Support langgraph checkpointer (#1005)

* Add checkpointer configuration to config.example.yaml

- Introduced a new section for checkpointer configuration to enable state persistence for the embedded DeerFlowClient.
- Documented supported types: memory, sqlite, and postgres, along with examples for each.
- Clarified that the LangGraph Server manages its own state persistence separately.

* refactor(checkpointer): streamline checkpointer initialization and logging

* fix(uv.lock): update revision and add new wheel URLs for brotlicffi package

* feat: add langchain-anthropic dependency and update related configurations

* Fix checkpointer lifecycle, docstring, and path resolution bugs from PR #1005 review (#4)

* Initial plan

* Address all review suggestions from PR #1005

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

* Fix resolve_path to always return real Path; move SQLite special-string handling to callers

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

backend/src/config/app_config.py

@@ -6,6 +6,7 @@ import yaml
 from dotenv import load_dotenv
 from pydantic import BaseModel, ConfigDict, Field
 
+from src.config.checkpointer_config import CheckpointerConfig, load_checkpointer_config_from_dict
 from src.config.extensions_config import ExtensionsConfig
 from src.config.memory_config import load_memory_config_from_dict
 from src.config.model_config import ModelConfig
@@ -29,6 +30,7 @@ class AppConfig(BaseModel):
     skills: SkillsConfig = Field(default_factory=SkillsConfig, description="Skills configuration")
     extensions: ExtensionsConfig = Field(default_factory=ExtensionsConfig, description="Extensions configuration (MCP servers and skills state)")
     model_config = ConfigDict(extra="allow", frozen=False)
+    checkpointer: CheckpointerConfig | None = Field(default=None, description="Checkpointer configuration")
 
     @classmethod
     def resolve_config_path(cls, config_path: str | None = None) -> Path:
@@ -92,6 +94,10 @@ class AppConfig(BaseModel):
         if "subagents" in config_data:
             load_subagents_config_from_dict(config_data["subagents"])
 
+        # Load checkpointer config if present
+        if "checkpointer" in config_data:
+            load_checkpointer_config_from_dict(config_data["checkpointer"])
+
         # Load extensions config separately (it's in a different file)
         extensions_config = ExtensionsConfig.from_file()
         config_data["extensions"] = extensions_config.model_dump()

backend/src/config/checkpointer_config.py

@@ -0,0 +1,46 @@
+"""Configuration for LangGraph checkpointer."""
+
+from typing import Literal
+
+from pydantic import BaseModel, Field
+
+CheckpointerType = Literal["memory", "sqlite", "postgres"]
+
+
+class CheckpointerConfig(BaseModel):
+    """Configuration for LangGraph state persistence checkpointer."""
+
+    type: CheckpointerType = Field(
+        description="Checkpointer backend type. "
+        "'memory' is in-process only (lost on restart). "
+        "'sqlite' persists to a local file (requires langgraph-checkpoint-sqlite). "
+        "'postgres' persists to PostgreSQL (requires langgraph-checkpoint-postgres)."
+    )
+    connection_string: str | None = Field(
+        default=None,
+        description="Connection string for sqlite (file path) or postgres (DSN). "
+        "Required for sqlite and postgres types. "
+        "For sqlite, use a file path like '.deer-flow/checkpoints.db' or ':memory:' for in-memory. "
+        "For postgres, use a DSN like 'postgresql://user:pass@localhost:5432/db'.",
+    )
+
+
+# Global configuration instance — None means no checkpointer is configured.
+_checkpointer_config: CheckpointerConfig | None = None
+
+
+def get_checkpointer_config() -> CheckpointerConfig | None:
+    """Get the current checkpointer configuration, or None if not configured."""
+    return _checkpointer_config
+
+
+def set_checkpointer_config(config: CheckpointerConfig | None) -> None:
+    """Set the checkpointer configuration."""
+    global _checkpointer_config
+    _checkpointer_config = config
+
+
+def load_checkpointer_config_from_dict(config_dict: dict) -> None:
+    """Load checkpointer configuration from a dictionary."""
+    global _checkpointer_config
+    _checkpointer_config = CheckpointerConfig(**config_dict)

backend/src/config/paths.py

@@ -176,3 +176,15 @@ def get_paths() -> Paths:
     if _paths is None:
         _paths = Paths()
     return _paths
+
+
+def resolve_path(path: str) -> Path:
+    """Resolve *path* to an absolute ``Path``.
+
+    Relative paths are resolved relative to the application base directory.
+    Absolute paths are returned as-is (after normalisation).
+    """
+    p = Path(path)
+    if not p.is_absolute():
+        p = get_paths().base_dir / path
+    return p.resolve()