diff --git a/src/kiro_crew/work_ledger.py b/src/kiro_crew/work_ledger.py
index 9e5f3f98a6e..4e5c5c32d4d 100644
--- a/src/kiro_crew/work_ledger.py
+++ b/src/kiro_crew/work_ledger.py
@@ -535,9 +535,21 @@ def _open_lock(path: Path) -> Iterator[None]:
     ``file_lock`` takes an already-open descriptor and fails CLOSED — it raises
     rather than entering the critical section unserialised — which is why nothing
     here has a lock-less fallback.
+
+    The lock file is created with ``touch`` and opened ``"r+"`` — WRITABLE, and
+    crucially WITHOUT truncation. ``msvcrt.locking`` needs a writable handle, so
+    ``"r"`` is not an option; but ``"w"`` TRUNCATES on open, and on Windows a
+    truncating open of a file whose first byte another thread or process already
+    holds under ``msvcrt.locking`` raises a sharing violation (``PermissionError``)
+    rather than waiting for the lock — so a second, contending acquirer crashes
+    before it ever reaches ``file_lock``, defeating the serialisation this lock
+    exists to provide. POSIX ``flock`` tolerates the truncate, which is why the bug
+    is Windows-only. Same reasoning, same fix as ``dashboard/handlers/mcp.py``'s
+    ``_McpFileLock``.
     """
     path.parent.mkdir(parents=True, exist_ok=True)
-    with open(path, "w") as handle:
+    path.touch(exist_ok=True)
+    with open(path, "r+") as handle:
         with file_lock(handle.fileno(), exclusive=True):
             yield
 
diff --git a/test/test_work_ledger.py b/test/test_work_ledger.py
index 5bb35aca1c2..6cf68988427 100644
--- a/test/test_work_ledger.py
+++ b/test/test_work_ledger.py
@@ -1375,6 +1375,35 @@ def bind(key: str, item_id: str) -> None:
         assert (item.worker_session_key == WORKER) == ((key, item_id) == binding)
 
 
+def test_acquiring_a_lock_does_not_truncate_the_lock_file():
+    """The lock-file open must be WRITABLE but MUST NOT truncate.
+
+    This is the property whose absence made
+    ``test_two_conductors_binding_one_worker_at_once_yield_exactly_one_binding``
+    fail on Windows only. ``msvcrt.locking`` needs a writable handle, so the fd
+    cannot be opened ``"r"``; but ``"w"`` truncates on open, and on Windows a
+    truncating open of a lock file whose first byte another holder already locked
+    raises a sharing violation instead of waiting — so the second, contending
+    acquirer crashes with a bare ``OSError`` before it reaches ``file_lock`` and
+    the bind it was serialising is never mutually excluded. POSIX ``flock``
+    tolerates the truncate, which is why the defect was invisible on Linux.
+
+    Truncation is the direct, platform-independent observable: seed the lock file
+    with bytes, acquire and release the lock, and assert the bytes survived. Under
+    the old ``open(path, "w")`` this test fails on every platform (the file is
+    emptied); under the ``touch`` + ``"r+"`` open it passes, and the same
+    non-truncating open is what stops the Windows sharing violation.
+    """
+    wl.ensure_conductor(CONDUCTOR, goal="g")
+    lock_path = wl.conductor_dir(CONDUCTOR) / wl._LOCK_FILE
+    lock_path.parent.mkdir(parents=True, exist_ok=True)
+    sentinel = b"held by a prior acquirer\n"
+    lock_path.write_bytes(sentinel)
+    with wl.conductor_lock(CONDUCTOR):
+        pass
+    assert lock_path.read_bytes() == sentinel
+
+
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