diff --git a/coworker/secrets.py b/coworker/secrets.py
index 6c1c0326..719164df 100644
--- a/coworker/secrets.py
+++ b/coworker/secrets.py
@@ -88,6 +88,18 @@ def _restrict_to_user(path: Path, *, is_dir: bool) -> None:
     os.chmod(path, 0o700 if is_dir else 0o600)
 
 
+def _write_user_only(path: Path, content: str) -> None:
+    """Write `content` to a file that is owner-only from its first byte.
+
+    `Path.write_text` creates the file with umask-default permissions (typically 0644) and a
+    later chmod leaves a window where the plaintext is readable by other local users. Opening
+    with mode 0600 closes that window on POSIX. Windows ignores the mode bits, so callers
+    still run the icacls-based `_restrict_to_user` afterwards."""
+    fd = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+    with os.fdopen(fd, "w", encoding="utf-8") as f:
+        f.write(content)
+
+
 def write_private_text(path: str | Path, content: str) -> Path:
     """Atomically write a user-only text file using the SecretStore's OS protections."""
     target = Path(path).expanduser()
@@ -97,7 +109,7 @@ def write_private_text(path: str | Path, content: str) -> Path:
     except OSError:
         pass
     tmp = target.with_name(target.name + ".tmp")
-    tmp.write_text(content, encoding="utf-8")
+    _write_user_only(tmp, content)
     _restrict_to_user(tmp, is_dir=False)
     os.replace(tmp, target)
     return target
@@ -188,6 +200,6 @@ def _write(self, store: dict[str, Any]) -> None:
         except OSError:
             pass
         tmp = self.path.with_name(self.path.name + ".tmp")
-        tmp.write_text(json.dumps(store, indent=2), encoding="utf-8")
+        _write_user_only(tmp, json.dumps(store, indent=2))
         _restrict_to_user(tmp, is_dir=False)
         os.replace(tmp, self.path)
diff --git a/tests/test_secrets.py b/tests/test_secrets.py
index c723fa95..d90ecdbb 100644
--- a/tests/test_secrets.py
+++ b/tests/test_secrets.py
@@ -8,7 +8,10 @@
 import sys
 import time
 
-from coworker.secrets import SecretStore
+import pytest
+
+import coworker.secrets as secrets_mod
+from coworker.secrets import SecretStore, write_private_text
 
 
 def test_put_get_round_trip(tmp_path):
@@ -79,6 +82,25 @@ def test_secrets_file_is_restricted(tmp_path):
         assert stat.S_IMODE(os.stat(path).st_mode) == 0o600
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="POSIX mode bits only")
+def test_secrets_never_world_readable_during_write(tmp_path, monkeypatch):
+    """Files holding secrets must be owner-only from creation, not chmod'd after the
+    plaintext already landed on disk (that ordering leaves a window where other local
+    users can read credentials, and a failed chmod leaves them exposed indefinitely).
+    Neuter the after-the-fact chmod and assert the mode is still 0600."""
+    monkeypatch.setattr(secrets_mod, "_restrict_to_user", lambda *a, **k: None)
+    old_umask = os.umask(0o022)  # permissive umask so creation-time perms do the work
+    try:
+        store_path = tmp_path / "secrets.json"
+        SecretStore(store_path).put("x", {"a": 1})
+        assert stat.S_IMODE(os.stat(store_path).st_mode) == 0o600
+
+        key_path = write_private_text(tmp_path / "id_key", "PRIVATE")
+        assert stat.S_IMODE(os.stat(key_path).st_mode) == 0o600
+    finally:
+        os.umask(old_umask)
+
+
 def test_delete(tmp_path):
     store = SecretStore(tmp_path / "secrets.json")
     store.put("x", {"a": 1})