fix(config): write config.json atomically via temp+replace

save_config truncated config.json in place on crash mid-write.
Route through _write_text_atomic like the pairing store so a failed write leaves the prior file intact.

nanobot/config/loader.py

@@ -11,6 +11,7 @@ from loguru import logger
 from pydantic import BaseModel
 
 from nanobot.config.schema import Config, _resolve_tool_config_refs
+from nanobot.utils.helpers import _write_text_atomic
 
 # Global variable to store current config path (for multi-instance support)
 _current_config_path: Path | None = None
@@ -85,8 +86,8 @@ def save_config(config: Config, config_path: Path | None = None) -> None:
             "proxy": config.providers.openai_codex.proxy,
         }
 
-    with open(path, "w", encoding="utf-8") as f:
-        json.dump(data, f, indent=2, ensure_ascii=False)
+    # Temp + replace so a crash mid-write cannot leave a truncated config.json.
+    _write_text_atomic(path, json.dumps(data, indent=2, ensure_ascii=False))
 
 
 def merge_missing_defaults(existing: Any, defaults: Any) -> Any:

tests/config/test_config_atomic_save.py

@@ -0,0 +1,36 @@
+"""Tests for atomic config.json writes."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from nanobot.config.loader import load_config, save_config
+from nanobot.config.schema import Config
+
+
+def test_save_config_round_trips(tmp_path: Path) -> None:
+    path = tmp_path / "config.json"
+    save_config(Config(), path)
+    loaded = load_config(path)
+    assert loaded.agents.defaults.model
+
+
+def test_save_config_preserves_existing_file_when_write_fails(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    path = tmp_path / "config.json"
+    save_config(Config(), path)
+    before = path.read_text(encoding="utf-8")
+
+    def boom(self: Path, target: Path) -> Path:  # noqa: ARG001
+        raise OSError("simulated crash before replace")
+
+    monkeypatch.setattr(Path, "replace", boom)
+    with pytest.raises(OSError, match="simulated crash"):
+        save_config(Config(), path)
+
+    assert path.read_text(encoding="utf-8") == before
+    assert json.loads(before)  # still valid JSON