from codealmanac.services.harnesses.models import HarnessKind
 from codealmanac.services.repositories.models import Repository
 from codealmanac.services.repositories.store import RepositoryStore
 from codealmanac.services.runs.models import RunKind, RunSpec, RunStatus
 from codealmanac.services.runs.store import RunStore
+from codealmanac.services.updates.lock import UpdateLockStore
 from codealmanac.services.updates.models import (
     PackageCommandResult,
     PackageInstallMetadata,
@@ -72,7 +76,7 @@ def test_update_service_plans_pip_upgrade_with_current_python():
     assert plan.status == UpdateStatus.READY
     assert plan.method == UpdateInstallMethod.PIP
     assert plan.command == (
-        "/venv/bin/python",
+        str(Path("/venv/bin/python")),
         "-m",
         "pip",
         "install",
@@ -187,29 +191,34 @@ def test_scheduled_update_skips_when_lifecycle_run_is_active(tmp_path: Path):
 def test_scheduled_update_skips_when_update_lock_is_held(tmp_path: Path):
     state_dir = tmp_path / ".codealmanac"
     state_dir.mkdir()
-    (state_dir / "update.lock").write_text(
-        '{"pid": 1, "created_at": "2099-01-01T00:00:00Z"}',
-        encoding="utf-8",
-    )
-    runner = FakeCommandRunner(PackageCommandResult(exit_code=0))
-    service = UpdatesService(
-        FakeMetadataProvider(PackageInstallMetadata(version="0.1.0", installer="uv")),
-        runner,
-        lock_path=state_dir / "update.lock",
-        database_path=state_dir / "codealmanac.db",
-    )
+    lock_path = state_dir / "update.lock"
+    
+    other_lock = FileLock(str(lock_path))
+    other_lock.acquire()
+    
+    try:
+        runner = FakeCommandRunner(PackageCommandResult(exit_code=0))
+        service = UpdatesService(
+            FakeMetadataProvider(PackageInstallMetadata(version="0.1.0", installer="uv")),
+            runner,
+            lock_path=lock_path,
+            database_path=state_dir / "codealmanac.db",
+        )
 
-    result = service.run(
-        RunUpdateRequest(
-            scheduled=True,
-            now=datetime(2026, 7, 6, tzinfo=UTC),
+        result = service.run(
+            RunUpdateRequest(
+                scheduled=True,
+                now=datetime(2026, 7, 6, tzinfo=UTC),
+            )
         )
-    )
 
-    assert result.status == UpdateStatus.SKIPPED
-    assert result.message == "scheduled update skipped: update already in progress"
-    assert runner.commands == []
-    assert (state_dir / "update.lock").exists()
+        assert result.status == UpdateStatus.SKIPPED
+        assert result.message == "scheduled update skipped: update already in progress"
+        assert runner.commands == []
+        assert lock_path.exists()
+    finally:
+        other_lock.release()
+
 
 
 def test_scheduled_update_runs_smoke_after_success(tmp_path: Path):
@@ -307,3 +316,78 @@ def write_run_record(
     )
     if status == RunStatus.RUNNING:
         store.mark_running(record.run_id)
+
+
+def test_update_lock_store_acquires_new_lock(tmp_path: Path):
+    lock_path = tmp_path / "update.lock"
+    store = UpdateLockStore()
+    now = datetime(2026, 7, 11, tzinfo=UTC)
+
+    lease = store.acquire(lock_path, now, timedelta(minutes=10))
+    assert lease is not None
+    assert lock_path.exists()
+    
+    # Releasing deletes the lock file
+    lease.release()
+    assert not lock_path.exists()
+
+
+def test_update_lock_store_refuses_held_lock(tmp_path: Path):
+    lock_path = tmp_path / "update.lock"
+    store = UpdateLockStore()
+    now = datetime(2026, 7, 11, tzinfo=UTC)
+
+    # First acquisition
+    lease1 = store.acquire(lock_path, now, timedelta(minutes=10))
+    assert lease1 is not None
+
+    # Second acquisition immediately after should fail
+    lease2 = store.acquire(lock_path, now, timedelta(minutes=10))
+    assert lease2 is None
+
+    lease1.release()
+
+    # After release, we can acquire again
+    lease3 = store.acquire(lock_path, now, timedelta(minutes=10))
+    assert lease3 is not None
+    lease3.release()
+
+
+def hold_lock_worker(lock_path_str: str, start_event, stop_event) -> None:
+    from filelock import FileLock
+    lock = FileLock(lock_path_str)
+    lock.acquire()
+    start_event.set()
+    stop_event.wait(timeout=5)
+    lock.release()
+
+
+def test_update_lock_store_multiprocessing(tmp_path: Path):
+    lock_path = tmp_path / "update.lock"
+    store = UpdateLockStore()
+    now = datetime(2026, 7, 11, tzinfo=UTC)
+    stale_after = timedelta(minutes=10)
+
+    import multiprocessing
+    start_event = multiprocessing.Event()
+    stop_event = multiprocessing.Event()
+
+    p = multiprocessing.Process(
+        target=hold_lock_worker,
+        args=(str(lock_path), start_event, stop_event),
+    )
+    p.start()
+
+    try:
+        # Wait for the worker to acquire the lock
+        assert start_event.wait(timeout=5)
+
+        # Now, try to acquire the lock in this process. It should fail.
+        lease = store.acquire(lock_path, now, stale_after)
+        assert lease is None
+
+    finally:
+        stop_event.set()
+        p.join(timeout=5)
+
+