fix(config): reload AppConfig when config path or mtime changes (#1239)

* fix(config): reload AppConfig when config path or mtime changes

- Track resolved path + mtime; invalidate cache on change
- Preserve set_app_config() injection behavior
- Add regression tests (test_app_config_reload.py)
- Document behavior in README and backend/CLAUDE.md

Signed-off-by: Gao Mingfei <g199209@gmail.com>

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Signed-off-by: Gao Mingfei <g199209@gmail.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/tests/test_app_config_reload.py

@@ -0,0 +1,81 @@
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+
+import yaml
+
+from deerflow.config.app_config import get_app_config, reset_app_config
+
+
+def _write_config(path: Path, *, model_name: str, supports_thinking: bool) -> None:
+    path.write_text(
+        yaml.safe_dump(
+            {
+                "sandbox": {"use": "deerflow.sandbox.local:LocalSandboxProvider"},
+                "models": [
+                    {
+                        "name": model_name,
+                        "use": "langchain_openai:ChatOpenAI",
+                        "model": "gpt-test",
+                        "supports_thinking": supports_thinking,
+                    }
+                ],
+            }
+        ),
+        encoding="utf-8",
+    )
+
+
+def _write_extensions_config(path: Path) -> None:
+    path.write_text(json.dumps({"mcpServers": {}, "skills": {}}), encoding="utf-8")
+
+
+def test_get_app_config_reloads_when_file_changes(tmp_path, monkeypatch):
+    config_path = tmp_path / "config.yaml"
+    extensions_path = tmp_path / "extensions_config.json"
+    _write_extensions_config(extensions_path)
+    _write_config(config_path, model_name="first-model", supports_thinking=False)
+
+    monkeypatch.setenv("DEER_FLOW_CONFIG_PATH", str(config_path))
+    monkeypatch.setenv("DEER_FLOW_EXTENSIONS_CONFIG_PATH", str(extensions_path))
+    reset_app_config()
+
+    try:
+        initial = get_app_config()
+        assert initial.models[0].supports_thinking is False
+
+        _write_config(config_path, model_name="first-model", supports_thinking=True)
+        next_mtime = config_path.stat().st_mtime + 5
+        os.utime(config_path, (next_mtime, next_mtime))
+
+        reloaded = get_app_config()
+        assert reloaded.models[0].supports_thinking is True
+        assert reloaded is not initial
+    finally:
+        reset_app_config()
+
+
+def test_get_app_config_reloads_when_config_path_changes(tmp_path, monkeypatch):
+    config_a = tmp_path / "config-a.yaml"
+    config_b = tmp_path / "config-b.yaml"
+    extensions_path = tmp_path / "extensions_config.json"
+    _write_extensions_config(extensions_path)
+    _write_config(config_a, model_name="model-a", supports_thinking=False)
+    _write_config(config_b, model_name="model-b", supports_thinking=True)
+
+    monkeypatch.setenv("DEER_FLOW_EXTENSIONS_CONFIG_PATH", str(extensions_path))
+    monkeypatch.setenv("DEER_FLOW_CONFIG_PATH", str(config_a))
+    reset_app_config()
+
+    try:
+        first = get_app_config()
+        assert first.models[0].name == "model-a"
+
+        monkeypatch.setenv("DEER_FLOW_CONFIG_PATH", str(config_b))
+        second = get_app_config()
+        assert second.models[0].name == "model-b"
+        assert second is not first
+    finally:
+        reset_app_config()