diff --git a/core/auth.py b/core/auth.py
index 9dfa0431c..8ea1cfac0 100644
--- a/core/auth.py
+++ b/core/auth.py
@@ -396,6 +396,15 @@ class AuthManager:
             current = self.get_privileges(username)
             for k, v in privileges.items():
                 if k in DEFAULT_PRIVILEGES:
+                    default = DEFAULT_PRIVILEGES[k]
+                    if isinstance(default, bool):
+                        ok = isinstance(v, bool)
+                    elif isinstance(default, int):
+                        ok = isinstance(v, int) and not isinstance(v, bool)
+                    else:
+                        ok = isinstance(v, list)
+                    if not ok:
+                        raise ValueError(f"Invalid value for privilege '{k}'")
                     current[k] = v
             self._config["users"][username]["privileges"] = current
             self._save()
diff --git a/routes/auth_routes.py b/routes/auth_routes.py
index fc47719b7..a86e1ca35 100644
--- a/routes/auth_routes.py
+++ b/routes/auth_routes.py
@@ -362,8 +362,16 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
         user = _get_current_user(request)
         if not user or not auth_manager.is_admin(user):
             raise HTTPException(403, "Admin only")
-        body = await request.json()
-        ok = auth_manager.set_privileges(username, body)
+        try:
+            body = await request.json()
+        except ValueError:
+            raise HTTPException(400, "Request body must be a JSON object")
+        if not isinstance(body, dict):
+            raise HTTPException(400, "Request body must be a JSON object")
+        try:
+            ok = auth_manager.set_privileges(username, body)
+        except ValueError as e:
+            raise HTTPException(400, str(e))
         if not ok:
             raise HTTPException(404, "User not found or is admin")
         return {"ok": True, "privileges": auth_manager.get_privileges(username)}
diff --git a/tests/test_set_privileges_validation.py b/tests/test_set_privileges_validation.py
new file mode 100644
index 000000000..fc02a9ad4
--- /dev/null
+++ b/tests/test_set_privileges_validation.py
@@ -0,0 +1,42 @@
+"""set_privileges rejects mistyped values instead of storing them (issue #6829)."""
+
+import pytest
+
+from tests.test_set_admin import _fresh_auth_manager
+
+
+@pytest.mark.parametrize("privs", [
+    {"can_use_bash": "no"},
+    {"can_use_bash": 1},
+    {"max_messages_per_day": "abc"},
+    {"max_messages_per_day": True},
+    {"allowed_models": "gpt"},
+    {"block_all_models": None},
+])
+def test_set_privileges_rejects_mistyped_values(tmp_path, privs):
+    _, mgr = _fresh_auth_manager(tmp_path)
+    mgr.create_user("bob", "pw-123456")
+    before = dict(mgr.get_privileges("bob"))
+
+    with pytest.raises(ValueError):
+        mgr.set_privileges("bob", privs)
+
+    assert mgr.get_privileges("bob") == before
+
+
+def test_set_privileges_accepts_well_typed_values(tmp_path):
+    _, mgr = _fresh_auth_manager(tmp_path)
+    mgr.create_user("bob", "pw-123456")
+
+    assert mgr.set_privileges("bob", {
+        "can_use_bash": True,
+        "max_messages_per_day": 25,
+        "allowed_models": ["gpt"],
+        "unknown_key": "ignored",
+    }) is True
+
+    privs = mgr.get_privileges("bob")
+    assert privs["can_use_bash"] is True
+    assert privs["max_messages_per_day"] == 25
+    assert privs["allowed_models"] == ["gpt"]
+    assert "unknown_key" not in privs