Fail fast on invalid config files

nanobot/config/loader.py

@@ -7,7 +7,6 @@ from pathlib import Path
 from typing import Any
 
 import pydantic
-from loguru import logger
 from pydantic import BaseModel
 
 from nanobot.config.schema import Config, _resolve_tool_config_refs
@@ -55,8 +54,7 @@ def load_config(config_path: Path | None = None) -> Config:
             data = _migrate_config(data)
             config = Config.model_validate(data)
         except (json.JSONDecodeError, ValueError, pydantic.ValidationError) as e:
-            logger.warning("Failed to load config from {}: {}", path, e)
-            logger.warning("Using default configuration.")
+            raise ValueError(f"Failed to load config from {path}: {e}") from e
 
     _apply_ssrf_whitelist(config)
     return config

tests/config/test_config_load_errors.py

@@ -0,0 +1,30 @@
+import json
+
+import pytest
+
+from nanobot.config.loader import load_config
+
+
+def test_load_config_missing_file_uses_defaults(tmp_path) -> None:
+    config = load_config(tmp_path / "missing.json")
+
+    assert config.agents.defaults.model
+
+
+def test_load_config_invalid_json_fails_fast(tmp_path) -> None:
+    config_path = tmp_path / "config.json"
+    config_path.write_text("{broken json", encoding="utf-8")
+
+    with pytest.raises(ValueError, match="Failed to load config"):
+        load_config(config_path)
+
+
+def test_load_config_invalid_schema_fails_fast(tmp_path) -> None:
+    config_path = tmp_path / "config.json"
+    config_path.write_text(
+        json.dumps({"tools": {"exec": {"timeout": -1}}}),
+        encoding="utf-8",
+    )
+
+    with pytest.raises(ValueError, match="Failed to load config"):
+        load_config(config_path)